10 A Synopsis of the principal 



Her mass, compared with that of the Sun considered aJ 



unity, is ■s-r-fiVsri-- 



[The proportion of light and heat, received by her from 

 the Sun, is 1-91 times greater than on our planet.] 



It is surrounded by an atmosphere, the refractive powers 

 of which difiVr very tittle from those of our atmosphere. 



As viewed from the Earth, Venus is the most brilHant of 

 all the planets ; and may sometimes l)e seen with the nakeJ 

 eye, at noon day. She is known as the morning and even- 

 ing star; and never recedes far from the Sun. Her elon- 

 gation, or angular distance, varies from 45° to 48". 



Her motion sometimes appears retrograde. The mean arc, 

 which she describes in such case, is about 16* 12'; and her 

 mean duration is about 42 days. This retrogradation com- 

 mences, or finishes, when she is about 28" 48' distant from 

 the Sun. 



Venus changes her phases, like the Moon, according to 

 her position with respect to the Sun and the Earth: which 

 causes a very considerable difi'erence in her brilliancy. 



Her mean apparent diameter is 17''jOj her greatest ap- 

 parent diameter is about 57",3. 



[Venus is sometimes seen to pass over the Sun's disk : 

 which can happen only when she is in her nodes, and 

 when the Earth is in the same longitude. Consequently 

 it can take place onlv in the months of June or Decem- 

 ber. Three of these transits have been already observed : 

 one in 1639, one in June 176I, arid one in June 1769. 

 There will not be another till the 8th of December 1874.] 



The Earth. 



The Eartk which we inhabit is also one of the planets 

 that revolve about the Sun. It performs its sidereal revolu' 

 tion in 363'^ 6'' 9' 11",3: but the time employed in going 

 from one tropic to the other is only 365'^ 3'' 48' 51",6. 

 The tropical year is about 11",2 shorter than it was at the 

 time of Hipparchus. 



Its mean distance from the Sun is 23578 times its own 

 aemi-dianieter : whence it is above 93 millions of miles di- 

 stant from that luminary. If this mean distance be taken 

 equal to unity, we shall have its distance at the perihelion 

 equal to "9832; and its distance at the aphelion equal to 

 10168." 



Thz -eccentricity of its orbit is -01 68: half the major axis 

 being considere«l as unity. The major axis, therefore, will 

 be to the minor axis of the orbit, in the proportion of 1 to 

 •99439. 



Its 



