ASTRONOMY. 195 



(Washington, 1879, 4to). The theory is founded on two remarkable and 

 hitherto unnoticed chance relations connected with the Saros. This 

 cycle takes account only of the mean motions of the Sun and Moon, but 

 in consequence of the eccentricity of the orbits the Sun may be 2° on 

 either side of its mean i)lace, and the Moon 5°. The relative position of 

 the two bodies may therefore vary 7° from their mean position at any 

 time, and recurring- eclipses might be expected to differ widely from the 

 predicted time, or might not occur at all. But, as a matter of fact, this 

 is uot the case, the irregularities being reduced almost to nothing by 

 the following remarkable relations. At the end of a Saros, not only 

 are the Sun, the Moon, and the node found nearly in their original rela- 

 tion, but the mean anomaly of the Moon happens to have the same value 

 to less than 3°, and the mean anomaly of the Sun to about 12°. There- 

 fore, not only the mean place of the Moon, but all its larger inequalities, 

 will return nearly to their original values at the end of the i^eriod. Tiiis 

 will hold true, not only with respect to the time of the eclipse, but also 

 with respect to its character, as the parallax and semi-diameter of the 

 Moon must also return nearly to their original values. On account of 

 the retrocession of 28'.0 in the the argument of latitude in each cycle, 

 the corresponding eclipses in successive cycles are subject to a progres- 

 sive change. A series of such eclipses commences with a very small 

 eclipse near one pole of the earth ; gradually increasing for about eleven 

 recurrences, it will become central near the same x)ole. Forty or more 

 central eclipses will then recur, the central line moving slowly towards 

 the other pole. The series will then become partial, and finally cease 

 altogether. The entire duration of the series will be more than a 

 thousand years, and a new series commences on an average at intervals 

 of thirty years. All eclipses may therefore be divided into sets, the 

 separate eclipses of each set being separated by intervals of one 18-year 

 cycle, and extending through sixty or seventy cycles. Moreover, from 

 the elements of the central eclipse of each set those of any other of the 

 same set may be readily found by applying the changes corresponding 

 to the number of intervals which separate it from the central one. This 

 circumstance Prof. ]S[ewcomb has utilized to form a series of tables 

 which give at once the circumstances of any eclipse between b. c. 700 

 and A. D. 2300. 



Phenomena attending Solar eelipses. — The whole subject of the charac- 

 teristics of solar eclipses is in so undecided a state that even those who 

 are most familiar with it are conscious of their incomplete knowledge 

 of the data. A general review of the evidence on special points has 

 been well nigh impossible. Like all comparatively new problems, the 

 different branches of the research have been developed in quite an 

 irregular way. On some lines of research our knowledge is very con- 

 siderable; on others, we have scarcely any trustworthy information. 



This was specially a hindrance in the study of the phenomena of solar 



