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Subject: Re: Timing Noon
From: George Huxtable (george@XXX.XXX)
Date: Wed Apr 10 2002 - 04:30:59 EDT
Peter Fogg had some kind words to say and then-
>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
at my position
>S33°44' E151°04, LAN occurs at 11h57m10s, the Zone Time is 10 hours
>ahead of GMT.
>I would be happy for anybody to check this data, any way you can, often
>wonder about just how accurate it is.
I have checked this prediction for local apparent noon on that date and at
Peter's location (presumably Thornleigh, near Sydney) on my own pocket
calculator. This was programmed using the data from Meeus "Astronomical
formulae for calculators", and hence Newcomb.
It gives me the same answer, within a second, as Peter obtained, i.e.
01:57:10 GMT.
Where does this time derive from? At Greenwich, the Mean Sun passes the
meridian at 12 noon. Thornleigh is 151°04 further East in longitude, so at
15° per hour, the mean Sun would transit the meridian there 10h 04min 16
sec earlier or at 01h 55min 44sec GMT. However, the equation of time is
then 1min 26sec, in the sense that the real Sun lags 1min 26sec behind the
mean Sun at that point of the year. The real Sun will then pass the
meridian of Thornleigh, and everywhere else on that same line of longitude,
at 01h 57min 10sec GMT. This is the GMT of Local Apparent Noon at
Thornleigh on that day.
>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.
I'm puzzled about this. If Peter had set up a couple of posts in his garden
on an exact North-South line, he would be able to tell when the Sun passed
the meridian. But otherwise, how can he tell it from the clock, even if
that clock has been set exactly to read GMT? If the clock reads 01:57:10,
then he knows the Sun must be on the meridian IF AND ONLY IF he is at a
longitude of E151°04. But if he isn't at that longitude, the Sun won't be
on the meridian. How can he tell if the Sun is on his meridian? He is
indulging in a circular argument, and assuming what he is trying to
measure, as I see it.
All Peter is able to measure accurately at noon is the Sun's altitude,
which taken with its declination will give him his latitude. Finding the
longitude to any accuracy requires a further measurement of Sun altitude,
earlier or later in the day (or both).
If I have completely missed the point of what Peter Fogg is explaining, I
hope he will forgive me and put me right.
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
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