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Navigating with a celestial computer

Dec 2, 2015

The reality cannot be avoided: In order to learn anything new and get better at it, effort must be applied. True, whether one is learning to cook, sew, sing or practice celestial navigation. There is no simple way of avoiding effort — and besides, why would we want to? So for those who want to learn to navigate in a traditional manner, you will have to exert some effort. But, and this is important, once you have made the initial effort, there are tools available that will make the practice of celestial sights as simple as brewing a pot of coffee.

Traditionalists insist that we use HO 229 because it is more accurate than HO 249. Others say the old HO 214 or HO 211 are the best methods. Actually, the choice of sight reduction tables makes very little difference. Find a sight reduction method that works for you and master it. Personally I am very much at ease using the air tables (HO 249) developed during WWII combined with the Nautical Almanac. William Buckley, who did a great service in popularizing celestial navigation in his books and videos, preferred the air almanac. All that matters is that you can complete the process comfortably when you are both tired and a little seasick.

But, after you learn the longhand method, it is time to simplify; by that I mean instead of going to the tables, let your fingers do the walking on a hand-held calculator. There are lots of apps available and navigation programs for the computer, but for the past 25 years I have gotten yeoman’s service from the celestial calculator called the Celesticomp V. This calculator, developed by John Watkins out of Washington, is the sine qua non of celestial calculators.

The Celesticomp V was programmed on a Sharp PC-1270 pocket computer with a ROM cartridge that fit in the back. The memory can’t be lost even if there is battery failure. This tool is such a workhorse that it is disconcerting to no longer have them in production (I think Sharp discontinued the model or went out of business). Celesticomp V can found only on eBay where they sell for about half the original price, which was about $290. I have sailed thousands of miles with this little tool and it has saved me hundreds of hours of time in the reduction of star, moon or planet sights. It will inform me of unknown stars, provided I enter the Hs and the bearing of the star. The prompts are simple, even for a Luddite like me. And here’s the thing: The easier it is to reduce shots, the more observations will be taken and the greater chance for increasing accuracy and all of that is good.

The point being is that we want to get up to speed and develop confidence as quickly as possible. The calculator, computer app or navigation programs won’t help you understand that when you put garbage in, you get garbage out. But it will easily point out anomalies in the sight reduction process. I combine the use of the calculator with the physical process of using plotting paper to plot my results. This way I can convert the results into LOP or fixes; the clarity of doing that is, for me at least, the best of both worlds.

To be sure, these computers and navigational apps and computer software programs are all marvelous tools, but they are just that — tools — that make the job easier in the same manner that a table saw will help simplify a woodworking project for a skilled carpenter. In both instances, there is no substitute for understanding and experience. A celestial calculator or any supplemental computer program will assist the skilled navigator who is already trained in the concepts and basic techniques of celestial navigation.

A tool like my Celesticomp — and I have three of them by the way, including a backup and a backup for the backup — has a permanent nautical almanac for the sun, moon, and the other planets and 173 stars, and has all kinds of bells and whistles to calculate great circle courses, rhumb line, course over ground, etc. It will calculate your time for morning twilights, nautical twilights and civil twilights, and can update the DR automatically.

It also does away with the need for precalculation of stars for twilight. All I have to do is shoot a star and record its true bearing and the time in GMT, along with the sextant altitude. If I don’t know the star, I enter a prompt called “unknown star,” and my trusty little Celesticomp reduces the HS to HO and spits out what the star is.

I highly recommend finding a calculator that is programmed to do celestial navigation and learning to use it, initially as an adjunct to the longhand method. As you develop confidence you will find you rely on it more and more — just remember to carry an extra set of batteries. Now go out there and have fun.

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Dec 2, 2015 03:47 pm
 Posted by  Stephen Hayes

Very timely. The news today included a story about the US Naval Academy reinstituting courses in celestial navigation because of concerns that the GPS system could be disabled by hackers or otherwise.

There are alternatives to your beloved Celesticomp. StarPilot runs on a couple of Texas Instruments calculators. You can buy the bundle already installed or by StarPilot separately for $129 and find a compatible calculator on eBay for cheap.

Your article (probably intentionally) overlooked the most important reason for buying the celestial calculator – for many of us it has been decades since we attempted trigonometry. A pre-programmed calculator limits our challenge to whether (or not) our eyesight is still good enoughTo get a reliable sight.

Dec 2, 2015 05:48 pm
 Posted by  Reg

For the past forty years I have carried a backup calculator in a Faraday bag to avoid loss due to nearby lightening strikes etc....This was the result of coming across a "Lost Sailor" about 600 miles SE from Hilo who was lost because he was caught by a storm and had a nearby lightening strike that cost him his electronic systems. I towed him in to Hilo and suggested that he attend the civilian Nav course at Pacific Maritime Academy in Honolulu before he continued on his way around the world....He solved the problem far more easily by informing me that the was leaving but now had a spare set on board. Never heard of a Faraday bag though. The good news is that he had changed his goals and was on his way back to Los Angeles, and that's a pretty big island so you can find it eventually by just following the compass East and then asking the first gas station you see when land comes into view as to just where you are....


Reg White

Dec 2, 2015 11:30 pm
 Posted by  gary217

Here is a link to another method using a special slide rule to do the computation and that provides a long term solution to celestial navigation:


Dec 3, 2015 04:33 am
 Posted by  SV Twilight

Thanks for the useful info. We are trying to ensure we keep our celestial skills current and are looking for easier ways than all the manual steps. Are there other options besides the calculator method recommended here. What about the Starpilot app ($50) and the associated Starpilot almanac ($15)?

I think it would be much easier and more practical (not to mention economical) for many of us to use those apps on our existing devices. That said, I have not used either and I would appreciate if ON could take a look into reviewing these products.

Keep up the great work!



Dec 13, 2015 10:46 pm
 Posted by  Patrick K.

I have had a Celesticomp V for several years and have been happy with it. However I understand that Celesticomp is now out of business and the ephemeris (internal nautical almanac) is now out of date. Is this true?

Mar 15, 2016 03:13 pm
 Posted by  John Stevenson

I wrote a program in MS Excel ( that also does the sight reduction and provides an ephemeris for the Sun, Planets, and Stars. One of these days I will get around to adding the Moon.
For me, the primary advantage of the electronic sight reduction is that the assumed position can be the current DR position rather than a lat and lon that fit the tables and simplify the arithmetic. Of course there are manual methods to do the same thing, but ...
FWIW, I started development of this program back in the 80s on a Radio Shack pocket computer, which I believe is the same computer on which the Celesticomp is based.

John Stevenson

Nov 21, 2017 02:40 pm
 Posted by  Karl

Like Mr. Stevenson I wrote a program to solve the navigational triangle. I used the equations found in the back pages of the Nautical Almanac. I debugged the program using examples from Gray's "100 Problems in Celestial Navigation."

I also skipped the plotting part by programming a least squares algorithm, These equations are also found in the back of the Nautical Almanac. The algorithm combines 3,4,5, or 6 LOPs. The new position can then be recycled as a new DR. This will yield new and hopefully improved LOPs.

When I compared my results with Mr. Gray's there were small differences of several miles. Since neither one of us were really there with a GPS I wonder whose more correct? Any ideas.

Aug 18, 2019 10:04 pm
 Posted by  armchair navigator

If you are trying to use this pocket computer and get error 4, there is an easy way to clear it if your RAM card is not corrupted.

On the back of the calculator is a hole labeled 'Reset All'. Using a pen, press the recessed button for a couple of seconds. The display should read '** **' after holding in the button long enough. Without touching any other buttons, press the '=' key. Cycle the power button and it should reload the program.

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