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British Mensa Travel Special Interest Group |
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The
Bloody Sun at Noon by Ken Gambier All in
a hot and copper sky, So wrote Samuel Taylor Coleridge in The Rime of the Ancient Mariner. Since I was a small boy, I have been fascinated by the idea of the sun being directly overhead. Perhaps it was the picture of the stoical man in his pith helmet, or of the flagpole casting no shadow on the ground. Maybe it was Noel Coward's song Mad Dogs and Englishmen. Whatever the reason, I have always wanted to be somewhere in the tropics when the sun passes overhead. On several occasions I have been close to achieving my ambition but I have never been absolutely certain that I have hit the right day and time. This set me calculating when and where it happens, and I was surprised by what I discovered. Although all the movement is of the earth going round the sun or revolving on its axis, it is better appreciated as a movement of the sun over the surface of the earth. The sun passes overhead only in latitudes between the Tropics of Cancer in the northern hemisphere and of Capricorn in the southern hemisphere. These latitudes are 23 degrees 28 minutes north and south of the equator so the sun moves 46 degrees 56 minutes twice a year, or every 182.5 days. If we consider "overhead" to be within a tolerance of 1 degree, for the sake of reasonable accuracy, the sun is overhead at each degree of tropical latitude for a period of about four days twice a year. This is easy enough to calculate and places near the equator have their two "visits" spaced about six months apart whereas places near the two tropics have their two "visits" in quick succession followed by a long period of nearly a year when the sun is lower in the sky. My calculations might not be strictly accurate as there is probably a stowing down as the sun approaches each tropic and turns to go the opposite way; however, I think this error is quite small and will not greatly alter which days the sun is overhead. However, what did surprise me in my calculations was the discovery of the very brief period each day when the sun is directly overhead. Taking the one degree tolerance again, the sun passes through this angle in about 4 minutes on each of the four-or-so days when it is due to be "overhead". This is such a short penod of time that one has to calculate quite accurately when it is due to happen. Moreover, it is unlikely to be noon on the clocks and watches in the relevant country because of the arbitrary corrections made for the longitude. Most countries fix their deviation from GMT from the longitude of their capital city or of a number of important cities in different time zones throughout the nation. China is a supreme example of dictatorial ruling by the capital; the entire nation follows Beijing time, though this means that, in western China, the sun rises and sets very late in the day. To work out when the sun will be overhead in any particular city, one has to calculate from the longitude the real time difference from GMT, then qualify it by the number of hours used in that country to regulate the clocks. In calculations for 26 places, only two were within 4 minutes of 12 noon - Manila and Harare. Singapore was over an hour early, the sun being overhead at 10.55am on 25 March and 16 September. The table below gives the results for all 26 cities. Latitude
North Overhead dates Longitude +/- GMT Hours Overhead time |