# Not a war, just a sextant question.



## PaulfromNWOnt (Aug 20, 2010)

Seriously, I'm just curious:

On E-bay I can buy a Davis plastic sextant for anywhere from 40 to 120 dollars depending more on model than condition. 

Would I really be that much farther ahead to do that than to buy a metal replica that I could "tweak" for around 60 to 80?

At this point, a small consistent error is something I could live with as I'm not planning to dock my boat with it. To help with my point, I'm currently using a non WAAS enabled gps. The way I see it is that as long as I'm using the correct datum, my gps is still more accurate than a pencil dot on a 1:50,000 topo map, and I'm good with that. While unaware that I was using the wrong datum in one circumstance I still managed to extrapolate my actual position.


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## killarney_sailor (May 4, 2006)

I think that it can be fun to use a sextant but for positioning it is really very inaccurate. If you get within a mile have done well - and I know I will get rained on by all of the 'what if all the GPS satellite fall out of the sky crowd?' If you just want to know where you are, get a backup GPS. I have a Garmin 12 that works just fine after many years. I turn it on about once a month while cruising - sometimes I have to tell it which part of which ocean we are in to help it get started.


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## Boasun (Feb 10, 2007)

Sextants are great for finding your position, Especially when you are far out on the Ocean. When you come in sight of land you transition to coastal piloting.

The nasayers of "archatic" methods of Navigation really don't care or want to invest the time in learning those skills. Now I do enjoy the gps. But in my case I have a C-Plath sextant that I'm comfortable in using. Was using sextants before GPS came into being and never had any confusion as to were I was at on this big blue marble. 

Recommend you acquire the best sextant you can afford and learn to use it. But it takes practice, lots of practice in building your skills. Remember that sextants were used for a few centuries before GPS came about.

And there have been many cases where mariners have run aground because they depended on the GPS far to many time.


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## chrisncate (Jan 29, 2010)

Boasun said:


> Sextants are great for finding your position, Especially when you are far out on the Ocean. When you come in sight of land you transition to coastal piloting.
> 
> The nasayers of "archatic" methods of Navigation really don't care or want to invest the time in learning those skills. Now I do enjoy the gps. But in my case I have a C-Plath sextant that I'm comfortable in using. Was using sextants before GPS came into being and never had any confusion as to were I was at on this big blue marble.
> 
> ...


I can't seem to get it (celestial nav) from fancy book lernin.. can you explain in simple human terms how you do the calculations after shooting the sight (the one thing I can do)?


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## Boasun (Feb 10, 2007)

There are formats that are easy to use. I have Weems & Plath (H.O. 229 Sight solver) that can ease your way into understanding the math involved. And there are a couple of books that simplify Celestrial navigation.


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## travlin-easy (Dec 24, 2010)

I've only tried two sextants during the past decade, including the Davis plastic sextant sold by West Marine. The best accuracy I experienced was approximately +/- 6 miles, which would be fine in the middle of the ocean. So, if both of my GPS systems failed, and I was well offshore when the failures occurred, then the GPS would be a handy gadget to have onboard. However, I've used Loran-A, Loran-C and GPS, all since their inception, and never experienced a failure. Either I'm darned lucky, or electronic navigation systems are very reliable.

I guess from my perspective, having a good, electronic backup system is a far better investment, especially if you're not going to spend endless days and weeks offshore. However, if you are going offshore, maybe crossing the oceans, then as stated above, purchase the best sextant you can afford--it may end up being your only backup, navigational system.

Good luck on whatever you decide upon,

Gary


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## chrisncate (Jan 29, 2010)

::sigh::

Still nothing. What _is_ it with the secret society of celestial.. 

Thanks anyway... I'll look at what you recommended.


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## travlin-easy (Dec 24, 2010)

Chris,

If you wish, I can email you a great book on learning how to use a sextant. It's a down to earth book that puts things in plain, easy to understand language. Just email me and I'll send it via return email.

Cheers,

Gary


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## chrisncate (Jan 29, 2010)

travlineasy said:


> Chris,
> 
> If you wish, I can email you a great book on learning how to use a sextant. It's a down to earth book that puts things in plain, easy to understand language. Just email me and I'll send it via return email.
> 
> ...


I will (and thank you), however I must warn you I have already bought a few books that billed themselves as easy to understand, and I couldn't understand them.. I am terrible at math, math theory, and anything to do with math...

I'll shoot you an email and give it a shot though...


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## travlin-easy (Dec 24, 2010)

It's on its way.

Good Luck,

Gary


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## chrisncate (Jan 29, 2010)

travlineasy said:


> It's on its way.
> 
> Good Luck,
> 
> Gary


Thank you sir  , if it's finally the book that makes me understand this alchemy, I will be forever in your debt.


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## PaulfromNWOnt (Aug 20, 2010)

Thanks for the replies folks.

I understand that the GPS is the way to go, but it's missing something. I think I just want something a little more organic (the only time I use this term is for sailing related stuff). While I can afford a decent sextant, if I had my way I'd just borrow one and take a course. My location precludes the course, and my thrifty nature prevents me from spending a gob of cash on a top-notch unit.

If/when I make a decision, I'll keep you posted how it works out.


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## SlowButSteady (Feb 17, 2010)

Chris,

Here is a *very basic* method (you'll have to refer to a book to fill in the details):

- Take the sighting by using the sextant to make the celestial body appear to touch the horizon; note the angle and the exact time of the sighting.

- Correct the angle for: sextant error, semi-diameter of the body, parallax, height of your eye above the horizon, refraction of the atmosphere, and number of beers you have had in the preceding eight hours. This is your observed elevation (Ho).

- Calculate the "declination" of the body (its angle relative to the equator) at the moment of your sighting (this is done via a nautical almanac)

- Calculate the LHA (Local Hour Angle) of your sighting

- Calculate the GHA (Greenwich Hour Angle) of the body at the time of your sighting

- Calculate the longitude of your sighting by comparing GHA to LHA

- Presume a latitude to the nearest degree, and use this to look up the elevation of the body at the time you took the sighting (Hc; from an almanac); also note the azimuth of the body at the time of the sighting (Z; again, from an almanac)

- Scribe a line on your chart (or worksheet) through your presumed latitude and calculated longitude at the azimuth angle (about +/- 10 miles is usually more than long enough, unless you've been unable to take a fix for some time)

- Compare Hc to Ho. The difference is the intercept (if this is too large, you may have to rejigger your presumed latitude, or cut down on the beer consumption).

- From the point through which you scribed Z measure a distance equal to the intercept, either toward the sun (if Hc > Ho) or away from the sun (if Hc < Ho). That is your fix (plus or minus and ocean or two).

- Do the same thing with two or three other bodies (in as short a period of time as possible) and you should have a nice tightly grouped set of marks, indicating a reliable fix.

- Now, compare your fix to that of your GPS. With a plastic sextant and a little practice you should be within 4 or 5 miles. With a nice sextant, within less than a mile. Double or triple the error if you're in a small boat on anything but a flat calm sea.


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## chrisncate (Jan 29, 2010)

Thanks sbs, I wish I understood any of what you wrote. I'm sure it's a great explanation, however it might as well been written in Mandarin as far as what I can comprehend. 

It's pretty clear I won't be able to learn this without someone actually showing me in person what all that stuff you wrote actually means. I know when I'm beat, and facing something I need someone to teach me "hands on".

Thanks for trying, it appears (what you wrote) to be the closest attempt at an actual nuts and bolts explanation I've gotten to date. It's intimidating and depressing that I have to master this (I want to use celestial as my primary means of navigation on passages) and I'm not actually sure that I can. It's rare when I encounter something I can't "self teach" myself, but here I am - stumped and totally on the outside looking in.

Thanks for taking the time, I appreciate it greatly (even though I don't understand any of it).


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## SlowButSteady (Feb 17, 2010)

chrisncate said:


> Thanks sbs, I wish I understood any of what you wrote. I'm sure it's a great explanation, however it might as well been written in Mandarin as far as what I can comprehend.
> 
> It's pretty clear I won't be able to learn this without someone actually showing me in person what all that stuff you wrote actually means. I know when I'm beat, and facing something I need someone to teach me "hands on".
> 
> ...


It is something of a byzantine art. Unfortunately, just like boats in general, celestial navigation also has its own lingo. However, the basic mechanism is pretty easy if you just take it step by step.

One basic thing to keep in mind is that as the earth spins on its axis all the stars "move" across the sky. The sun, moon, and planets also move, but in somewhat more complicated patterns. If we just consider a star for the moment, and setting aside the effects of various errors and the atmosphere, all stars appear to move in a nice smooth arc as the Earth spins. Thus, the star's angle above the horizon (its elevation) will be dependent upon the _*time of day*_ and _*the location on the Earth from which one makes the observation*_. IF one can determine one of these two variables, one can calculate the other. Since watches accurate to within a few seconds cost only a few bucks these days, a good estimate of time is readily available to everyone. The rest is just looking up numbers in books with teeny-tiny print, and adding and subtracting those numbers in the correct order to estimate ones position.

Here's a link that may help: website; PDF of printed version


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## Minnewaska (Feb 21, 2010)

I will take a crack at celestial nav 101 over a cup of coffee. For starters, forget the sextant. 
Learning how to use it has nothing to do with understanding what is going on.

You are trying to figure out two things. First, your distance north or south of the equator (latitude). Second, your distance east or west from an artificial starting point on the planet (longitude) That was arbitrarily set at Greenwich England.

Latitude first. This would be easier with a model, but can you picture yourself standing on the equator and pointing at the sun high in the sky at noon? Now picture yourself standing in Canada at the exact same time on the exact same day and point at the sun. The angle of your arm would change because you moved but what you are pointing at didn't. You don't need to know much math. The angle changes as you move away from the equator. Once you know that angle, you can look up in a published table to know how far you are from the equator.

Now longitude. Set your watch to noon when the sun is at its highest point in the sky as you are standing in Greenwich England. Now, wherever you are on the globe, just check your watch when the sun is at its highest point. That will tell you how long it has been since it was noon in Greenwhich England. If your watch says 3:00, the sun has traveled for three hours since it was right above Greenwich. Since the sun travels fully around the earth in 24 hours, you are 3/24ths of the distance around the globe from Greenwich. Again, you just look that time up in a table and it will tell you how far you are west of Greenwich.

The sextant is only a device to measure that angle for latitude. When that angle reaches its highest for the day, it is noon and you check your watch to see how far from Greenwhich you are.

The techniques for doing this precisely with a sextant are a separate matter, but have nothing to do with the concept.

How was that?


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## wingNwing (Apr 28, 2008)

Minne - "That" was perfect. Coincidentally, last weekend we went to an event where they demonstrated various medieval navigation tools for latitude pre-sextant (astrolabe, backstaff, and something (I don't have the name of it in front of me, but it was basically just a calibrated block of wood and a string). Even with these, we got ourselves within 20 miles...all based on exactly the underlying theory you described. It's much simpler to explain now that we know that the earth revolves around the sun rather than vice-versa.


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## JimsCAL (May 23, 2007)

Keep in mind that a sextant is only really useful in the open ocean. You basically can get positions three times a day - noon, dusk and dawn. Not really useful for coastal cruising. There are really two separate skills - (1) taking the sight using the sextant, and then (2) doing the calculations to get the position. The math for the sight reduction is complicated (spherical trig) so most use tables (HO249, HO214, etc) or a computer program or preprogrammed calculator to crunch the numbers. Definitely a skill that takes lots of practice to get even moderately good at. A nice mental exercise, but not something the average sailor is going to need. Just get a spare (or two) GPS units.


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## cb32863 (Oct 5, 2009)

Sometimes it is just _fun _to do it the old fashioned way. That's why I learned how to do it and I sail on an inland lake. Just wanted to know how they did it back in the old days. All the flames aside from both sides of the argument. I learned because *I* wanted to know how it was done, I find it interesting on how it all works, and enjoy it.

EDIT: I have a plastic Davis one myself, it may go without saying but, try to keep the plastic one out of the sun. They are not very accurate when they warp from being cooked by the sun.


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## PaulfromNWOnt (Aug 20, 2010)

Minnewaska that was the best way to describe the way I understand it. Copied and pasted into a doccument for future reference, thank you.

My understanding is that most (all) of the tables/almanacs are available online, and some of them are even customizable to show only specific time frames to reduce the number of pages you need to print out.

I think I will go with a low cost yet pretty metal sextant and have a go. If the functionality suffers, I can still use it as a cheap conversation starter.


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## johnnyandjebus (Sep 15, 2009)

Minnewaska said:


> I will take a crack at celestial nav 101 over a cup of coffee. For starters, forget the sextant.
> Learning how to use it has nothing to do with understanding what is going on.
> 
> You are trying to figure out two things. First, your distance north or south of the equator (latitude). Second, your distance east or west from an artificial starting point on the planet (longitude) That was arbitrarily set at Greenwich England.
> ...


Minnewaska.
The above is about the most useful, basic, simple and easy to understand explanation of CN I have ever read. It ought to have a sticky attached to it.

John


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## chrisncate (Jan 29, 2010)

First, thank you for the great explanation. It's really good.



> Latitude first. This would be easier with a model, but can you picture yourself standing on the equator and pointing at the sun high in the sky at noon? Now picture yourself standing in Canada at the exact same time on the exact same day and point at the sun. The angle of your arm would change because you moved but what you are pointing at didn't. You don't need to know much math. The angle changes as you move away from the equator. Once you know that angle, you can look up in a published table to know how far you are from the equator.


So this angle is determined by the sextant - you read the degrees and note the exact time you are reading the degrees as soon as you have lined up your sight? Am I understanding this correctly?



> Now longitude. Set your watch to noon when the sun is at its highest point in the sky as you are standing in Greenwich England. Now, wherever you are on the globe, just check your watch when the sun is at its highest point. That will tell you how long it has been since it was noon in Greenwhich England. If your watch says 3:00, the sun has traveled for three hours since it was right above Greenwich. Since the sun travels fully around the earth in 24 hours, you are 3/24ths of the distance around the globe from Greenwich. Again, you just look that time up in a table and it will tell you how far you are west of Greenwich.


So.. you need the sextant for latitude, and the tables for longitude - is this correct? What happens next? How do you combine both pieces of data in order to find where you are? I realize you probably put this answer in here already, I'm really trying to understand exactly what you are saying.



> The sextant is only a device to measure that angle for latitude. When that angle reaches its highest for the day, it is noon and you check your watch to see how far from Greenwhich you are.


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## SlowButSteady (Feb 17, 2010)

Good summary, Minnewaska. The only problem with "the noon sighting" is that it actually consists of a series of observations as the sun rises and then falls around local noon. And, of course, it only can be used with the sun. This means that the sun and horizon must be visible and clear for about 10 or 20 minutes (or more, depending on how accurate ones dead reckoning has been) around local noon.

The "line of position" method is what I outlined above. It was first developed (using slightly different methodology) around the middle of the 19th century (rather fortuitously, and under the most challenging of circumstances; see here) and allows one to fix position any time two or more celestial bodies and the horizon are visible (usually dawn and dusk). With practice, one can fix position in 5 minutes or so twice a day, usually with a bit higher accuracy than the noon position method.

Even if just the sun and horizon are visible, one can still combine ones dead reckoning position with a single line of position to get a pretty good estimate of ones actual position (although, this is not an actual "fix" of position).


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## PaulfromNWOnt (Aug 20, 2010)

chrisncate:

My understanding is that you take a series of sights, and mark down the time for each of them. If you plot these sights and times on a graph, you will then be able to eyeball the time when the sun was at it's highest point overhead. Using the appropriate almanac/table/form that time will indicate your longitude.

Let me see if I can confuse myself even more:

You use the sextant to determine latitude directly-ish with one sight, but you take multiple sights to make sure that your solar noon is correct to determine longitude accurately...???

Kinda??

Sorta??

Hmm...


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## PaulfromNWOnt (Aug 20, 2010)

SlowButSteady said:


> Good summary, Minnewaska. The only problem with "the noon sighting" is that it actually consists of a series of observations as the sun rises and then falls around local noon. And, of course, it only can be used with the sun. This means that the sun and horizon must be visible and clear for about 10 or 20 minutes (or more, depending on how accurate ones dead reckoning has been) around local noon.
> 
> The "line of position" method is what I outlined above. It was first developed (using slightly different methodology) around the middle of the 19th century (rather fortuitously, and under the most challenging of circumstances; see here) and allows one to fix position any time two or more celestial bodies and the horizon are visible (usually dawn and dusk). With practice, one can fix position in 5 minutes or so twice a day, usually with a bit higher accuracy than the noon position method.
> 
> Even if just the sun and horizon are visible, one can still combine ones dead reckoning position with a single line of position to get a pretty good estimate of ones actual position (although, this is not an actual "fix" of position).


Dood.... I love the idea of accuracy, and not having to rely on only one time of day, but I'm still at the baby steps and hand-holding stage. I'm going to add your post to Minne's for future reference.

Thank you.


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## SlowButSteady (Feb 17, 2010)

PaulfromNWOnt said:


> Dood.... I love the idea of accuracy, and not having to rely on only one time of day, but I'm still at the baby steps and hand-holding stage. I'm going to add your post to Minne's for future reference.
> 
> Thank you.


I'm not knocking Minnewaska's post at all. Conceptually, that is how celestial nav was done for about 100 years. However, once the line of position methods were developed they were used almost exclusively until electronic navigation (LORAN, GRS, et cetera) were developed.


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## smurphny (Feb 20, 2009)

I have a cheap plastic sextant in case GPS ever disappears. Being math-challenged, I had a hard time understanding the concepts. Some publications that focus on the trig make my eyes glaze over and I'm convinced they are written by people whose mission it is to discourage most people from attempting celestial nav. There is no way I will EVER understand the math. BUT, understanding the basic idea of the navigational triangle is not too difficult. Once I understood the 3-D picture of global position, assumed position, zenith distance, azimuth, and the hard-to-get-your-head-around idea that the rays of a body hit the entire surface of the earth in a parallel fashion, the actual process slowly became clear. Understanding the triangle is the basis for all of it. I also bought the great little computer program by Omar Reis which has everything needed to accomplish the task of figuring out where you are.

A good publication is Mary Blewitt's _Celestial Navigation for Yachtsmen_ although imo it could explain some concepts a little better.


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## AdamLein (Nov 6, 2007)

I like Minnewaska's explanation as well. A noon sighting---which I really don't think fell out of use at all (all the cruising books I've read mention noon sights or no sights)---is one example of a special special case of CN, which is the meridian passage---the point when a star crosses a meridian.

Meridians are lines of longitude. At any point in time, a given celestial body's center is over exactly one meridian. "Noon" is when you and the sun are on the same meridian, so noon is the sun's meridian passage. But every star has a meridian passage, and you can do a "noon sight" for any star.

The other method mentioned here is the "line of position" method. This method is based on more complex math that simplifies to basic addition in the case of a meridian passage. You may have heard about the navigational triangle. Imagine taking an orange and drawing two dots somewhere on the skin, but not on the stem or navel. One of the dots is your position on the Earth and the other is the star's position in the sky (or rather, the spot directly below it on the Earth). You can now draw a triangle on the orange by connecting the navel (aka "north pole") with these two dots using great circles. That's the navigational triangle. There's some fancy math for solving these triangles, or you can use tables. Either way, a lot of work.

But not in all cases is the resulting triangle really a triangle. The special case is when the navel and the two dots are lined up. If you draw a great circle through those three points, you are drawing a meridian; both the meridian you are on, and the meridian the star is on. So this is a meridian passage. Now instead of this crazy curvy triangle and the complex math/tables that you need for it, you just have a simple line. Here's how you solve it:

The distance on the line from the north pole to you is CL = 90° - your latitude, or "colatitude".
The distance on the line from the north pole to the star is CD = 90° - the star's declination, which is just a fancy word for latitude on the sky; this is "codeclination".
The distance on the line from you to the star is ZD = 90° - the star's altitude in the sky, measured by your sextant; this is "zenith distance".

Depending on the order the points come in, you may end up adding or subtracting, but once you have this picture in your head, it should be easy to follow. For example if you're farther north than the star (i.e. you're looking south the see the star), then CD = CL + ZD. If you're looking north, there are two possibilities, based on whether the star is higher or lower in the sky than Polaris. You can just draw the picture to figure out the formula, or you can stick to using stars in the south*.

This is how you get latitude from a meridian passage; longitude is conceptually even simpler and has already been explained. Note that it works regardless of whether the star is the sun, a planet, or any other very distant light source (more corrections are needed for the moon). So if I were navigating by CN, I would go for three sights per day: meridian passage of a bright star or planet in the morning and evening twilight (you need to be able to see the horizon) and a noon sight. If I were feeling particularly masochistic I might do the fancy intercept method, but not if I had any real responsibilities at the time, which I expect would be always.

I admit I'm being a bit hard on the intercept method. You don't actually have to do any of the fancy math, because it's all in the tables for you. You only have to do basic arithmetic. But it still is more work than a meridian passage.

*I am simplifying a bit. In principle if you're in the southern hemisphere you will probably be looking north for noon sights, and if you're in the tropics, your noon sights might be any which way. In any case, having that picture in your mind of those three points---north pole, you, star---lined up on a meridian tells you everything you need to know to do CN by meridian passage.


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## killarney_sailor (May 4, 2006)

The best way to learn celestial is to actually do it. Take a sight and one of the standard ways of reducing it and see how you do. Likely there will be some issues with the calculations or perhaps the actual sight (it is a bit of an artform on a small boat). Then take some more sights and keep plugging away at it. When I tried to learn it from a book I found it quite hard to visualize everything. When you do it, step-by-step it makes much more sense and it is easier.


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## Minnewaska (Feb 21, 2010)

SlowButSteady said:


> Good summary, Minnewaska. The only problem with "the noon sighting" is that it actually consists of a series of observations as the sun rises and then falls around local noon. And, of course, it only can be used with the sun. This means that the sun and horizon must be visible and clear for about 10 or 20 minutes (or more, depending on how accurate ones dead reckoning has been) around local noon.
> 
> The "line of position" method is what I outlined above. It was first developed (using slightly different methodology) around the middle of the 19th century (rather fortuitously, and under the most challenging of circumstances; see here) and allows one to fix position any time two or more celestial bodies and the horizon are visible (usually dawn and dusk). With practice, one can fix position in 5 minutes or so twice a day, usually with a bit higher accuracy than the noon position method.
> 
> Even if just the sun and horizon are visible, one can still combine ones dead reckoning position with a single line of position to get a pretty good estimate of ones actual position (although, this is not an actual "fix" of position).


I agree, although, this alternate method is much harder for a 101 level course.

I also agree that you must take several sites around noon, to determine exactly when local noon ocurred. As the angle keeps increasing, it must be getting closer to noon and once it begins to decrease again, you've passed noon. However, I was trying to avoid how to use the sextant when trying to explain why this all works.

We're on the same page, I know.


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## Minnewaska (Feb 21, 2010)

chrisncate said:


> So this angle is determined by the sextant - you read the degrees and note the exact time you are reading the degrees as soon as you have lined up your sight? Am I understanding this correctly?
> 
> So.. you need the sextant for latitude, and the tables for longitude - is this correct? What happens next? How do you combine both pieces of data in order to find where you are? I realize you probably put this answer in here already, I'm really trying to understand exactly what you are saying.


Close, but not exactly. Different tables for both latitude and longitude, although longitude can be done with a formula too.

It can also depend on the tables you use. If you understand the concept of your angle to the sun being a reference for latitude, then the "latitude" tables will give the resultant latitude for every angle on a given day. Since those angles would also be different at each hour of the day, the table you use will likely only have noon, as you need that for longitude anyway.

As other posters have pointed out, you keep taking the noon sight until you know you've approached and passed noon. You know this because the angle kept getting bigger, then suddenly gets smaller right as you pass noon. Mark the time then and you can either use a formula to determine longitude or look that time up in another table.

I didn't want to get too deep into the sextant until the idea makes sense, because the art of using the sextant is really a separate discussion. For example, if you take your noon sights every few minutes around the time it looks like the sun is highest, you will have rising and lowering angles, but would only be lucky to have actually taken a sighting at exactly noon. You may need to extrapolate abtome between to sightings. This is where error enters the method. The sun is moving around the planet at roughly 1,000 miles per hour. Each second that your local noon time is off will introduce about a third of a mile of inaccuracy in longitude. That could be your technique, but could also be error in your watch. That's why quality timepieces were so critical back in the day.

I better stop there and see if I've answered your question.


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## Squidd (Sep 26, 2011)

So what happens if your watch stopped working..?


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## travlin-easy (Dec 24, 2010)

Squidd said:


> So what happens if your watch stopped working..?


The error factor increases dramatically, but you could still function to some degree by taking at least three daily sightings and having the tables available.


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## SlowButSteady (Feb 17, 2010)

Squidd said:


> So what happens if your watch stopped working..?


If you have absolutely no time-keeping device, you're hosed (unless you can use the much more complicated "lunar method", which most old-time navigators ditched once accurate chronometers became cheap enough). But, you can set a watch, or clock, to the correct time (or just note the error) via the SSB time signal. Before radio, ships often had multiple chronometers and used their average as the best estimate of GMT. Surveying ships, such as HMS Challenger, often had a dozen or more chronometers in an effort to get as good an estimate of GMT as possible.


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## SlowButSteady (Feb 17, 2010)

travlineasy said:


> The error factor increases dramatically, but you could still function to some degree by taking at least three daily sightings and having the tables available.


Huh?? Without a chronometer one can estimate latitude (most simply via the elevation of Polaris). However, finding longitude requires knowing GMT; unless the "lunar method" is used, but that's an order of magnitude (or two) more complicated.


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## Squidd (Sep 26, 2011)

I'm just saying cause the watch is one of the first things I take off when I go sailing... 

That and turn off the cell phone...


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## CapnBilll (Sep 9, 2006)

Without the watch how do you know when it's happy hour?


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## bljones (Oct 13, 2008)

If you have to ask when happy hour is, you ain't part of the solution.


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## bljones (Oct 13, 2008)

Hey, do they have sextents at the base camp on Everest?
or can you pretty much do it in any tent you want?


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## TQA (Apr 4, 2009)

In practice for a noon sight which is the easiest one to understand you take several sights noting the exact time and angle, before and after local noon.

Then EITHER plot those onto a piece of graph paper this allows you to eliminate obviously bad sights and gives you an arc OR use an everaging program on a programmable pocket calculator. [I used the latter method for some years.]

Use the max angle and time for your noon sight calculation.

I used a cheap plastic Ebbco and found it OK for sun sights but no use for star sights.

As the plastic ones are a LOT lighter than the likes of a Plath you may find them easier to use and if you keep them out of the sun and always put them back in their box they stay ' in tune ' pretty well. A tleast mine did.

While I have not experienced the cheap chinese metal ones myself gossip says they are not good and the deviations vary dramatically.


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## chamonix (Jun 25, 2010)

Looks like I'm going to have to learn how to use a sextant. Intresting story. At one point in my life I decided I needed to learn how to shave with a staight razor. Not sure why, something about all that steel from razors just being discarded, and a feeling that it was a skill worth knowing. So I did. Learned two things. One was why moustache's and goatee's were so popular until the modern razor came along, and the secound was why straight razors disapeared after the modern safety razor came along. Still think its a skill worth knowing and still shave that way every now and then to keep my hand in.
So at some point I'm going to have to learn how to use a sextant and I'm going to get the exact same lesson.


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## Capt Len (Oct 9, 2011)

Nobody. has mentioned dip and tllt of the earth in taking a noon site. If you are doing this at equinox (sun over the equator) and you allow for dip, how high you are (off the water) )horizon). and correct for cheap sextant ,your reading, subtracted from 90 , should approximate where you're at latitudinaly. Another day and you use the sine table for that day in the almanac and add in the winter, subtract in summer, max at solstice (northern). If you've timed this at GMT, that's the longitude. 24 hours=360degrees=earth's circumference at equator. Having a mental model of whats what makes it easier to build on when it gets rough. Worked for me.


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## killarney_sailor (May 4, 2006)

*Not an unreasonable analogy except for the blood part*



chamonix said:


> Looks like I'm going to have to learn how to use a sextant. Intresting story. At one point in my life I decided I needed to learn how to shave with a staight razor. Not sure why, something about all that steel from razors just being discarded, and a feeling that it was a skill worth knowing. So I did. Learned two things. One was why moustache's and goatee's were so popular until the modern razor came along, and the secound was why straight razors disapeared after the modern safety razor came along. Still think its a skill worth knowing and still shave that way every now and then to keep my hand in.
> So at some point I'm going to have to learn how to use a sextant and I'm going to get the exact same lesson.


I don't rember ever cutting myself while taking sights, although I might have increased my blood pressure and chance of stroke waiting for a break in the clouds when I really needed to get a sight.


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## AdamLein (Nov 6, 2007)

Capt Len said:


> If you are doing this at equinox ... your reading should approximate where you're at latitudinaly.


For example, suppose I'm standing on the equator on the equinox. At noon, the sun is directly overhead, so altitude is 90°. You're saying that the latitude of the equator is 90°?

The result you got is the colatitude, not the latitude. Subtract from 90° to get the latitude.



> Another day and you use the sine table for that day in the book and add in the winter, subtract in summer, max at solstice (northern).


Sine tables? Seriously? Does the almanac even have sine tables? Why not just look up the declination?


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## SlowButSteady (Feb 17, 2010)

AdamLein said:


> ...
> Sine tables? Seriously? Does the almanac even have sine tables? Why not just look up the declination?


So one can calculate (via spherical trig) some of the terms directly. For instance, LHA can be calculated by plugging the corrected observation of the celestial body, its declination (from an almanac), and ones dead reckoning latitude into an equation known as "Bowditch's Third Method". Of course, even a pretty cheap calculator has trig functions in it, so the trig tables are usually unnecessary. But, trig and logarithm tables are included if one wants to just use pencil and paper.


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## AdamLein (Nov 6, 2007)

SlowButSteady said:


> So one can calculate (via spherical trig) some of the terms directly. For instance, LHA can be calculated...


Hm, okay, but the cited example was latitude by noon sight. Surely it's easier to use the solar declination from the almanac than to compute a declination using a sine table! The sine table method would require several extra calculations, each of which has a chance of error.


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## smurphny (Feb 20, 2009)

Here's a link to a downloadable Pdf. file, a "SHORT" explanation on how to use a sextant. HaHaHaHaHa....... http://www.celnav.de/page2.htm


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## SlowButSteady (Feb 17, 2010)

AdamLein said:


> Hm, okay, but the cited example was latitude by noon sight. Surely it's easier to use the solar declination from the almanac than to compute a declination using a sine table! The sine table method would require several extra calculations, each of which has a chance of error.


One gets declination from the tables. The declination is the angle of the celestial body relative to the equator at the time of the observation. One simply notes the time of the observation and looks up the declination at the top of the hour before the observation and at the top of the hour after the observation and interpolates to estimate the declination. Local Hour Angle (LHA) can then be calculated using spherical trig.


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## Capt Len (Oct 9, 2011)

My understanding is that the declination for the sun is a sine function; the tables have already worked it for you I was trying to draw a picture of why. for a simple noon shot. Sorry about the colatitude.I forgot to count down from 90 N .I haven't done it since 1967.


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## joethecobbler (Apr 10, 2007)

If you enjoy using the sextant or would like to , try googling making a sextant , it's quite interesting. The first one I bought was a $20 brass chinese model sold by harbor frieght . I purchased it just to play with not expecting any degree of accuracy from it. It is also agood teaching tool when teaching celestial nav to my daughter. and if it gets dropped, no harm done.


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## MedSailor (Mar 30, 2008)

Minnewaska said:


> I will take a crack at celestial nav 101 over a cup of coffee. For starters, forget the sextant.
> Learning how to use it has nothing to do with understanding what is going on.
> 
> You are trying to figure out two things. First, your distance north or south of the equator (latitude). Second, your distance east or west from an artificial starting point on the planet (longitude) That was arbitrarily set at Greenwich England.
> ...


Miniwaska's post =









I believe I have now been enlightened. I've always been able to solve math that I can visualize (trig-geometry, Newtonian physics equations), but never math where I can't "see" the significance of the variables in my mind's eye (integral calculus) . This post may single-handedly allow me to understand the "easy" celestial navigation books. Thanks!

MedSailor


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## PaulfromNWOnt (Aug 20, 2010)

This thread keeps on getting better. I can really appreciate all the insight into the "Dark Arts" of celestial navigation. It's also nice to know that you don't have to be a rocket scientist to get started.


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## chrisncate (Jan 29, 2010)

Minnewaska said:


> Close, but not exactly. Different tables for both latitude and longitude, although longitude can be done with a formula too.
> 
> It can also depend on the tables you use. If you understand the concept of your angle to the sun being a reference for latitude, then the "latitude" tables will give the resultant latitude for every angle on a given day. Since those angles would also be different at each hour of the day, the table you use will likely only have noon, as you need that for longitude anyway.
> 
> ...


Ok, I _think_ I understand the noon site and it's significance (off by just seconds and the error(s) pile up), and this leads me to ask - how the heck does one do celestial nav by shooting sites of _other_ bodies - stars, the moon... ? Should I not even think about that yet? Yikes...

Back to the lesson at hand - so... you have your noon site (let's say) within 30 seconds of actual noon at your location - You then check the longitudinal table for the difference between the time where you are at local noon, compared to when it was noon GMT? Do I have that right? God this is confusing, and I;m not even sure I worded that right.. (please be patient, this is very difficult when discussing this in theory without "hands on" show and tell)..

?



> Now longitude. Set your watch to noon when the sun is at its highest point in the sky as you are standing in Greenwich England. Now, wherever you are on the globe, just check your watch when the sun is at its highest point. That will tell you how long it has been since it was noon in Greenwhich England. If your watch says 3:00, the sun has traveled for three hours since it was right above Greenwich. Since the sun travels fully around the earth in 24 hours, you are 3/24ths of the distance around the globe from Greenwich. Again, you just look that time up in a table and it will tell you how far you are west of Greenwich.


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## Geoff54 (Oct 30, 2011)

This covers the absolute basics but is very easy to understand (has pictures  ).
http://www.davisnet.com/product_documents/marine/manuals/00026-710_IM_00025.pdf


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## SlowButSteady (Feb 17, 2010)

chrisncate said:


> Ok, I _think_ I understand the noon site and it's significance (off by just seconds and the error(s) pile up), and this leads me to ask - how the heck does one do celestial nav by shooting sites of _other_ bodies - stars, the moon... ? Should I not even think about that yet? Yikes...
> 
> Back to the lesson at hand - so... you have your noon site (let's say) within 30 seconds of actual noon at your location - You then check the longitudinal table for the difference between the time where you are at local noon, compared to when it was noon GMT? Do I have that right? God this is confusing, and I;m not even sure I worded that right.. (please be patient, this is very difficult when discussing this in theory without "hands on" show and tell)..
> 
> ?


No tables needed. Once you determine local noon, relative to GMT, you know your longitude.

Let's say you determine that local noon (when the sun is at its greatest elevation) is at 1505 GMT (3:05 pm, Greenwich Mean Time). So that means that at your observation point noon was 3 hours and 5 minutes after noon at the Prime Meridian (0˚ longitude). There are 360˚ of longitude and 24 hours in a day, so "noon" moves to the west 15˚ an hour (or the Earth spins at 15˚ per hour, however you want to think of it). Three hours and five minutes equals 3.083 hours, 3.083 hrs x 15˚/hr = 46.25˚, or 46˚15' west longitude. If you happened to be on the equator (latitude = 0) you would be just off the coast of Brazil, about 100 nm east of the mouth of the Amazon River.


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## Minnewaska (Feb 21, 2010)

SlowButSteady said:


> No tables needed. Once you determine local noon, relative to GMT, you know your longitude.
> 
> Let's say you determine that local noon (when the sun is at its greatest elevation) is at 1505 GMT (3:05 pm, Greenwich Mean Time). So that means that at your observation point noon was 3 hours and 5 minutes after noon at the Prime Meridian (0˚ longitude). There are 360˚ of longitude and 24 hours in a day, so "noon" moves to the west 15˚ an hour (or the Earth spins at 15˚ per hour, however you want to think of it). Three hours and five minutes equals 3.083 hours, 3.083 hrs x 15˚/hr = 46.25˚, or 46˚15' west longitude. If you happened to be on the equator (latitude = 0) you would be just off the coast of Brazil, about 100 nm east of the mouth of the Amazon River.


That's how it works, although, you can get a table to avoid having to do the math. Better yet, I got curious and it looks like there are iPad apps that will solve the math, if you provide the angles and time. One even says you can use the iPad for the sightings, which has to be a scam or inaccurate, but might be fun.


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## Minnewaska (Feb 21, 2010)

chrisncate said:


> Ok, I _think_ I understand the noon site and it's significance (off by just seconds and the error(s) pile up), and this leads me to ask - how the heck does one do celestial nav by shooting sites of _other_ bodies - stars, the moon... ? Should I not even think about that yet? Yikes...
> 
> Back to the lesson at hand - so... you have your noon site (let's say) within 30 seconds of actual noon at your location - You then check the longitudinal table for the difference between the time where you are at local noon, compared to when it was noon GMT? Do I have that right? God this is confusing, and I;m not even sure I worded that right.. (please be patient, this is very difficult when discussing this in theory without "hands on" show and tell)..
> 
> ?


The method to use other celestial bodies is analogous to doing a triangulation off known points ashore. If you can draw a line across a chart that comes from any point you are looking at, you know you are on that line somewhere. Take another sighting, draw another line and theoretically you are where the two intersect. The method usually requires three lines and similar errors to the noon sight method will cause them not to intersect perfectly, forming a little triangle near where they each intersect. Chances are, you are inside the triangle somewhere.

What get very complicated is determining where these celestial bodies are, because, unlike shooting a sight to a fixed land mass, they move (and you move). All expect one, the north star, Polaris. (technically it moves a little)

Yes, seems like you are getting the noon sight longitude concept.


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## PaulfromNWOnt (Aug 20, 2010)

Minnewaska said:


> That's how it works, although, you can get a table to avoid having to do the math. Better yet, I got curious and it looks like there are iPad apps that will solve the math, if you provide the angles and time. One even says you can use the iPad for the sightings, which has to be a scam or inaccurate, but might be fun.


I think some smart programming dude might be able to come up with a rudimentary sextant for either the ipad or iphone. Using the built in level feature, you could have a virtual "horizon" on the screen, then use the camera to do your sights. The device would automagically capture time and do the averages to get proper noon.

Bingo.... here you are!

Would I use it for nav purposes? I think I'd trust my lack of skill over a level on an electronic device, but it would be neat.


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## smurphny (Feb 20, 2009)

Declination is simply the latitude of the sun/star/planet where the body is at 90 degrees to the Earth's surface--directly overhead at a certain moment. This is one of the known variables in your equation. It changes continually which is why an accurate timepiece is needed. The ray, theoretically shone through the Earth's center forms the same zenith distance as the one at our position. This gives the second known variable. It's important because it gives you the distance, in degrees of one leg of the triangle- from your position to the gp of the body.

All these calculations have corrections. The Earth doesn't rotate at a constant rate so the time=longitude calculation needs to be corrected by the "equation of time" found in the almanac (or program). Height of eye (dip), parallax, sextant error, watch error, refraction/semi-diameter(altitude corr.) all need to be plugged into the calculations to get any sort of accuracy. The computer programs do most of this. If you had to figure lops by hand, it would be tedious and prone to a lot of error. 

A good way to practice is using a cookie tray filled with water next to a window (or outside) at home. If you can position the water surface so that the image on the water and the one through the sextant mirror can be seen and aligned, it will give you a 2X reading of the observed altitude. Knowing the lat/lon of your spot, you can do the calcs and then check your figures against known position.


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## TQA (Apr 4, 2009)

smurphny said:


> A good way to practice is using a cookie tray filled with water next to a window (or outside) at home. If you can position the water surface so that the image on the water and the one through the sextant mirror can be seen and aligned, it will give you a 2X reading of the observed altitude. Knowing the lat/lon of your spot, you can do the calcs and then check your figures against known position.


Remember you also need to know your altitude pretty accurately when you use this method.


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## chrisncate (Jan 29, 2010)

SlowButSteady said:


> No tables needed. Once you determine local noon, relative to GMT, you know your longitude.
> 
> Let's say you determine that local noon (when the sun is at its greatest elevation) is at 1505 GMT (3:05 pm, Greenwich Mean Time). So that means that at your observation point noon was 3 hours and 5 minutes after noon at the Prime Meridian (0˚ longitude). There are 360˚ of longitude and 24 hours in a day, so "noon" moves to the west 15˚ an hour (or the Earth spins at 15˚ per hour, however you want to think of it). Three hours and five minutes equals 3.083 hours, 3.083 hrs x 15˚/hr = 46.25˚, or 46˚15' west longitude. If you happened to be on the equator (latitude = 0) you would be just off the coast of Brazil, about 100 nm east of the mouth of the Amazon River.


Ok, now I think I get (at least this part of) it, excellent explanation 

I'm still looking over and trying to understand all the helpful info from everyone who has helped out in this thread, but I think I am closer than ever to understanding this dark art. I'm hoping that once I have a sextant in hand and begin to try to learn by doing, it becomes clear on all accounts.

Thanks everyone, I know this thread has helped more than just me!


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## smurphny (Feb 20, 2009)

TQA said:


> Remember you also need to know your altitude pretty accurately when you use this method.





TQA said:


> Remember you also need to know your altitude pretty accurately when you use this method.


Yes, sorry, should have mentioned that. That's a thing to remember when on a lake as well. At first, I couldn't figure out why my readings were out by a number of degrees until this dawned on me. I'm up here in the mountains, at about 1450' and it is really impossible to get an accurate enough height above sea level. All I can do is to use readings from my little Brunton weather gizmo, which has an altimeter, the gps, or topographic map gradient data but none of these is really accurate enough to use. I can either reverse-engineer dip correction which is possible because I have a known, accurate actual position or, what I've done (probably classified as cheating): have adjusted my sextant to known altitude without any corrections which, it would seem, eliminates dip and index error. I also worry that through the double-paned window glass, there is likely some unknown amount of light refraction, but again, adjusting the sextant to eliminate error probably addresses this as well. I could probably go outside but as I write this, it is 10 below 0 I can use Omar Reis' nice little navigation program, using my exact lat/lon from gps to get a reading for some body, set the sextant to that number and quickly go over and do a sight adjusting the MIRROR to fit the correct (2X) altitude. Doing this I can run through the exercise using different sights, even doing intersecting lops if I can see two or more useful stars, pretending I don't know what the answer will be and using a fictitious assumed position. Just have to remember to readjust the sextant when back down on the water.


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## flyingwelshman (Aug 5, 2007)

chrisncate said:


> Thanks everyone, I know this thread has helped more than just me!


It is certainly removing the scales from my eyes!

Now, unfortunately, I am tempted to get hold of a sextant and take a course.....


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## PaulfromNWOnt (Aug 20, 2010)

Yes, this thread has certainly gone a long way in simplifying things. There is much talent on sailnet, and thankfully also a willingness to share.


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## cb32863 (Oct 5, 2009)

Here is a link to Starpath. The course I took, ASA 107, at my sailing school used the book from this course. I found it to be an excellent resource and a very easy read. Give it a look, I think you can get just the book.


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## Squidd (Sep 26, 2011)

Been an informative thread... while I may never use the skills outlined at least I know "how" it all works and is no longer a black art shrouded in mystery...

Course I still don't know what time it is....


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## MedSailor (Mar 30, 2008)

Since this thread is becoming quite the "how-to" and seems to be getting "stickier" by the minute, I'll ask the contributing minds a question. 

Can you post for us please a book list, with short reviews of each, of books that will help the celestially incompetent? I also, have had the problem of it all turning to Greek after 5 pages or so, but some books must be better than others. 

Pictures and cartoons = good. 
Copious Greek math symbols = bad. 

Please post your recommendations for us, and a review of their strengths and weaknesses. 
Cheers!

MedSailor


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## chrisncate (Jan 29, 2010)

Squidd said:


> Been an informative thread... while I may never use the skills outlined at least I know "how" it all works and is no longer a black art shrouded in mystery...
> 
> Course I still don't know what time it is....


 Well, you could always point the boat north and use the mast as your sundial...

(Why didn't they do that before time pieces again?..)


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## FoolishMuse (Oct 12, 2006)

The absolute, very best Sextant Users Guide in the whole world can be downloaded for free here:
http://www.bethandevans.com/pdf/sextant.pdf

Excuse my modesty

Andy


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## travlin-easy (Dec 24, 2010)

Gotta' agree with Andy--it's a great book and the price is right.

Gary


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## smurphny (Feb 20, 2009)

MedSailor said:


> Since this thread is becoming quite the "how-to" and seems to be getting "stickier" by the minute, I'll ask the contributing minds a question.
> 
> Can you post for us please a book list, with short reviews of each, of books that will help the celestially incompetent? I also, have had the problem of it all turning to Greek after 5 pages or so, but some books must be better than others.
> 
> ...


I've read through Blewitt's _Celestial Navigation for Yachtsmen_ carefully. It seemed to be the one I could best understand but different learning styles will probably favor different publications. Have also been absolutely stymied by Umland's _A short Guide to celestial Navigation_. The little guide that comes with the Davis sextants is helpful and simple. The "help" files that come with the Omar Reis program are really helpful. Discussion boards like this are almost as helpful as anything else. If there was a course given by anyone around here, I would have and would still do it because being shown is usually quicker than reading about it.

In studying this on my own for a couple of years, the biggest Aha moment came when I finally grasped the picture of the 3D triangle. At least it made sense of the basic principle and gave meaning to terms like line of position.

The most astounding thing to me is how FAST planets move! Last night Venus scooted across the southern sky almost too fast for the sextant. Almost needed a shotgun.


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## ronspiker (Jun 27, 2001)

My son bought me a book on how to use a sextant. I borrowed a friends and have played with it a bit. I actually like it. I think it is a nice backup to GPS and I like the math and all the calculations, a bit of a geek. I think its fun. Plus if the electronics do go out its a nice safety measure.


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## erps (Aug 2, 2006)

> Please post your recommendations for us, and a review of their strengths and weaknesses.
> Cheers!


I learned from Basic and intermediate Celestial Navigation by WM. Bruce Paulk. The book goes over the tools required then practical celestial problems from one of his trips from California to Hawaii. He used a method that only required the Nautical Almanac. Since then, I learned the method using the H.O. tables, which I thought was a lot easier.

I can't say whether this particular book is better or worse than others, but it worked for me. I live near you. I could loan you the book if you would like to see it for yourself.


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## INMA (Sep 13, 2011)

Answering the original question, buying an instrument made to be accurate and made to last in a marine environment is better value than a cheap copy made in a shed in a third world country.

The Davis can sit in a yacht for any length of time and the mirrors and pivots should function properly, even after some exposure. The cheap copy will soon be stuffed sitting in a yacht for a few years.

Why would you bother buying a decorative item and try to use it for real work.


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## hellosailor (Apr 11, 2006)

A metal "replica" sextant is a toy that is designed to be turned into a table lamp, wall art, or presentation plaque. Please, don't even think about trying to navigate with one. I'm sure it will work as well as two crossed sticks but...probably never as well as the plastic. Which suffers from thermal expansion, but that can be compensated for.

There's also a whole world of real metal sextants, real instruments, from $300-500 with several decent metal models (ARES, ASTRA) and a lot of used ones that originally sold for 2-3x more. If you buy a used one, learn how to check it out and adjust it, or buy from a reputable source.

Accuracy? Pursuit of accuracy is a wonderful thing as a mental game, like chess. But on a small craft in a moving ocean, if you can get your position refined within two mautical miles, that's good enough for government work. And with proper care, the Davis or the inexpensive metal ones will all get you that.

"Replica" is a code word for "toy."


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## AdamLein (Nov 6, 2007)

chrisncate said:


> Ok, I _think_ I understand the noon site and it's significance (off by just seconds and the error(s) pile up), and this leads me to ask - how the heck does one do celestial nav by shooting sites of _other_ bodies - stars, the moon... ?


Like I said above, the "noon site" is just a meridian passage of the sun. You can use the meridian passage method for any celestial body to get both latitude and longitude.


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## chef2sail (Nov 27, 2007)

This is a great thread.... very enlightening and the best sim ple explaination I have seen about things.

Dave


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## PaulfromNWOnt (Aug 20, 2010)

INMA said:


> Why would you bother buying a decorative item and try to use it for real work.


People do it all the time with items that are much larger, and orders of magnitude more expensive than sextants. My original question was answered about 3 pages back though, thank you.

On to another post about books: I really think that this thread is a great explanation of celestial nav, and I wonder if it would look good in booklet form.

While I have no real fears of the GPS system going down, I like the mental exercise involved in using astral bodies to determine my location. Realistically though.... a $150.00 GPS is likely a little more accurate than a $500 sextant even though you need to carry spare batteries.

Hellosailor helped seal the deal for the Davis.


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## AdamLein (Nov 6, 2007)

PaulfromNWOnt said:


> While I have no real fears of the GPS system going down, I like the mental exercise involved in using astral bodies to determine my location. Realistically though.... a $150.00 GPS is likely a little more accurate than a $500 GPS even though you need to carry spare batteries.


For sure. Fundamentally sailing is not about doing things using the most advanced technology available; we do it for fun, and CN is fun


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## PaulfromNWOnt (Aug 20, 2010)

AdamLein said:


> For sure. Fundamentally sailing is not about doing things using the most advanced technology available; we do it for fun, and CN is fun


Can we get an AMEN!


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## hellosailor (Apr 11, 2006)

chrisncate-
I'm not sure if you got the direct answer to this along the way...but actually "how the heck does one do celestial nav by shooting sites of other bodies - stars, the moon... ?" Shooting anything with the sextant doesn't really do anything by itself. The sextant would be a lousy nutcracker, without "sight reduction tables". 
The easiest way to see that is with a real globe and a piece of string. The tables tell you exactly where some object (planet, moon, star) is above the surface of the earth as some specific time. Sort of a "local noon" for each object at every possible time. If you were at that location, the object would be dead above you, 90 degrees above the horizon. If it is at any other angle...that just means you are not directly under it. You are offset from that position by xxx miles, and by using spherical trigonometry (which seems perfeclty useless back in grade school) you can work backwards saying "if the angle is x, then I am y miles away from that spot." Now you draw a big cicrle on your globe, you know you are somewhere on that circle y miles away from that spot.
Do it again with a second body, and the two circles should intersect in two places. Do it a third time, and all three should intersect more or less in only one place. (Realistically, a sloppy triangular splotch not a pure intersection.)
Which is how a navigator says "We're here-ish."

If you don't mind a little slop, really, you can navigate with a globe. But it really helps put the whole thing in perspective. 

And if you don't mind a little more "electronic reliance" a sight reduction calculator or program makes for much faster work than sight reduction tables, with possibly less chance for error. It all comes down to measuring angles, then marking them on charts. And without the sight reduction tables (based on incredible amounts of calculations down by professionals, one hopes) it doesn't work well at all.


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## sneuman (Jan 28, 2003)

chrisncate said:


> I can't seem to get it (celestial nav) from fancy book lernin.. can you explain in simple human terms how you do the calculations after shooting the sight (the one thing I can do)?


Chris,
Concentrate on the noonsite. It's easiest and the calculation is dumbell easy (which, for me at least, is a good thing):

CELESTIAL REASONING: Quick & Dirty Noon Sights - www.boats.com


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## hellosailor (Apr 11, 2006)

chris-
Look for a copy of "Mixter's Primer of Navigation" it is literally a primer, like "Dick and Jane", that explains the actual theory and what you are measuring and calculating behind all kinds of navigation, not just the sextant. There should also be some web sites that break it out in "Oh DUH!" terms not just calling for calculations and tables, and other books.
Once you've seen the physical relationship, what you are actually working the numbers on, it WILL all gel. The frightening thing is that 'radians' and 'spherical trig' all of a sudden become incredibly clever and useful concepts. (G)


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## wingNwing (Apr 28, 2008)

I like Schlereth's book: Amazon.com: Celestial Navigation in a Nutshell (Seafarer Books) (9781574090581): Hewitt Schlereth: Books for easy understanding. (Disclaimer: my very first time with a sextant, I stood on the shore so I can't blame the rocking of the boat ... but I still plotted myself about 20 miles inland.  )

Proof of concept: last weekend we attended a demo of 16th century navigation techniques (among other things). Using a compass, sundial, and protractor, we shot a noon sight and got within 50 miles of our actual location. Only about double the error of my plastic Davis? Not a very impressive showing for 400 years of innovation!


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## PaulfromNWOnt (Aug 20, 2010)

Wing'n'wing, that is exactly what got me to thinking about buying a sextant. These guys did things hundreds of years ago with ancient technology that we are terrified of doing ourselves with all the new stuff the banks can afford to buy for us.

I guess I want to get back to the days of "Wooden Ships and Iron Men" without getting any splinters.


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## SlowButSteady (Feb 17, 2010)

wingNwing said:


> ...Using a compass, sundial, and protractor, we shot a noon sight and got within 50 miles of our actual location. Only about double the error of my plastic Davis? Not a very impressive showing for 400 years of innovation!


A Davis sextant should be more accurate than you infer. I can usually fix my position within less than 5 miles (sometimes less than 2 miles) with an old Davis and using 3 or 4 LOP sightings. Of course, that's standing on the beach or on an anchored boat. Actually underway the error often dramatically increases, depending on sea state.


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## SlowButSteady (Feb 17, 2010)

PaulfromNWOnt said:


> Wing'n'wing, that is exactly what got me to thinking about buying a sextant. These guys did things hundreds of years ago with ancient technology that we are terrified of doing ourselves with all the new stuff the banks can afford to buy for us.
> 
> I guess I want to get back to the days of "Wooden Ships and Iron Men" without getting any splinters.


Yes. The "trick" is two-fold: first, one has to be able to settle for knowing ones position to within a few miles, rather than a few feet; and second, one has to be really, REALLY good at dead reckoning (as the time/distance between fixes can be days and/or hundreds of miles). The latter is probably the most important consideration. Using dead reckoning the error increases with time, particularly in bad weather. Since bad weather is exactly when celestial fixes are few and far between, estimating ones position becomes doubly problematic at these times.

Let's say I'm able to get a celestial fix to within 5 miles, using LOP sightings, at dusk. It the weather turns nasty and I'm not able to get a good fix for another 23-24 hours, my dead reckoning could easily be 20 or 30 miles off. Likewise, if the storm lasts several days, my dead reckoning estimate of my position could be well over 100 miles off, maybe more. An error of +/- 5 or 10 miles on the open ocean is trivial, but an error of +/- 100 or 200 miles could be deadly.


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## wingNwing (Apr 28, 2008)

SlowButSteady said:


> A Davis sextant should be more accurate than you infer. I can usually fix my position within less than 5 miles (sometimes less than 2 miles) with an old Davis and using 3 or 4 LOP sightings. Of course, that's standing on the beach or on an anchored boat. Actually underway the error often dramatically increases, depending on sea state.


Agreed, and with practice I did get better, I could consistently get within a 5-10 mile range. But I was comparing my first time with a sextant, to my first time with the "primitive" methods.


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## hellosailor (Apr 11, 2006)

Actually a beach can be as bad as a rocking boat. There are usually atmospheric distortions caused by the heat rising off the coastal boundary (land/water heating and cooling differently, etc.) that make sightings from a beach or shoreline notoriously inaccurate.

And in some places, the coastal charts are also simply wrong. The US 1927 survey is sometimes still used and I found several structures (a brick tower and stadium) were 1/3 mile off their real positions, comparing the chart to the GPS. 1/3 mile isn't too bad, when the original survey was rod-and-chain and no one ever bothered updating it.


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## captflood (Jan 1, 2011)

GREETINGS EARTHLINGS In this worl of digital time which system is correct for every four seconds your off true time (GMT) your one mile out in calculations! Try to aline the meridian passage (for your area) and get some practice dropping stars yoiu can use a faulse horizon.. SING A SONG OF SEXTANTS, LET GRANNY HAVE A TRY,FOUR AND TWENTY COSINGNS MULTIPLIED BY PIE, WHEN THE SUM IS ADDED UP, WHAT IS YOUR POSSITION? SOMWHERE IN THE CARPARK BEHINDE THE SEA-MANS MISSION .GO SAFE


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## chrisncate (Jan 29, 2010)

hellosailor said:


> chrisncate-
> I'm not sure if you got the direct answer to this along the way...but actually "how the heck does one do celestial nav by shooting sites of other bodies - stars, the moon... ?" Shooting anything with the sextant doesn't really do anything by itself. The sextant would be a lousy nutcracker, without "sight reduction tables".
> The easiest way to see that is with a real globe and a piece of string. The tables tell you exactly where some object (planet, moon, star) is above the surface of the earth as some specific time. Sort of a "local noon" for each object at every possible time. If you were at that location, the object would be dead above you, 90 degrees above the horizon. If it is at any other angle...that just means you are not directly under it. You are offset from that position by xxx miles, and by using spherical trigonometry (which seems perfeclty useless back in grade school) you can work backwards saying "if the angle is x, then I am y miles away from that spot." Now you draw a big cicrle on your globe, you know you are somewhere on that circle y miles away from that spot.
> Do it again with a second body, and the two circles should intersect in two places. Do it a third time, and all three should intersect more or less in only one place. (Realistically, a sloppy triangular splotch not a pure intersection.)
> ...


Between all the great replies/resources posted here, I THINK (think) I might basically understand how it works now, and I'm really looking forward to trying it "in the field".. Regarding more electronics... I'm open to the calculator, as I'll probably need one no matter what (I am just plain horrible at math, and I'm not sure I could do the calculations without on in fact). I do plan on trying to learn it without the aid of a calculator though, so we'll see..


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## PaulfromNWOnt (Aug 20, 2010)

I don't know if this older thread will pick up any traffic, but a friend directed me to an iphone/ipad app that contains a "sextant", as well as a multitude of nav tools. It's called "Spyglass", and a google search for spyglass sextant will turn up information on this app.

It's a 2 dollar app, but lacking an ianything I can't try it out myself. The youtube videos make it seem like a very versatile little app.

Anyone want to try it out and get back to us?


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## AdamLein (Nov 6, 2007)

PaulfromNWOnt said:


> I don't know if this older thread will pick up any traffic, but a friend directed me to an iphone/ipad app that contains a "sextant", as well as a multitude of nav tools. It's called "Spyglass", and a google search for spyglass sextant will turn up information on this app.
> 
> It's a 2 dollar app, but lacking an ianything I can't try it out myself. The youtube videos make it seem like a very versatile little app.
> 
> Anyone want to try it out and get back to us?


I'm a total sucker for this sort of thing but the app doesn't seem to be available on Android.

However I have played with an Android app, StarStruck Navigation, that I thought did a pretty good job. I got in touch with the developer to report a bug and request a couple of features and he was pretty quick in responding.

With StarStruck, you point your phone at (either align normal vector with, or much easier, sight along the edge) a star and click "record". Do it several times and you can then ask it do plot a fix on the map using the intercept method. Pretty straightforward.

Alternatively you can manually enter in altitudes.

Finally it acts as a crude starfinder for your location.

I have no commercial interest.


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## Minnewaska (Feb 21, 2010)

I may give it a try for giggles. Since my iPad has an internal gps, I have to admit to being suspicious of the app. I guess I would feel better if it was wrong, but then........


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## AdamLein (Nov 6, 2007)

M: fwiw Star Struck was pretty wrong. About as wrong as you'd expect, sighting along the edge of a phone at a star.


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## Minnewaska (Feb 21, 2010)

AdamLein said:


> M: fwiw Star Struck was pretty wrong. About as wrong as you'd expect, sighting along the edge of a phone at a star.


Ok then, great. I think..... 

I went to the app store and found several CN apps. Spyglass really isn't a CN app per se, but a nav app that has a CN feature. There were a few others, such as one from Navimatics (maker of Charts and Tides), that looked liked they could be more robust, but I lost interest in doing the research on which to try. It is cool to have an app that has perpetual lifetime sight tables, even if you took your own sightings.


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## sneuman (Jan 28, 2003)

I'd be more surprised if it worked and was anywhere in the ballpark. A sextant is a very precise instrument for a reason. You can't just substitute sighting down the edge of a cell phone, no matter how "smart" that phone is.


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## svzephyr44 (Jun 26, 2000)

Suggestions:

1. Don't try and figure out the math. Just get a book that gives you a step by step worksheet that you can understand. You are trying to get a position. You don't need to understand celestial mechanics - just how to read the tables and plug in the numbers. If you do that often enough you might have an "ah ha" moment.
2. Use your GPS position as the DR (dead reckoning) start position at first. This eliminates the steps of using your log to figure out where you estimate you are from your last fix.
3. If possible find someone who knows how to do the math and have them walk you through the steps a few times.
4. Consider getting a celestial navigation computer. This solves all of your problems. I would bet, although I have not looked, that there might even be a web site that would take your sights and reduce them to Lines of Position (LOP's)
5. Relax - relax - relax. Those of us that do this do it for the psychic benefits. We actually enjoy it (hard to believe but true.) Don't go overboard (not an intended pun but until I remembered my harness I almost did once.) Some of the old navigation systems, like staring at the sun through a forked stick, are hazardous to your health, but this one is pretty safe. 

A hint - all you are doing is taking three bearings on stars. Think of it as if you were in an enclosed harbor with a hand held compass and you were taking bearings on three landmarks. You would then transfer the reciprocal bearings to lines on your chart and where they cross is where you are. For reasons that are unimportant it is very difficult to measure the angle from 000 degrees of a star. Celestial navigation is actually a work around for that problem.

Don't worry about the quality of your sextant unless you actually get good at this and do it every day. Taking the sights is what introduces the errors. I don't know anyone who can take decent sights without doing it very frequently. If possible start on land or when you are at anchor to minimize the impact of trying to take a sight from a pitching boat. It also helps if you have a scribe to record the time and sight angles.

Most important have fun.


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## PaulfromNWOnt (Aug 20, 2010)

Zephyr, I love the Zen approach to CN. I figure as long as I'm getting a consistent error I'm doing OK.

Minne, I also like the idea of perpetual tables. I suppose if the electronics go then we're in serious trouble by not having paper tables, but hey.... I'd be in trouble anyway. heh heh


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## hellosailor (Apr 11, 2006)

Anyone who has used Google Sky Map can tell you that using a smartphone for anything astronmical can be a rude surprise. Mine kept saying "West" when I was looking SOUTH.

It turns out that this is a common error, the magnetic sensors in the smartphones need to be regularly recalibrated (you make voodoo figure 8 passes with the phone) in order to get anything near reality with them.


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## killarney_sailor (May 4, 2006)

*An interesting wrinkle*



Minnewaska said:


> I may give it a try for giggles. Since my iPad has an internal gps, I have to admit to being suspicious of the app. I guess I would feel better if it was wrong, but then........


Get suspicious if your positions are too accurate or if the app suggested that the GPS feature is required for offshore use. I agree with someone else, sighting along the edge of the IPad is just not that accurate. You might get accuracy to one degree (if lucky) but that won't do much for a useful position.


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## AdamLein (Nov 6, 2007)

hellosailor said:


> Anyone who has used Google Sky Map can tell you that using a smartphone for anything astronmical can be a rude surprise. Mine kept saying "West" when I was looking SOUTH.
> 
> It turns out that this is a common error, the magnetic sensors in the smartphones need to be regularly recalibrated (you make voodoo figure 8 passes with the phone) in order to get anything near reality with them.


I've had similar problems with Google Sky Map. In my case it had no idea what direction I was facing and would routinely put solar system objects in incorrect _relative_ positions, as though it were plotting for the wrong date, but displaying the correct date. Another problem was that it kept forgetting where I said I was. These things seem to have been resolved now.

Anyway, a "sextant app" only uses the accelerometer, AFAIK. They do have sensitivity problems but surprisingly I've rarely seen the Hs that the app gives be _terribly_ wrong.


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## svzephyr44 (Jun 26, 2000)

*Celestial Calculator*

Put this in a page named "something.html" and open it with your web browser. It works with the Sight Reduction Tables for Marine Navigation downloadable for free from Maritime Safety Information Also you can get a bunch of useful calculators for free at the same site.
..............................................


</******>



Pub 229

This page allows calculation of the data in the Sight Reduction 
Tables for Marine Navigation (Pub. No. 229).

The main purpose of this calculator is to facilitate the practice 
of celestial navigation at sea. A secondary purpose is to 
provide the solutions of a spherical triangle of which two sides 
and the included angle are known and it is necessary to find the 
values of the third side and adjacent angle.



 
Latitude:



(degrees)

Same name as declination
 
(minutes)

Contrary name as declination

 
Declination:



(degrees)
 
(minutes)

 
Local Hour Angle (LHA):



(degrees)
 
(minutes)

  










Altitude (Hc):

(degrees)


(minutes)

Azimuth (Z):

(degrees)


(minutes)


-->
.......................................................


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