Elements of Astronomy . 23 



of time thaii four minutes : but the Moon may be hid from 

 our view fi>r a much longer period. 



[Tiie number of eclipses in a year cannot be less than 

 two, nor mure than seven.] 



Eclipses generally return in the same order and mag- 

 nitude at thl- end of 'J23 lunations. For, in 2-23 mean sy- 

 nodical rr'.olutions, there are 6585'' ?'' 4-2' 3i",7 : and. in 

 6585 ■ I- 41' 43". 6 there are ig mean synodical revolutions 

 of the I\i;;on's nodes. Therefore, at the end of 6585"^ 7'' 

 4£' 31", 7, the Moon's mean longitude will be only 28' 32'' 

 behind tlie mean place of her nodes. In 65B5 days there 

 are 16 J.ii;an vears and 11 days, if there are /oz^r leap years 

 in that period': but if there are five leap years, they form 

 no more than IS Julian years and 10 days. 



The ulmosphere of the Moon, if it has anv, must be ex- 

 tremely attenuated ; and must be more rare than that which 

 we can produce with our best air-pumps. 



The Lis.ht of the Moon is 300000 times more weak than 

 that of the Sun. Its rays, collected bv the aid of powerful 

 olasses, do not produce any sensible effect on the thermo- 



meler. 



The refraction of the ravs of light, at the surface of our 

 Earth, m'^iist be at least 1000 times greater than at the sur- 

 face of the Moon. 



Volcanoes and mountains are discovered on her surface, 

 by the aid of the telescope. 



A bodv projected from the surface of the Moon, with 

 a momentum that would cause it to proceed at the rate of 

 about 8-200 feet in the first second of time, and whose direc- 

 tion should be in a line which at that moment passed through 

 the centre of the Earth and Moon, would not fall again to 

 the surface of the Moon ; but would become a satellite to 

 the Plarth. Its primitive impulse might, indeed, be such 

 as to cause it even lo precipitate to the Earth. The stones, 

 which have fallen from the air, may be accounted for in 

 this manner. 



Satellites of Jupiter. 



By the aid of the telescope we may discover four satel- 

 lites revolvimr round Jupiter. The sidereal revolutions of 

 these bodies are given in tlie following table : together with 

 their mean distances from Jupiter, the semi-diameter of 

 that planet's ev']ualor being considered as unity ; and like- 

 wise their masses, compared with Jupiler considered also 



as unitv. ^ ,,. 



B 4 Satellite. 



