APPEARANCES OF THE CELESTIAL BODIES. 421 



into 12 parts, called digits, each of which contains 30 minutes : thus if one 

 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 times of the moon's rising on two successive days, is, in Lon- 

 don, 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 : 

 the greatest possible difference is 1 hour 17 minutes. But it happens every 

 month that the difference becomes greater and less by turns, and when the 

 least difference is at the time of the full moon, it is usually called the har- 

 vest 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 first three never pass the shadow without being plunged 

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

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

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

 hind the body of the planet, and this circumstance constitutes an occul- 

 tation : hence it happens that we can never see both the immersion of the 

 first satellite into the shadow of Jupiter, and its emersion from it ; but 

 both the immersion and emersion of the three outer satellites are sometimes 

 observable. The ring of Saturn exhibits a variety of forms according to 

 its angular position : it disappears to common observation when either its 

 edge or its dark side is presented to us : but to Dr. Herschel's telescopes it 

 never becomes invisible ; the light reflected from the planet being probably 

 sufficient for illuminating in some measure the side not exposed to the sun's 

 direct rays. 



The comets are seen for a short time, and are again lost to our view ; 

 their tails are in general situated in the planes of their orbits, following 

 them in their descent towards the sun, and preceding them in their ascent, 

 with a slight degree of curvature in their forms ; they must also appear to 

 us more or less arched, and of different extent, according to their distances, 

 and to the angular position of the orbits with respect to the ecliptic. 



The proportion of the light afforded by the different heavenly bodies has 

 been variously estimated by various authors ; but there is little reason to 

 call in question the accuracy of the experiments and calculations of Mr. 

 Bouguer. He states the intensity of the moon's light as only one three 

 hundred thousandth of that of the sun. These calculations have been ex- 

 tended by Euler and by Lambert ; Eulert considers the direct light of the 

 sun as equal to that of 6560 candles of a moderate size, supposed to be 

 placed at the distance of 1 foot from the object : that of the moon to the 



* Wargentin, A New Method of determining the Longitude from the Eel. of Jup. 

 Sat. Ph. Tr. 1766, p. 278. 

 t Hist, et M&n. de Berlin, 1750, p. 280. 



