07 July 2010

Celestial Navigation Fun!

Like a lot of wooden boat nuts, I was probably born in the wrong century. I like the Blue Moon's gaff rig, I use kerosene lamps, and I much prefer paper charts to electronic ones (my paper charts never 'go blank', as Roger Fitzgerald's did at a critical moment during the 2010 Jester Challenge. How awkward!)

I also like celestial navigation.

From the 1728 Cyclopaedia
image wikimedia commons

Why, in this GPS age, would I enjoy anything so unlikely? Why would I want to laboriously calculate my position on planet Earth by looking at real stars, when the artificial stars of the GPS system are happy to do all the sights and calculations for me?

It's not because I think the GPS system is 'unreliable', or I fear the bad guys will shoot the satellites out of the sky. In fact, I figure the odds are pretty darn slim that I'm going to need my GPS just as WW III starts.

And it's not because my simple Garmin 72 GPS needs a backup. Frankly, the best backup for a GPS is... another GPS.

No, the reason I like celestial navigation is the best one of all: FUN.

Now I admit, this is the same sort of fun that people get from doing the Sunday Times crossword puzzle, or writing a computer program, or building a wooden boat, for that matter. It's hard to explain this type of fun to people who like doing everything the easy way, but if you're reading this blog, you probably share this odd pleasure.

(Secret bonus pleasure: it's also fun to lord it over the 99% of sailors who don't know celestial navigation, and never will.)

It's always been that way. The common sailor, no matter how experienced, has always held in awe the Captain and his ability to find his way across the trackless oceans by looking at the stars.

Jim Hawkins, overhearing the conversation, below
image wikimedia commons

Remember Treasure Island, when Long John Silver's band of thieves were impatient to get rid of Captain Smollett and take over the ship? What did the ruthless but practical Long John remind them?

'What I say is, when are we going to do it?'  growled the coxswain impatiently.

'When! by the powers!' cried Silver. 'I'll tell you when. The last moment I can manage; and that's when. If was sure of you all, sons of double Dutchmen, I'd have Cap'n Smollett navigate us half-way back again before we struck.'



Ah, the inner secret of sailing! The one that garnered respect, even from pirates. Yup. That's what I call fun.

I brought my sextant along on the Blue Moon, but I haven't had much chance to use it, what with being fairly close to shore, or in the ICW, or tucked into an anchorage at dusk and dawn. Whenever I had a chance to 'shoot' the Sun or a nice star, there always seemed to be a shore under it, instead of the horizon. And everyone knows you need a clear view of the horizon to get a shot, right?

Well, a couple days ago, I just happened to read about the 'Short Dip' tables. I'll explain these in more detail later, but the short version is that these special tables let you take sights even when you don't have a clear view of the horizon. At least in certain circumstances.

This got me fired up about using my sextant, but of course, I'm totally out of practice. And celestial navigation isn't something you can do without practice. At least I can't.

So I've decided to use this break from the Blue Moon to brush up my calculations, and test out those Short Dip tables. And I'm going to blog about it, so maybe I won't have to drag out those impenetrable and stupefyingly dull books the next time I need to refresh my memory.

photo wikimedia commons

If you've always wanted to learn how to steer by the stars, this might be a good way to get your feet wet. I'm going to start off with the simplest sight possible -- a noon sun sight -- and show how to calculate your position with the minimum amount of math and mumbo-jumbo possible. If I do my job right, you should say to yourself, "hey, this celestial navigation stuff isn't so hard, after all!"

If enough readers get into it, maybe I'll even add a new feature to my Blue Moon blog -- a 'Where Am I?' feature. I'll take sights and print them in my blog, and you can use them to figure out where I am.

Now, come on... what could be more fun than that???

Well, if you are interested, shoot me an email at john@unlikelyboatbuilder.com, just so I know I'm not completely crazy. If there's enough interest, I'll get into taking other, more complicated, types of shots.

No sextant necessary. The only tools you'll need to get started are a pencil, some graph paper, and maybe a simple calculator.

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Thanks for your interest!

-- John



  1. Great - please post more about navigating by the stars.
    Some years back I bought a plastic sextant and a book to teach myself, learned how to take a noon day sight, but never progressed much beyond that.


  2. Really do enjoy your posts. Please continue blogging.

    I have navigated worldwide using Inertial, Omega and now GPS. I, for one, will have fun working out your plots. But then, I do have fun with the NYT crossword puzzle.

    Thanks again for your posts.

  3. Share your interest in celestial navigation. please continue to share. can't wait for your saga and my "virtual" cruise to continue.

  4. Also, don't forget that within sight of land, you can use your sextant for taking observed angles between charted objects on the shore. Two of these a known (dead reckoning) distance apart will fix your position. Also, measuring the angular height of, say, a tall chimney or radio mast, will give you a "distance off".

  5. Being landlocked on Lake Ontario l have wanted to cut free of the electronics and learn the stars..my first stumbling block was this ...is a siteing taken on a landlocked body of water accurate without taking the elevation of the lake above sea level into consideration?

  6. Dave: Do you have to correct for height above sea level? Good question. I don't believe so, but I will do some research and get back to you. -- John

  7. Dave: I can't find any reason why you'd have to do apply any special correction for height above sea level. There are corrections for temperature and air pressure, but these are more for surveyors and other high precision, land-based navigators. The corrections are too small for sailors to worry about.

  8. John l was considering that you must make corrections for dip or your eye level as it were, above the surface of the water. The figure would be different if you were on a small cruising boat as compared to a battleship. I would had have assumed that the sea level is the constant and that any difference from the surface should be dealt with.
    Not being argumentative but that is how l reasoned it.

  9. No, dip is the height of your eye above the surface of the water that you are looking at.

    How high the water is above or below mean sea level doesn't matter. Otherwise, you'd have to correct for tide in places that have 13' tides, for instance.

  10. Hello John,
    Yes, Celestial Navigation is fun! I hold the grade of Navigator in the Power Squadron, and taught CN for several years.
    That was until we relocated 16 years ago and I have not uncased my Plath since. I need to refresh, big time.
    We studied Ageton, H.O. 214, H.O. 229 but focused on the "Nautical Almanac Site Reduction Method"
    I look forward to your blog with great interest.

  11. Dick,

    I'm going to be taking it real slow and easy for awhile for those sailors who have tried CN before, but were thwarted by the math and/or those miraculous sleeping agents disguised as celestial navigation books.

    Should be a snap for you!

  12. Thats it John! I never considered the tides would effect the siteing.
    Dip is only considering the height above the water not the height above sea level

  13. John - of interest - John A. learned with a plate of oil on the roof of the Brooklyn Eagle building before going to the Pacific in WWII.

  14. As far as the "sleeping agents disguised as celestial navigation books". I stumbled across one that caught my attention and actually made sense to me. It's called "Celestial Navigation for the Clueless", found at http://www.ncsail.org/publications.html.
    It is available either in paperback form or as a PDF download, best of all you can download a free sample that contains the first 30 or so pages of the book that walks you through a basic polaris sight for latitude and a basic noon sun sight for figuring longitude.
    In my opinion it is well worth the $10 for the download. ($14.95 for the paperback version)
    It easy to understand for the absolute beginner in cel-nav (that's me) I would recommend it highly for any one wanting to learn celestial navigation.

  15. Of course, the NYTimes crossword puzzle is or can be fun, Cel Nav is another game, even more enjoyable, of so it strikes me.

    John, I believe it is Dip Short, not Short Dip.

    GPS is great, but the sextant requires no electricity, batteries tending to go dead at the most inconvenient moment. That having been said, I'd rather get a fix within a couple of NM's with a sextant and the Daily Pages of the Nautical Almanac, than rely on something, the operation of which I have virtually no understanding of. I do have a rather basic GPS unit. I also have learned to shoot the sun, moon and can sometimes manage a star or two. Have obtained three body fixes, standing on the beach, that plotted to well inside 5 NM's of an electronic fix (GPS).

    As for Celestial Navigation for the Clueless", I have a copy, which one of these days, he laughingly says, I will finish reading.

    In conclusion, for those whose place of residence is out of sight of the sea, as unfortunately mine is, there is recourse. Think Artificial Horizon, ala a dish of oil, mercury if you can find that these days or a dish of water, with wind shields, akin to the Davis Artificial Horizon.

  16. John:

    I'm somewhat familiar with your blog, having saved entries for some toime now. Unfortunaatqely, one com[puter that had them asll cr5ashed, and I lost a bunch pof data, saved styuff, that sort ofd thing, your old entries included.

    Could you point me toward a place where I might find them?



    1. Alan: everything is still on this website. Are you looking for anything in particular?

  17. Magical post, can I be the elephant in the room and ask what a sextant actually is?


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