t^lements of Astronomy . & 



Us mean duration is about 23 days: but tliere is great dif- 

 ference in this respect. This retrogradation commenceB or 

 finishes when he is about 18° distant from the Sun. 



Mercury changes his phases, Hke the Moon, according to 

 his various positions with regard to the Earth and Sun : but 

 this cannot be discovered without the aid of a telescope. 

 His mean apparent diameter is 6",9. 



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

 which can only happen when he is in his nodes, and when 

 the Earth is in the same longitude. Consequently this 

 phasnomenon can take place only in the months of May or 

 November. The first observation of this kind was made 

 by Gassendi in November 1631 : since which period they 

 have been frequent. The next appearance of this kind will 

 be in November 1815.] 



Venus. 



Venus performs her sidereal revolution in 224'' 16*" 49' 

 li",2: and her mean synodical revolution in about 584 

 days. 



Her mean distance from the Sun is -723; that of the 

 Earth being considered as unity. This makes her mean di- 

 stance nearly 68 millions of miles. 



The eccentricity of her orbit is '0069; half the major 

 axis being considered as unity. She is the least eccentric 

 of all the planets. 



Her mean longitude, at the commencement of the pre- 

 sent century, was in 0' 10^ 44' 35",0. 



The longitude of her perihelion was, at the same time, 

 in 4» 8=" 37' 0",9. The line of her apsides has a sidereal 

 motion, contrary to the order of the signs, of 4' 27,"8 in a 

 century. But, if referred to the ecliptic, this motion will 

 appear (on account of the precession of the equinoxes) to 

 proceed according to the order of the signs at the rate of 

 47", 4 in a year, or 1° I9' 2'i2 in a century. 



Her orbit is inclined to the plane of the ecliptic in an 

 angle of 3^ 23' 32 ",7: which angle decreases about 4",6 iu 

 a century. 



Her orbit, at the commencement of the present centurv, 

 crossed the ecliptic in 2= 14° 52' 38 ',g. But the nodes hsivc 

 an apparent motion in longitude of 31",4 in a year, or 52' 

 20", 2 ni a century. 



The rotation on her axis is performed in 23^ 21' 7",2: 

 but the inclination of her axis is not known. 



[The diameter of V^enas is 7702 miles: consequently she 

 is nearly as large as the Earth.] 



Her 



