I well remember the first time that I worked out my position using celestial navigation – that cross on the chart represented more than my position – now I had joined the big boys, I was a proper navigator. There is a lot of mystique surrounding astro, or celestial navigation as it is often called, and reading texts about it early on only seemed to perpetuate the myth that you had to be some sort of mathematical wiz to actually fix you position on the earth surface using a sextant.
To find our position we must be able to find our latitude and longitude. In the case of GPS this is done automatically usually with a legend or icon on an electronic chart. This takes very little effort on behalf of the operator beyond turning on the unit. These days one would think that there is little point in using observations of planets and stars to find our position and you may be right most of the time but understanding celestial navigation gives the small boat navigator enhanced confidence should all the electronics fail, plus it’s just a cool skill to master.
To get started with celestial navigation I strongly suggest starting with the sun and becoming proficient in what’s known as the noon sight and then if you have the interest progressing from there to other stars and planets. Truth be told if you can use the sun there is almost no point going on from there unless you really want to. By accurately knowing the angle of the angle between the sun and the horizon at noon where you are, called local noon, and the time it’s more than possible to get a very accurate position of where the boat is.
In simple terms the navigator measures the suns altitude using a sextant at local noon then subtracts the reading recorded on the sextant from 90 degrees, which is the total amount of degrees in each hemisphere between the pole 0degrees and the equator, which is 90 degrees. The number that you end up with is called zenith distance, ZD for short. Next we look in the nautical almanac to see what is the declination (the angular distance of the sun north or south of the equator) of the sun was to the nearest hour when noon occurred at your location. Add the suns declination to ZD, if you and the sun are in the same hemisphere, subtract if not. The resulting 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 and due north if south of the equator. We do this with the sextant. There are several ways of doing this, the most often quoted is to keep taking a sight of the sun until it stops rising, it appears 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 or so before we expect midday to occur, record the time then 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 reading will be local noon. For instance if you recorded 57.40’16 at 10.34 and the sextant reading once again true at 10.42 local noon would be 10.38. We then refer to the almanac from which we can 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 one hour before or after GMT (Greenwich Mean Time) which is also where 0 degrees is.
All this is fairly basic and involves measuring angles with a sextant, noting the accurate time and looking up data in the almanac. The one fly in the ointment is that there are several corrections that have to be taken into account. The first of these is index error and relates to the error that is inherent in the sextant itself, which in many cases are so slight as to be inconsequential. The second is dip, which is the height of eye of the observer above the horizon. On a small boat this may be fairly small but on the bridge of a ship could be 80 feet or more. The last one is time error, which you need to add or subtract if your watch runs at all fast or slow.
A note on sextants.
It is possible to buy a fairly cheap plastic sextant, which gives acceptable results, but I’ve found that they do need careful handling as they can go out of adjustment fairly easily. If you are planning on doing much celestial navigation then you may well find that you want to invest in a quality instrument. It’s possible to spend $2000 on a sextant but it’s also possible to buy an Astra111B a popular model that is perfectly adequate and robust for considerably less. EBay is good place to look too, and there are some bargains to be had. Buy the best that you can afford and it will last several lifetimes.
Get used to handling the sextant, practice really does make perfect, start ashore before you try to do on the boat. The boat motion is another factor that makes taking sights difficult.
For further reading might I suggest: Celestial Navigation For Yachtsmen by Mary Blewitt