The Story of Eclipses. George F. Chambers

10 8½ 223 times 29.5 = 6585.32 = 18 10 7¾ 19 times 346.6 = 6585.78 = 18 10 18¾

      The interpretation to be put upon these coincidences is this: that supposing Sun and Moon to start together from a Node they would, after the lapse of 6585 days and a fraction, be found again together very near the same Node. During the interval there would have been 223 New and Full Moons. The exact time required for 223 Lunations is such that in the case supposed the 223rd conjunction of the two bodies would happen a little before they reached the Node; their distance therefrom would be 28′ of arc. And the final fact is that eclipses recur in almost, though not quite, the same regular order every 6585⅓ days, or more exactly, 18 years, 10 days, 7 hours, 42 minutes.[5] This is the celebrated Chaldean “Saros,” and was used by the ancients (and can still be used by the moderns in the way of a pastime) for the prediction of eclipses alike of the Sun and of the Moon.

      At the end of a Saros period, starting from any date that may have been chosen, the Moon will be in the same position with respect to the Sun, nearly in the same part of the heavens, nearly in the same part of its orbit, and very nearly indeed at the same distance from its Node as at the date chosen for the terminus a quo of the Saros. But there are trifling discrepancies in the case (the difference of about 11 hours between 223 lunations and 19 returns of the Sun to the Moon’s Node is one) and these have an appreciable effect in disturbing not so much the sequence of the eclipses in the next following Saros as their magnitude and visibility at given places on the Earth’s surface. Hence, a more accurate succession will be obtained by combining 3 Saros periods, making 54 years, 31 days; while, best of all, to secure an almost perfect repetition of a series of eclipses will be a combination of 48 Saroses, making 865 years for the Moon; and of about 70 Saroses, or more than 1200 years for the Sun.

      These considerations are leading us rather too far afield. Let us return to a more simple condition of things. The practical use of the Saros in its most elementary conception is somewhat on this wise. Given 18 or 19 old Almanacs ranging, say, from 1880 to 1898, how can we turn to account the information they afford us in order to obtain from them information respecting the eclipses which will happen between 1899 and 1917? Nothing easier. Add 18^y 10^d 7^h 42^m to the middle time of every eclipse which took place between 1880 and 1898 beginning, say, with the last of 1879 or the first of 1880, and we shall find what eclipses will happen in 1898 and 17 following years, as witness by way of example the following table:—

			d.	h.	m.	Error of Saros by Exact Calculation.
Moon.	1879	Dec.	28	4	26 p.m.
(Mag. 0.17)	18		10	7	42
(Mag. 0.16)	1898	Jan.	8	12	8 a.m.	(civil time) +3 m.
			d.	h.	m.
Sun.	1880	Jan.	11	10	48 p.m.
(Total)	18		10	7	42
(Total)	1898	Jan.	22	6	30 a.m.	(civil time) −1 h. 7 m.
			d.	h.	m.
Moon.	1880	June	22	1	50 p.m.
(Mag. Total)	18		11	7	42
(Mag. 0.93)	1898	July	3	9	32 p.m.	+35 m.
			d.	h.	m.
Sun.	1880	July	7	1	35 p.m.
(Mag. Annular)	18		11	7	42
(Mag. Annular)	1898	July	18	9	17 p.m.	+1 h. 10 m.
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