This is the third article in a series concerning the use of the noon sight as a means of finding longitude. Although most often associated with the determination of latitude, the meridian passage of the sun — if it can be timed precisely — can be utilized for ascertaining longitude. The timing, however, is the tough part. As any navigator who has ever done a noon sight knows, at the moment of its maximum altitude, the sun appears to hang in the sky. This makes it very difficult to mark the exact time that meridian passage occurs. If, on the other han d, we could know the exact time of meridian passage then we can enter that time — in GMT — into the Nautical Almanac and find the GHA of the sun. Then we can find our longitude since the Local Hour Angle of the sun at Meridian Passage is 0°. In this case the GHA is actually the longitude of the observer. Of course, there are qualifications for this assertion, but for all intents and purposes, an accurate longitude can be found using a noon sight. The proviso being that we need to know the exact time of the meridian passage and can shoot it at that moment.
This brings us to an idea known as the “noon curve.” What we do in this case is graphically plot the rise and fall of the sun beginning at about 20 minutes before calculated noon, and continuing for another 20 minutes after the meridian passage. The graph is constructed with the sextant altitude on the left side of the page and the time at the bottom. Perhaps 10 shots of the sun are taken as it rises and another 10 as it descends. At each shot time, a pencil mark is made on the page that corresponds to the altitude at that moment. When completed, and the points joined and faired, they should look like a sine curve. The top of the curve is the maximum altitude of the sun and should be marked with an exact time. Actually the top of the curve will be flattened a bit, this is where the sun hangs before losing altitude.
The exact time and altitude of the sun can thus be found for meridian passage. It is true that this method requires a great deal of time, and a calm sea helps as well. In one sense it is almost a contradiction of the raison d’etre of the noon sight which was developed as a method that was quick and simple and required very little time or math. Still it is important to know how to wring every bit of information from the sight. There might come a time when you may need it.