406 LECTURE XLII. 



and a synodical revolution, during which she returns to the same position 

 with respect to the earth and sun, in 29 days 12f hours ; a period which 

 constitutes a lunation, or a lunar month. Her orbit is inclined to the 

 ecliptic in an angle of a little more than five degrees, hut this inclination is 



liable to great variations : the place of its nodes is also continually caji \ 

 ing, their motion being sometimes retrograde, and sometimes direct, but on^ ^ 

 the whole the retrograde motion prevails. The form of the moon's orbit is 

 irregularly elliptic, and the velocity of its motion deviates considerably 

 from the Keplerian law of the description of equal areas in equal timer; ; 

 the apsides, or the extremities of the greater axis of the ellipsis, which ,>re 

 called the apogee and perigee, have on the whole a direct motion. Frory a 

 comparison of modern observations with the most ancient, the mean motion 

 of the moon is found to be somewhat accelerated. 



The moon revolves on her own axis with a very equable motion, and 

 the period of her rotation is precisely equal to the mean period of her 

 revolution round the earth ; so that she always presents to us the same 

 portion of her surface, excepting the apparent librations produced by her 

 unequal velocities in her orbit, and by the position of her axis, which is 

 inclined 1 43' to the ecliptic, and sometimes as much as 7 to her own 

 orbit. Her distance from the earth is about 240,000 miles ; her diameter 

 -^r of that of the earth, or 21GO miles ; and the weight of bodies at her 

 surface is supposed to be about one fifth of their weight at the surface of 

 the earth, 



The surface of the moon presents to us, when viewed with a telescope, 

 a great diversity of light and shade, the principal features of which are 

 visible even to the naked eye. Many of these inequalities resemble very 

 strongly the effects of volcanos ; several astronomers have imagined that 

 they have seen volcanos actually burning in the unenlightened part of the 

 planet ; and Dr. Herschel's instruments have enabled him to obtain satis- 

 factory evidence of the truth of the conjecture.* The appearance of a 

 perforation, which Ulloa supposed that he observed near the margin of 

 the Moon's disc, in a solar eclipse, has been attributed by some to a volcano 

 actually burning. Dr. Halley and Mr. Weidlerf have also observed 

 flashes of light on the dark part of the moon, considerably resembling the 

 effect of lightning. The height of the lunar mountains has been com- 

 monly supposed to exceed very considerably that of the mountains of the 

 earth ; but Dr. Herschel J is of opinion that none of them are so much as 

 two miles high. The names, which have been given by astronomers to 

 various parts of the moon's surface, are of some utility in the observation 

 of the progress of an eclipse. 



Of the satellites of Jupiter, some are a little larger, and others smaller 

 than the moon : they all revolve in planes inclined between 2 and 3 to 

 the orbit of the planet, and they are therefore always seen nearly in the 

 same line. It is inferred, from some periodical changes of light which they 



* An Account of the Volcanos in the Moon, Ph. Tr. 1787, Ixxvii. 229. See also 

 Ph. Tr. 1794, pp. 84, 429, 435. 



t Ibid. 1739, p. 228. J Ibid. 1780, p. 507. 



Marii Mundus Jovialis, ito, Nuremb. 1611. Herschel, l'h. Tr. 1797, p. 332. 



