The Story of Eclipses

by George F. Chambers

[7] See p. 28 (post) for an explanation of this word.

[8] In Mrs. D. P. Todd’s interesting little book, Total Eclipses of the Sun (Boston, U.S., 1894), which will be several times referred to in this work, two maps will be found, which will help to illustrate the successive northerly or southerly progress of a series of Solar eclipses, during centuries.

[9] In his and Professor Holden’s Astronomy for Schools and Colleges, p. 184.

[10] See p. 19 (ante).

CHAPTER IV.

MISCELLANEOUS THEORETICAL MATTERS CONNECTED WITH ECLIPSES OF THE SUN (CHIEFLY).

One or two miscellaneous matters respecting eclipses of the Sun (chiefly) will be dealt with in this chapter. It is not easy to explain or define in words the circumstances which control the duration of a Solar eclipse, whereas in the case of a lunar eclipse the obscuration is the same in degree at all parts of the Earth where the Moon is visible. In the case of a Solar eclipse it may be total, perhaps, in Africa, may be of six digits only in Spain, and of two only in England. Under the most favourable circumstances the breadth of the track of totality across the Earth cannot be more than 170 miles, and it may be anything less than that down to zero where the eclipse will cease to be total at all, and will become annular. The question whether a given eclipse shall exhibit itself on its central line as a total or an annular one depends, as has been already explained, on the varying distances of the Earth and the Moon from the Sun in different parts of their respective orbits. Hence it follows that not only may an eclipse show itself for several Saros appearances as total and afterwards become annular, and vice versâ, but on rare occasions one and the same eclipse may be annular in one part of its track across the Earth and total in another part, a short time earlier or later. This last-named condition might arise because the Moon’s distance from the Earth or the Sun had varied sufficiently during the progress of the eclipse to bring about such a result; or because the shadow just reaching the Earth and no more the eclipse would be total only for the moment when a view perpendicular upwards could be had of it, and would be annular for the minutes preceding and the minutes following the perpendicular glimpse obtained by observers actually on the central line. The eclipse of December 12, 1890, was an instance of this.

If the paths of several central eclipses of the Sun are compared by placing side by side a series of charts, such as those given in the Nautical Almanac or in Oppolzer’s Canon, it will be noticed that the direction of the central line varies with the season of the year. In the month of March the line runs from S.W. to N.E., and in September from N.W. to S.E. In June the line is a curve, going first to the N.E. and then to the S.E. In December the state of things is reversed, the curve going first to the S.E. and then to the N.E. At all places within about 2000 miles of the central line the eclipse will be visible, and the nearer a place is to the central line, so much the larger will be the portion of the Sun’s disc concealed from observers there by the Moon. If the central line runs but a little to the N. of the Equator in Winter or of 25° of N. latitude in Summer, the eclipse will be visible all over the Northern Hemisphere, and the converse will apply to the Southern Hemisphere. It is something like a general rule in the case of total and annular eclipses, though subject to many modifications, that places within 200-250 miles of the central line will have partial eclipse of 11 digits; from thence to 500 miles of 10 digits, and so on, diminishing something like 1 digit for every 250 miles, so that at 2000 miles, or rather more, the Sun will be only to a very slight extent eclipsed, or will escape eclipse altogether.

The diameter of the Sun being 866,000 miles and the Moon being only 2160 miles or 1⁄400th how comes it to be possible that such a tiny object should be capable of concealing a globe 400 times bigger than itself? The answer is—Distance. The increased distance does it. The Moon at its normal distance from the Earth of 237,000 miles could only conceal by eclipse a body of its own size or smaller, but the Sun being 93,000,000 miles away, or 392 times the distance of the Moon, the fraction 1⁄392 representing the main distance of the Moon, more than wipes out the fraction 1⁄400 which represents our satellite’s smaller size.

During a total eclipse of the Sun, the Moon’s shadow travels across the Earth at a prodigious pace—1830 miles an hour; 30½ miles a minute; or rather more than a ½ mile a second. This great velocity is at once a clue to the fact that the total phase during an eclipse of the Sun lasts for so brief a time as a few minutes; and also to the fact that the shadow comes and goes almost without being seen unless a very sharp watch is kept for it. Indeed, it is only observers posted on high ground with some miles of open low ground spread out under their eyes who have much chance of detecting the shadow come up, go over them, and pass forwards.