NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Timing Noon
From: George Huxtable
Date: 2002 Apr 10, 09:30 +0100
From: George Huxtable
Date: 2002 Apr 10, 09:30 +0100
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. ------------------------------ george@huxtable.u-net.com George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. Tel. 01865 820222 or (int.) +44 1865 820222. ------------------------------