Places at or near the Earth’s equator enjoy the best opportunities for seeing total eclipses of the Sun, because whilst the Moon’s shadow travels eastwards along the Earth’s surface at something like 2000 miles an hour, an observer at the equator is carried in the same direction by virtue of the Earth’s axial rotation at the rate of 1040 miles an hour. But the speed imparted to an observer as the result of the Earth’s axial rotation diminishes from the equator towards the poles where it is nil, so that the nearer he is to a pole the slower he goes, and therefore the sooner will the Moon’s shadow overtake and pass him, and the less the time at his disposal for seeing the Sun hidden by the Moon.
It was calculated by Du Sèjour that the greatest possible duration of the total phase of a Solar eclipse at the equator would be 7m 58s, and for the latitude of Paris 6m 10s. In the case of an annular eclipse the figures would be 12m 24s for the equator, and 9m 56s for the latitude of Paris. These figures contemplate a combination of all the most favourable circumstances possible; as a matter of fact, I believe that the greatest length of total phase which has been actually known was 6½m and that was in the case of the eclipse of August 29, 1886. It was in the open Atlantic that this duration occurred, but it was not observed. The maximum observed obscuration during this eclipse was no more than 4m.
Though total eclipses of the Sun happen with tolerable frequency so far as regards the Earth as a whole, yet they are exceedingly rare at any given place. Take London, for instance. From the calculations of Hind, confirmed by Maguire,[11] it may be considered as an established fact that there was no total eclipse visible at London between A.D. 878 and 1715, an interval of 837 years. The next one visible at London, though uncertain, is also a very long way off. There will be a total eclipse on August 11, 1999, which will come as near to London as the Isle of Wight, but Hind, writing in 1871, said that he doubted whether there would be any other total eclipse “visible in England for 250 years[12] from the present time.” Maguire states that the Sun has been eclipsed, besides twice at London, also twice at Dublin, and no fewer than five times at Edinburgh during the 846 years examined by him. In fact that every part of the British Isles has seen a total eclipse at some time or other between A.D. 878 and 1724 except a small tract of country at Dingle, on the West coast of Ireland. The longest totality was on June 15, 885, namely, 4m 55s, and the shortest in July 16, 1330, namely, 0m 56s.
Contrast with this the obscure island of Blanquilla, off the northern coast of Venezuela. The inhabitants of that island not long ago had the choice of two total eclipses within three and a half years, namely, August 29, 1886, and December 22, 1889; whilst Yellowstone, U.S., had two in twelve years (July 29, 1878, and January 1, 1889).
Counting from first to last, Du Sèjour found that at the equator an eclipse of the Sun might last 4h 29m, and at the latitude of Paris 3h 26m. These intervals, of course, cover all the subordinate phases. The total phase which alone (with perhaps a couple of minutes added) is productive of spectacular effects, and interesting scientific results is a mere matter of minutes which may be as few as one (or less), or only as many as 6 or 8.
As a rule, a summer eclipse will last longer than a winter one, because in summer the Earth (and the Moon with it), being at its maximum distance from the Sun, the Sun will be at its minimum apparent size, and therefore the Moon will be able to conceal it the longer.
Footnotes:
[11] Month. Not., R.A.S., vol. xlv., p. 400. June 1885.
[12] Johnson makes the eclipse of June 14, 2151, to be “nearly, if not quite, total at London.” Possibly it was this eclipse which Hind had in his thoughts when he wrote in the Times (July 28, 1871) the passage quoted above.