. Discovery. Science. DISCOVERY 299 eclipse would be both annular and total, the former in one part of its track, the latter in another. This is a very rare phenomenon, and is due to a slight variation in the moon's distance from the earth during the progress of the eclipse, the earth being all the time close to the apex of the lunar shadow. Or if the earth were actuaUy at the apex, the shadow would just exactly reach it, with the result that a total eclipse would be observed at the moment when a perpendicular upward view could be obtained, but this would be preceded and followed by some minut
. Discovery. Science. DISCOVERY 299 eclipse would be both annular and total, the former in one part of its track, the latter in another. This is a very rare phenomenon, and is due to a slight variation in the moon's distance from the earth during the progress of the eclipse, the earth being all the time close to the apex of the lunar shadow. Or if the earth were actuaUy at the apex, the shadow would just exactly reach it, with the result that a total eclipse would be observed at the moment when a perpendicular upward view could be obtained, but this would be preceded and followed by some minutes of the annular phase. One such eclipse took place on December 12, 1890 (see Fig. 5). An interesting series in the twenty-nine-year period is that containing the historical eclipse of Nineveh, which was visible in the year 763 on June 15. This is recorded in the Assyrian tablets. Adding the period we are considering, we find other eclipses on May 27, 734 , May 7, 705 , a series contiiuiing. THE EARTH Annular Fig. 5.—showixg three positions of the shadow-cone. to our own times in the eclipses of 1814, July 17 (total) ; 1843, June 27 (annular) ; 1872, June 6 (annular) ; 1901, May 18 (total) ; the last-mentioned being the ninety-third of the series since that of Nineveh. Between each of these eclipses the lunar perigee— that point in the moon's orbit at which it is nearest to the earth—moves through about 242-4 degrees, so that after three such intervals it has advanced through 727 degrees, and, since 720 degrees is twice round the circle, it is thus only 7 degrees in advance of its original position. For a long period of, say, a thousand years or more, every third eclipse of the series— every one recurring at 3 x 29, 87 years — will therefore be of the same character. Thus those of 1814, 1901, 1988, in the above series are total. The dates mentioned above involve a change from the so-called Old Style to the New. The latter was introduced in 1582,
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