ON THE APPEARANCES OF THE CELESTIAL BODIES. 53,9 



cone of total darkness extending to somewhat less than two thirds of the 

 m/)on'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 diameters arc 

 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 appar- 

 ent diarmeter 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 attri- 

 buted to the effect of the sun's atmosphere, since that of the moon is pro- 

 bably too inconsiderable to produce the appearance: a red streak 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 observation 

 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 spectator on the moon, 

 an eclipse of the earth, or a transit of the moon's shadow over the earth'* 

 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 eartb after 2^3 lunations, or revolutions of the moon, which are per- 

 formed in 18 years of 365 days each, 15 days,7 hours, and 43|: minutes ; so that 

 after a period of about 18 years, the series of eclipses reeommences nearly in 

 the, same order. This circumstance was observed by the ancients, and i& 



VOL. I. 3y 



