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Celestial navigation 101: master the basics first

I well remember the first time I worked out my position using celestial navigation. That cross on the chart represented more than my position — I had joined the big boys. I was a proper navigator. There is a lot of mystique surrounding celestial navigation, and reading about it early on only seemed to perpetuate the myth that you had to be some sort of mathematical whiz to actually fix your position on the earth’s surface using a sextant.

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To find our position we must be able to find our latitude and longitude. GPS has reduced this to little more than pushing a button and finding an icon on an electronic chart. You may think there is little point in using the antiquated method of observing planets and stars to locate position, but understanding celestial navigation gives the small-boat navigator enhanced confidence should the electronics fail. And there’s probably a pretty good reason the Navy returned to teaching cadets this ancient skill. Cyber crime is on the rise, and what would happen if the satellite system we depend upon for positioning were taken down? A Plan B is a good idea. Even if you never use it, celestial navigation is a cool skill to master.

I strongly suggest starting with the sun and becoming proficient in what’s known as the noon sight, then, if you have the interest, progressing to other stars and planets. By accurately measuring the angle between the sun and the horizon at noon where you are, called local noon, and the time, it’s possible to get a fairly precise position.

The navigator measures the sun’s altitude using a sextant at local noon, then subtracts the reading recorded on the sextant from 90 degrees, which is the total number of degrees in each hemisphere between the pole, 0 degrees, and the equator, 90 degrees. The number you end up with is called zenith distance, or ZD. Next we look in the nautical almanac to see what the declination — the angular distance of the sun north or south of the equator — of the sun was to the nearest hour at noon at your location. Add the sun’s declination to ZD if you and the sun are in the same hemisphere, subtract if not. The answer will be your latitude.

To work out longitude it is essential to know the precise moment that local noon occurs. The sun will be due south at this point if we are in the northern hemisphere, due north if south of the equator. There are several ways to do this. The most common is to keep taking a sight of the sun with the sextant until it stops rising — it should appear to hang at it’s highest point for a minute or so before it starts to fall. The other method is to take a sight of the sun approximately 10 minutes before we expect midday to occur and record the time without adjusting the sextant. Keep taking sights until the sun starts to descend and the sight corresponds to the one you took earlier. Again, record the time. The average of these two readings will be local noon. For instance if you recorded 57.40’16 at 10.34 and the sextant reading was once again true at 10.42 local noon would be 10.38. Referring to the almanac, we can then work out what is known as Greenwich hour angle.

There are 360 degrees around the earth, and it takes 24 hours for the earth to revolve, which means that each 15 degrees adds one hour before or after Greenwich Mean Time, which is also 0 degrees.

All of this is fairly basic and involves measuring angles with a sextant, noting the accurate time and looking up data in the almanac. There are several corrections that have to be taken into account. The first is index error, and this relates to the error that is inherent in the sextant, which in many cases are so slight as to be inconsequential. The second is dip is the height of the observer’s eye above the horizon. On a small boat this may be fairly small, but on the bridge of a ship it could be 80 feet or more. The last is time error, which you must add or subtract if your watch runs fast or slow.

Sea motion can make taking sights difficult. Start taking sights ashore before you try to do it on a boat, and get used to handling the sextant. Practice really does make perfect.

None of this is difficult if you follow the steps and are patient. And the first time you lose the GPS offshore, you’ll feel like a genius.

Buying a sextant

You can buy an inexpensive plastic sextant, which gives acceptable results, but I’ve found that these must be handled carefully, as they can go out of adjustment easily.

If you plan to use celestial navigation often, invest in a quality instrument. You can spend $2,000 on a sextant, but an Astra 111B is a popular model that you can buy for around $700. It’s a perfectly adequate and robust instrument. Buy the best you can afford, and care for it well. It will last several lifetimes.

This article originally appeared in the June 2016 issue.