530 Lt;CTUUE XLIV. 



mentioned by Ptolemy and by Pliny. When the full moon happens within 

 74.° of the node, there must be a lunar eclipse and there may be an eclipse 

 at the distance of 13" from the node. An eclipse, of the sun may happen 

 when the moon changes, or comes into conj unction with the sun, at any dis- 

 tance within 1 r-j° of the node. The mean number of eclipses which occur 

 in a year is about 4; and there are sometimes as many as 7: there must 

 necessarily be two solar eclipses, but it is possible that there may not be 

 even one lunar. In speaking of the magnitude of the part of the sun or 

 moon eclipsed, it is usual to consider the whole diameter as divided into 

 12 parts, called digits, each of which contains 30 minutes: thus if o n 

 fifth part of the diameter were dark, the extent of the eclipse would be 

 called 2 digits 12 minutes. 



The moon travels through the heavens with a motion contrary to their 

 apparent diurnal revolution. Hence she rises and sets, on an average, about 

 three quarters of an hour later every day. The least possible difference 

 between the time sof the moon's rising on two successive days, is, in London, 

 17 minutes; and this circumstance occurs once in about 19 years, which is 

 nearly the period of the moon's nodes with respect to the heavens : t\^e great- 

 est possible difference is 1 hour 17miivute3. But it happens every month that 

 the difference becomes greater and less by turns, and when the least differ- 

 ence is at the time of the fuU moon, it is usually called the harvest moon. 

 In parts nearer to the poles, the moon often rises at the same hour on two 

 succeeding days. 



The eclipses of the satellites of Jupiter exhibit appearances extremely in- 

 teresting for their utility in identifying the same instant of time in different 

 places. On account of the small inclination of their orbits to the plane of 

 Jupiter's orbit, the fii-st three never pass the shadow without being plunged 

 into it, and the fourth but seldom; while those of Saturn are much less fre- 

 quently liable to be eclipsed, on account of their greater deviation from the 

 plane of his ecliptic. These satellites are also frequently hidden behind the 

 body of the planet, and this circumstance constitutes an occultation: hence 

 it happens that we can never see both the immersion of the first satellite into 

 the shadoM' of Jupiter, and its emersion from it; but both the immersion and 

 emersion of the three outer satellites are sometimes observable. The ring of 



