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Subject: Re: Timing Noon
From: Peter Fogg (ffive@XXX.XXX)
Date: Mon Apr 08 2002 - 21:24:44 EDT
Good stuff, George. Fascinating and informative, the sort of occasional
gem I hope for and greatly appreciate receiving from the Nav list. I
follow your drift and take your points. However ...
The almanac contained within the electronic nav calculator I use gives
me a precise moment for meridian passage of the Sun, according to the
Lat/Long and date entered.
For example, tomorrow Wednesday the 10 April, 2002
(I was only briefly tempted to calculate for the 1 Avril but its too
late for that confusion) at my position
S33°44' E151°04, LAN occurs at 11h57m10s, the Zone Time is 10 hours
ahead of GMT.
At Abingdon, UK (easy commuting distance to Oxford, I note)
N51°41' W1°17', LAN occurs at 12h06m27s
and at a point at the entrance to Chesapeake Bay, USA,
N37° W76°, LAN occurs at 12h05m16s, Zone Time is 5 hours behind GMT.
I would be happy for anybody to check this data, any way you can, often
wonder about just how accurate it is.
If correct, its a great asset, since it also gives me my precise GHA,
and thus Longitude, at the moment of meridian passage, as confirmed by
my (corrected for error) ship's clock. Which, mated with the horizontal
LOP given by the Sun's altitude, gives me a precise and reasonably
accurate (compared to a running fix) position in the middle of the day,
by means of one easy observation - although in practice I would always
make as many sights as possible over a few minutes and average them out.
Here the slowness of meridian passage becomes an advantage.
And on the subject of accuracy, the traditional problem of knowing the
Latitude well enough but being rather vague about where Longitude might
lie is reversed - Longitude is spot-on but Latitude is subject to my
human errors of observation and calculation.
Trying to think this through a little further, by using an accurate
annalemma (I have one crude annalemma, and another method using graphs
- they are, at best, accurate to the nearest minute) could I then reset
my clock, if need be, to GMT?
Would the result be at least as accurate as the result from successful
lunar sights? And greatly simpler?
Peter Fogg
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