Celestial (nautical) navigation

Miscellaneous and Personal Stuff I Must Share
1

Well, I did not want to call this thread “ask the navigator” or “ask the sailor” for several reasons, one being that probably the first question posted I may not know how to answer…

So I’ll state it this way: I am knowledgeable about the history of navigation at sea and very specially about celestial navigation. I have read about it, learnt it and taught it.

If anyone has any questions I will try to answer them. If I do not know the answer, I will try to find it. And all are welcome to contribute.

Today, of course, celestial navigation is useful only as a backup for GPS. I have many friends who try to convince you that, for bluewater sailing, you should know astronav in case your GPS fails.

Well, for the price of an average sextant you can buy four GPS units… how many backups do you need? As I said to a friend who was admiring my sextant. He said something like “it must be wonderful using this on your boat” and i said: “Are you crazy? Do you think I am going to risk damaging that sextant by taking it on the boat? That sextant does not leave the house! I’d rather lose four GPSs!”

Anyway, besides any specific questions or topics I will post some trivia and fun facts.

2

Where does one purchase a celestial / nautical almanac these days?

Is there a preferred supplier of such things?

Have you ever attempted to use an astrolabe?

How exactly does the astrolabe differ from the sextant?

3

How much does the average sextant cost?

4

Oh yes, and I remember reading in Maiden Voyage, by Tanya Aebi(sp?), that she used a cheap sextant, at first, for celestial navigation and it was all screwed up. What, exactly, differentiates a cheap sextant from a quality one?

5

If I studied at it, without any equipment, looking up on an average clear night, about how accurately would I be able to guess my Latitude, the time of night, and the Day of Year?

Also, I know the Big Dipper spans darn near close to 25°, and that the diameter of the full Moon is around ½°. Are there any other good landmarks (so to speak) that give you handy angular distances?

6

As I understand it, a sextant is basically a glorified ‘plumb-line attached to a protractor’. Is this essentially what ancient (i.e., pre-sextant) mariners used to determine latitude? And how was longitude determined at all, especially before the development of accurate timepieces and the establishment of a ‘zero-point’.

Also, what is ‘dead-reckoning’?

7

Okay. I’ve always wanted to learn Celestial Navigation.Planetariums give courses on it, so I might do it yet. In the meantime…
How accurately can you personally fix your position? What’s your margin of error typically, and at best?

And for comparisons sake, how accurate have you found consumer GPS equipment to be? When you compare it on a good map does it agree?

8

Wow, I did not expect such an interest… Ok here I go:

Tymp
>> Where does one purchase a celestial / nautical almanac these days?

The Nautical Almanac is published jointly in London by Her Majesty’s Stationary Office and in Washington DC by the Naval Observatory. It can be purchased from the Government Printing Office for $31.
Here are the details.

>> Is there a preferred supplier of such things?

Well, I always bouth it from the GPO but there other almanacs published. There are commercial editions published which contain the same info but are somewhat cheaper because they contain ads. The format is the same. You can also buy almanacs published by other countries and they have different formats and have been calculated independently

To tell the truth, I have not purchased a printed almanac in some years because I have some computer programs that are good enough. They are not as precise as the NA but they are good enough and save me that cost.

>> Have you ever attempted to use an astrolabe?

Yes, I have used an astrolabe and have great admiration for the people who used it for actual observations because, even on land, it is not easy to use. The best you can expect to do is about one degree.

>> How exactly does the astrolabe differ from the sextant?

Well they are totally different. I am working on a web page describing these intruments and this would be perfect to answer your question if it were ready but it has been the graphics that are slowing me down. I will try to get it done and post a link here soon. This is a case where one picture is worth a million words.

For now let me put it this way: A sextant is a circle that hangs from a ring and takes the vertical. A rotating arm is pointed at the Sun and you read the altitude of the sun on the graduated circle. You cannot get any simpler than that. It could also be used at night but there was a slightly different instrument used for this and called a nocturnal.

The Sextant on the other hand uses a couple of mirrors so that you can see in one line of sight both the horizon and the astrobody. Note that the astrolab does not use the horizon. I will post a basic explanation of the sextant in the next couple of days for you to see.

I hope this answers your questions for now.

9

Democritus

>> How much does the average sextant cost?

Well this is a bit like asking how much does a car cost but I’ll give it a try.

The Davis Mark3 at about $40 is almost a toy and meant for learning but 16th century people would have killed to have something like that. I have done some actual navigation with it and it works… of course, it makes you feel better to have the GPS for backup :slight_smile:

The Davis Mark 15 costs about $125 and is quite better in that it has more features and is of better quality. For example it has a telescope which makes the sighting more precise whereas the mark 3 just has a visor (no optics).

The Mark 25 (good photo BTW) is similar to the Mark 15 but with a couple of things added and costs $200. I have also used it in practice and, even though it may not match the precision of a metal frame sextant, I would use it happily for any ocean crossing on a small boat.

If you are being tossed about on the deck of a small boat, the precission of your sight is already limited and a better instrument means nothing except losing a more expensive piece of equipment if it would fall overboard.

Then you go on to metal sextants.
Celestaire sells the
Astra which is made in China for under $500 whereas a Plath goes for $3000.

Personally I would not spend more than $1000 on a sextant unless I could have sex with it :slight_smile:

The old joke: A man goes into a sporting store and asks “do you have sextants?” and the attendant, somewhat confused, says, “weell, … we do have tents, but what you do in them is up to you”
>> What, exactly, differentiates a cheap sextant from a quality one

A sextant is an instrument designed to measure angles very precisely. It has to be machined and adjusted with great precission. A cheap plastic sextant will change dimensions with temperature more than a metal one which is much more rigid. So you are talking quality. Both sextants are basically the same thing.

I hope that answers your question. feel free to ask more.

10

Shoot! lemme try those links again:
Celestaire sells the Astra which is made in China for under $500 whereas a Plath goes for $3000.

11

Achernar

Achernar (Alpha Eridanus), as we all know (don’t we?), is a navigational star, magnitude 0.6 (very bright), SHA=336, dec=S57. Given it’s souther declination you have to be south of lat 33N to see it. I have been south of 33N but never sailing so I do not think I have ever seen it. Anyway, let’s get down to business…

>> If I studied at it, without any equipment, looking up on an average clear night, about how accurately would I be able to guess my Latitude, the time of night, and the Day of Year?

Well, this is tough because it depends on how good you are at estimating angles, where you are in the world and other factors. But I will give you some very general information:

Latitude is easier in the northern hemisphere. Subtract from 90 the H of Polaris over the horizon and that’s your latitude. In other words, the Zenital distance of the celestial pole is your latitude. If you want more precission you need some corrections because Polaris is not at the celestial pole. 16th century mariners knew this and applied the necessary corrections. I have a facsimile reproduction of a 16th century navigation manual where this is explained.

If you know the day of the year and have a protractor, you can look at the stars and tell time to about 5 or 10 mins. with no instrument it depends too much on the ability to measure angles and we are all pretty bad… let’s say one hour. If you know the time, you can tell the date but if you do not know either then you are stuck.

>> Also, I know the Big Dipper spans darn near close to 25°, and that the diameter of the full Moon is around ½°. Are there any other good landmarks (so to speak) that give you handy angular distances?

Well, “up to a certain point” (as the joke says meaning “no”). Let’s start with the Big Dipper. From Dubhe to Alkaid is about 27 degrees (I am not measuring, just looking at the chart) which I suppose is what you are referring to, but strictly speaking Talitha belongs to the big dipper (or rather Ursa Major) and is about 47 degress away. But I know what you mean. The diameter of the moon and the sun are roughly 30’ as you say but they do change somewhat depending on the distance.

To be honest I do not know any more rules of thumb although you can easily make your own depending on what part of the world you live in. While I am quite versed in navigation and I do like stargazing, I do not have the knowledge of the skies and constellations other people have.

I do have an introductory book to astronomy that had exactly this chart of "landmarks"but unfortunately I do not have it at hand and I will not see it again for a few months…

I do have a half baked web page that I started long ago about telling time by looking at the stars. Maybe I will finish it and post it in the next few days. You guys are giving me some homework! :slight_smile:

I hope this begins to answer your questions but feel free to ask more.

12

Mauve Dog

>> As I understand it, a sextant is basically a glorified ‘plumb-line attached to a protractor’. Is this essentially what ancient (i.e., pre-sextant) mariners used to determine latitude? And how was longitude determined at all, especially before the development of accurate timepieces and the establishment of a ‘zero-point’.

Again, “up to a certain point” (meaning absolutely not!) A sextant is nothing like that. I will try to post a basic explanation of a sextant within the next 24 hrs. A sextant measures the angle between the horizon and the astrobody. You are probably thinking of the astrolabe which is a circle which is oriented by gravity, with an arm that is free to rotate.

Another instrument was a quadrant which was a quarter of a circle with a plumb bob hanging from the corner (center of the circle). maybe this is what you are thinking of. I will try to post schematics of this also.
>> Also, what is ‘dead-reckoning’?

Dead reckoning is called “dead” because most of those who used it ended up dead… har har (yeah, I know it’s not even close to begin to be funny).

OK, they say the term “dead reckoning” is derived from “deduced reckoning”. Basically what it means is keepin track of the position of the ship solely by taking into account where you were and what directions and speeds you have been doing, therefore, no outside references like landmarks, astro nav etc. It comes down to distance = time * speed. you draw it on the chart and find out where you are.

In the old times speed was measured with the chip log and hourglass and course with the compass, stars etc. it was a rough thing by today’s standards. I will try to give more details about the chip log and glass later.

More questions?

13

Wow, these are great answers and definitions sailor! Thanks!

14

CalMeacham

>> I’ve always wanted to learn Celestial Navigation

I hear people say that all the time but most of the time it is wishful thinking because when they realise it is not as simple as they wished, they lose interest fast. Rather than “taking a course” (fast learn, faster forget) I would recommend buying a book and studying slowly over time and letting things sink in. It does take a while. A course would probably be much more productive if you already know some of the basics.

Here is one internet introduction. The problem with most books and tutorials is they drown you in so much stuff you have no clue what you are doing. You lose sight of the big picture. Here is another site.

The principle of celestial navigation is this: If you see a body H degrees above the horizon, then you are located in a circle on the surface of the earth that is centered at the point on the surface of the earth located just under that body (called GP, geographical position) and has a radius (angular radius, from the center of the earth) equal to 90-H. This is so obvious when we talk about the pole star and yet, when you talk about any other body, people get confused.

Imagine you had a globe where you could move around the lat/lon grid so the pole of this grid could be anywhere. You place the pole where the star was when you measured it and your “latitude” is 90-H. As simple as that (except you cannot carry a globe big enough and some other minor considerations, so you have to use some math).
>> How accurately can you personally fix your position? What’s your margin of error typically, and at best?

well, IMHO if you are doing better than 1’ (one nautical mile) you are doing pretty good but this depends on so many factors… it is one thing to take a sight from the deck of a small boat being thrown about and you cannot expect much accuracy there and it is another to take a sight from the deck of a huge ship in fair weather. It also depends on the number of sights you can get, the more the better, as you can average them out. It is difficult to give a number and navigating is an art as much as a science. the navigator should know what kind of confidence to give to each source. Let’s say this: In ideal conditions with enough sights you can get down to a fraction of a mile. In bad conditions you can consider yourself happy if you are within a 5 or 10 NM radius. In the open ocean this is not so much, and near land you should have some landmarks.

One of the milestones of celestial navigation was when captn. Sumner had not been able to take a sight for days due to bad weather and was very concerned because he was approaching the coast of England (or maybe Ireland). He plotted his estimated longitudes using as argument several latitudes and he saw all the points were in a straight line. Today we know it as an LOP (line of Position) and we know it is part of a circle (the “latitude” I mentioned).

>> And for comparisons sake, how accurate have you found consumer GPS equipment to be? When you compare it on a good map does it agree?

As you know the degradation of GSP called selective availability has been turned off and now GPS is even more precise than it was. It varies but can be as good as 50 feet. Still, you need to have eyes and brains and you cannot navigate a narrow channel blindly relying on the GPS.

When you get to this level of precission you get into a whole new set of problems. The charts were not made by God and many of them are mistaken because they were done with the limited instruments of the time. North American charts are pretty good but you can find charts of remote places that are pretty bad and the best GPS in the world ain’t gonna help if you hit a reef or an island because your chart says there is water there.

Then there is something called datum. Nobody has measured the actual distance to the inch between San Francisco and Greewich and every time they measure it they come up with something different. But the GPS tell you how far you are from Greenwich and from the equator and you just want to know how far you are from Alcatraz. So there are different “datums” and every chart will tell you which one it used and"(usually) GPS receivers will let you choose a datum.

The short answer is that GPS (and Loran where available) are way more precise and realiable than astronav.

I know I am not giving the short simple answers you would like but i hope it still clarifies this.

15

Mauve Dog to expand on dead reckoning. As I said it basically involves keeping track of course, speed and time. The course was reckoned by the compass.

Time was measured by an hour glass that was turned every half hour. The sailor at the helm or whoever was in charge of this chore would turn the hourglass and ring the bell the same number of times (half hours) that had transpired. The time on the ship was called as the number of bells (at 3 bells… etc)

0.5 hr = Ding
1.0 Hr = Dingding
1.5 Hr = dingding,… ding
2.0 hr = dingding,… dingding
2.5 hr = dingding,… dingding,… ding
Etc.

The seaman’s day is traditionally divided into periods of duty of 4 hours also called watches. Eight bells therefore signalled the end of the current watch and the beginning of the following watch when the count started anew.

Thus there would be six four-hour watches in a day but as this would entail the crew to stand the same watches every day the evening watch from 1600 to 2000 is divided into two 2-hour watches known as the first and last dog watches. This way, the watches each crew stands rotate every day. Beginning at 20:00 (8 PM) the names of the watches are: First, Middle, Morning, Forenoon, Afternoon , First Dog and Last Dog.

During every watch the men were kept constantly at work working the sails or doing maintenance chores. Only during the dog watches were they allowed to relax if there was no work to be done immediately. If the circumstances required it “all hands” would be summoned on deck.

But getting back to dead reckoning. Time was thus reckoned by the hourglass. Speed was estimated in different ways. One was plain estimation. Another was to watch some floating object and measure how long it took to pass between two points of the ship. But the “precise” method was to use the chip log and sand glass.

The chip log was a triangular piece of wood than was weighted with lead so it would float vertically in the water. From the three corners you had three short lines that joined and then a long line with knots in it.

Here is how it worked: the log is heaved overboard by the stern and the line starts running out. You need to have a leader so the log is far enough astern where the water is not disturbed by the wake. One man is loosely feeling the line as it goes out. As he feels the first knot pass he calls “hut!” and the other man instantly turns the glass. When the sand runs out, he calls “hut!” and the man on the line instantly grabs it.

Now, of the three lines that hold the log, one is held in such way it will release if subjected to a sudden jerk, so now it has released and the chip is floating flat and can be recovered.

Measure the distance, divide by time, and you have the speed. Now, if you have a 20 second glass and you space the knots 33.75 feet, then you get the speed directly in nautical miles which are called “knots” and now you know why. One knot is one nautical mile per hour.

In the documentary filmed in 1929, “Peking around cape Horn” you can see this operation being done. (And IIRC the ship was doing something like 18 knots under sail!!)

16

Ok, I have started a page about nautical instruments here. I still have some way to go but I hope to do some more work in the next couple of days. let me know if you have any questions.

17

Well, Achernar, you can always use distance across sky = 180[sup]o[/sup]!

As for telling time by the stars, I’ve been able to get to within a half-hour. And as to the price of instruments, my astrolabe only cost 59 cents… It consists of a protractor, a drinking straw, and a weight on a thread. I’m still writing the calculator software for it.

Yes, sailor, I realize that I haven’t got nearly the precision of one of your $30-3000 dollar toys.

By the way, in dead reckoning, how do you correct for the speed of the currents?

18

Chronos, in DR you can just draw vectors to adjust for current but you have to know or assume the drift as you cannot measure it.

You can make some cheap measuring instruments with a protractor etc and with practice and knowledge get where you want to go. 16th century explorers had less than that to work with and they got around.

Talking about sextants I should add you can buy a used plastic sextant for 1/3 to 1/2 the price at some stores that sell used nautical stuff. I have seen them at Bacon’s and Ocean Outfitters, both in Annapolis. It helps if you know sextants because they may be missing some essential part.

Cruising World recently published an article comparing sextants and this page and this page discuss that article and defend opposing views. One interesting point: he says a plastic sextant is as good as a metal one but people take better care of a $600 metal sextant than a $150 plastic sextant. So the solution is take a marker and on the case of the plastic sextant write in big letters “$600” and that way you’ll take better care of it.

Starpath has celestial navigation courses and materials.

19

The US naval Observatory has a page where you can input some data and it will generate the almanac data. If you need to practice or just a few hours or days worth of data, you can generate and print the pages here.

Locations for the Government Printing Office