In the last Navigation newsletter, I wrote an article explaining how to obtain longitude from a noon sight using the GHA method. Basically this method is based on the fact that at the time of meridian passage of the sun, Greenwich Hour Angle (GHA) is equal to the DR longitude of the observer. The resulting Local Hour Angle (LHA) is thus equal to zero degrees — which is a definition of meridian passage. This method, although reliable, has its drawbacks. This is because the hanging time of the sun, when it reaches the meridian, is difficult to determine. It follows that the exact time of meridian passage is questionable. Fortunately, there is another method of finding longitude from a noon sight that eliminates this problem.
Although I scoured Mixter’s, Dutton’s, Leaky and Blewett for information concerning this technique, it was only in Bowditch that I found any mention of the procedure. I learned this application from my friend and navigational mentor, Eben Whitcomb, years ago while shipping aboard the schooner Harvey Gamage.
Instead of relying on the GHA we instead take at least two timed shots of the sun, while it is ascending, and when it is descending. I’m certain that there are many variations on the theme, but I will explain the procedure that I use.
Roughly 15 minutes (it doesn’t need to be exactly 15 minutes) before the calculated time of Local Apparent Noon (LAN) I take either a lower or upper limb shot of the sun and mark the time. I then record the sextant altitude and the time.
I record LAN as usual, so I can obtain my latitude. Then after the time of LAN, I pick up the sextant — which I then set to the angle of the shot I took 15 minutes before LAN — and when the sextant altitude of the sun is the same going down as it was when it was rising, I mark and record the time. I then put the sextant away and prepare to calculate the exact time of LAN.
The procedure for finding the exact time of LAN is simple: just add the two times of the two shots and divide the result by two. This will yield the time of LAN that you can use to enter into the almanac to find the necessary information to get the longitude. Let’s do an example:
The day is April 15th. We are at a DR position of 35° 25’ N and 60° 18’ W. We want to calculate the longitude from the meridian passage of the sun. We first see on the daily pages that the time of LAN is 12 hours and no minutes. This would be for the standard meridian of a time zone (0°, 15°, 30°, etc.) We think we are at 60° 18’ so we have to see how long it takes the sun to move 18’. Entering the Arc to Time Conversion table we find that it takes 1 min. 12 sec., so we can estimate that the time of LAN for our DR will be at 12 hours 1 min, 12 sec. I always convert the local time to GMT so we add 4 hours to the time of LAN, making it 16:01:12. At 15:45:08 GMT we take a sextant sight and record the altitude at X°. After we find LAN altitude of the sun, we reset the sextant to X° and at 16:29:10, the sun is once again at that altitude.
We next take the times and add them and then divide by two:
32:14:18 / 2 =
16:07:09. This is the time of LAN.
We next go to the daily pages of the Nautical Almanac for 16 hours on April 15:
16 hours = 60° 01.3’ GHA
+07 min 09 sec +01° 47.3’ =
Longitude at time of sight = 61° 48.6’. Remember the GHA is equal to the longitude of the observer at the time of LAN so we can convert GHA into longitude. It is also interesting to notice that the longitude puts us further to the west of our DR, and we should adjust our plot accordingly.
We will notice that if this method is used, as it is, we have not discussed latitude. Latitude from a noon sight is easy to obtain, but the point of this discussion is to see how we can establish longitude from the noon sight. I welcome your comments.
About the Writer
Contributing Editor David Berson writes the Nav Problem page in every issue of Ocean Navigator. He is also the owner and operator of Glory, an electrically powered excursion boat, in Greenport, N.Y.
Question for David? email@example.com