420 LECTURE XLIV. 



of which the moon remains still visible to us, the cone of total darkness 

 extending to somewhat less than two thirds of the moon's distance. But 

 it has sometimes happened, probably from the effect of clouds occupying 

 the greatest part of our atmosphere, that the moon has totally disappeared. 

 (Plate XXXIV. Fig. 496.) 



When the sun is eclipsed, it depends on the situations of the earth and 

 moon in their orbits, whether the sun or moon subtends the greatest angle 

 as seen from the earth ; since at their mean distances their apparent dia- 

 meters are each about half a degree. If the sun's apparent diameter is the 

 greater, the eclipse, when the centres coincide, must be annular, the margin 

 of the sun's disc being still visible in the form of a ring : when the moon's 

 apparent diameter is greater than the sun's, the eclipse, if central, becomes 

 total ; but still a ring of pale light is seen round the disc,* which has been 

 attributed to the effect of the sun's atmosphere, since that of the moon is 

 probably too inconsiderable to produce the appearance ; a red streak t is 

 also sometimes observed at the margin, before the actual emersion of the 

 sun. The degree of darkness depends on the situation of the place of obser- 

 vation within the shadow, on account of the greater or less illumination of 

 the atmosphere within view : sometimes a considerable number of stars 

 may be seen during a total eclipse of the sun. 



It is obvious that, since the earth is much larger than the moon, the 

 whole shadow of the moon will only pass over a part of the earth's surface : 

 and that no solar eclipse can be visible in the whole of the hemisphere 

 turned to the sun : while lunar eclipses, on the contrary, present the same 

 appearance wherever the moon is visible. In the same manner, to a spec- 

 tator on the moon, an eclipse of the earth, or a transit of the moon's shadow 

 over the earth's disc, would have nearly the same appearance wherever he 

 might be stationed ; but an eclipse of the sun by the earth would be total 

 to that part of the moon's surface only, which to us appears dark at the 

 same time. (Plate XXXIV. Fig. 497. . .499.) 



The moon's nodes arrive very nearly at the same situation with respect 

 to the earth after 223 lunations, or revolutions of the moon, which are per- 

 formed in 18 years of 365 days each, 15 days, 7 hours, and 43f minutes ; 

 so that after a period of about 18 years, the series of eclipses recommences 

 nearly in the same order. This circumstance was observed by the ancients, 

 and is 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 conjunction with the sun, 

 at any distance within 17l 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 



* Duillier, Ph. Tr. 1706, p. 2241. Halley, ibid. 1715, p. 245. Lahire, Hist, et 

 M6m. 1715, p. 161, H. 47. Ulloa, ibid. 1779, p. 105. 

 t Ph. Tr. 1706, p. 2240 ; 1748, p. 490. 



