
April 2008
Celestial
Navigation
Another
method of obtaining longitude from a noon sight
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:
15:45:08
+16:29:10 =
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’ =
61° 48.6’
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.
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