8 A Synopsis of the principal 



There are also some inequalities which affect the elliptical 

 motion of the planets. That of the Earth is a little altered. 

 But they are most sensible in .Jupiter and Saturn : for it 

 appears that the duration of their revolution round the Sun 

 is subject to a periodical variation. 



Mercury. 



Mercury is the nearest planet to the Sun : its mean dis- 

 /ance being '387, that of the Earth being considered as unity. 

 This makes his mean distance above 36 millions of miles. 



He performs his sidereal revolution in 87'' "23^ lo' 43'',9 

 and his mean synodical revolution in about 1 16 days. 



The eccentricity of his orbit is '2055 ; half the major axis 

 being taken equal to unity. 



His mean longitude, at the commencement of the pre- 

 sent century, was in 5' 23° 56'' 27",0. 



The longitude of his perihelion wasj at the same time, in 

 V 14° 21' 46"59. The line of the apsides has a sidereal mo- 

 tion, according to the order of the signs, equal to 9' 43", 6 

 in a century. But, if referred to the ecliptic, this motion 

 win (owing to the precession of the equinoctial points) be 

 equal to 55", 8 in a year ; or to 1** 33' 13", 6 in a century. 



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

 angle of 7° 0' 9'',> : which angle is subject to a small in- 

 crease of about 1 8', 2 in a century. 



His orbit, at the commencement of the present century, 

 crossed the ecliptic in 1' 15^ 57' 30", y : having a sidereal 

 motion, to the westward every century, of 13' 2", 2. But, 

 if referred to the ecliptic, the place of the nodes will (on 

 account of the precession of the equinoxes) fall more to 

 the eastward by 42",3 in a year, or l° lo' 27", 8 in a century. 



The rotation on his axis is accomplished in l** O'' 5' 28",3. 

 But the inclination of its axis is not known. 



\^T\\e, diameter of Mercury is about 3123 miles: which, 

 compared with the Earth's diameter considered as unity, is 

 about -3944.] 



His mass, compared with that of the Sun considered a3 

 unity, is _^i^— . 



[The proportion of li^^bt and heat received from the Sun 

 is about 663 times greater ihan on our planet.] 



As seen from the Earth, Mercury never appears at anv 

 great distance from the Sun ; either in the morning or the 

 evening. His elongation, or angular distance, varies from 

 16M2' to 28^ 48'. 



His course sometimes appears retrograde. The mean 

 arc, which it describes in this case, is about 13" ity ; and 



US 



