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From: Lu Abel (no email)
Date: Sat Jun 04 2005 - 16:56:48 EDT
Gordon Talge wrote:
> The "Noon Sun Shot" was a staple. However, due to the fact that the
> Sun tends to "hang" in the sky on the meridian with little apparent
> change in altitude means that finding longitude by noting the time
> of passage, while in theory should work, in reality is not practical.
>
> One method has the observer, taking a shot at maybe 10 minutes before
> LAN and noting the altitude and the watch time, doing a noon shot
> and resetting the sextant to the altitude that was noted 10 minutes
> before and watching the Sun until it is at this same altitude and
> noting the watch time. Since the Sun must have been at LAN midway
> between these two times, you then calculate that time and using the
> equation of time from the NA get your longitude.
I don't know how many on this list have actually taken a noon sight. I
have.
A noon sight is part of the requirements for US Power Squadron's
Navigation course (advanced celestial nav) which I have taken. I found
(at ~40N Lat, sight taken during the early summer) that not only did I
get a spot-on latitude value, but by graphing a series of sights over
about 10 minutes (5 min on either side of LAN) I got quite an excellent
value for longitude.
The sun being "nearly stationary" in the sky is relative. For an early
navigator using a backstaff and lucky to get latitude to 1 degree, the
sun truly appeared to be hanging in the sky at LAN. But looking at it
through a modern sextant, I found the vertical movement around LAN quite
perceptible. I also freely confess this sight was taken on dry land,
so I wasn't dealing with trying to bring down the sun on a heaving
ship's deck.
Lu Abel
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