Noon sight longitude

Always looking to save time, I have become a great proponent of the noon sight longitude fix. I say fix in this case because with a few sextant observations you can, on a good day, get both a latitude and longitude from the sighting of the sun at local apparent noon (LAN). The process is not difficult and can be reviewed in Bowditch, but I have been using it for years. I probably picked it up from Eben Whitcomb of the schooner Harvey Gamage in the early 1980s.

The problem with the LAN shot is marking the exact time that the sun reaches its apogee. It can be precalculated, but it is almost impossible to know of the exact time. This time is essential for a sight. Remember, a four-second difference in time is equal to an error of one nautical mile, making time of the essence. So how to get an exact time for LAN? One way is to average the times when the sun reaches a specific height on both its ascension and while it is descending. This method is sometimes referred to as “using the noon curve.” The method that I prefer is the easiest, though not necessarily the most accurate. I choose to use just two sights and average their time with the sextant set at the same height. For a more complex and accurate method, read Frances Wright’s book Learn to Navigate.

Basically we want to predict the exact time of LAN. It is difficult to do since the sun has the tendency to “hang” at maximum height. In order to better calculate at what time LAN actually occurs, we can take a series of sun shots recording the time before and after LAN. My preferred method is to take just two shots, each about one half hour before the precalculated LAN time. Dorothy LaFond, profiled in the Jan/Feb 2012 issue of Ocean Navigator, uses this technique, which she describes in her celestial navigation workbook Celestial Navigation by The Altitude Intercept Method. LaFond has used this technique at sea and is not dissatisfied with the results.

Let’s do a shot just for longitude and see if it works, we will use the following numbers: We have calculated that LAN on Nov. 15 will be at around 16:21 GMT based on our dead reckoning (DR) position. We are, in this instance, not interested in the latitude but only the longitude. At about one half hour before we calculate LAN we take a lower limb shot of the sun. We mark the time and record our Hs. In this case the time is 15:52:20 and the Hs is 38° 58.6’. I leave the sextant set to that angle and at 16:50:45 the sun is exactly at that Hs once again.

Next, I average the times of the shot and as close as I can get it is 16:21:32. All these times, by the way, are GMT which is what is always used for celestial navigation.

The next part is the fun. We have to convert the time into arc and thus arrive at our longitude. Remember that local hour angle (LHA) is equal to the difference between Greenwich hour angle (GHA) and west longitude. Also recall that a definition of meridian passage is that it occurs when the LHA is equal to 0°— in other words when our west longitude is equal to the GHA. So we go to the daily tables of the Nautical Almanac and convert 16:21:32 to the following:

GHA 16 hours    63° 51.6’
+21’:32 sec        5° 23.0’
GHA            69° 14.6’

So the sun’s GHA is 69° 14.6’ and we can say with a degree of certainty that that is our longitude as well. Of course, this information must be treated with the understanding that it may not be totally accurate. I would say, though, that it could be used as an estimated position and would be more accurate than the existing DR longitude.

For more by David Berson:
Some tricks of the trade

The benefits of using H.O. 249 for Selected Stars

Dodge Morgan: First American non-stop circumnavigator

At sea again

Voyaging couple led the way

By Ocean Navigator