444 PHYSICAL SCIENCES. 



the masses of the planets P, P', &c., be respectively multiplied by the num- 

 bers representing the areas or spaces p S m, p' S m', &c., the sum of the 

 whole will be greater for the invariable plane than it would be for any 

 plane that could pass through S, the centre of gravity of the system. 



NOTE 82, p. 24. The centre of gravity of the solar system lies within 

 the body of the sun, because his mass is much greater than the masses of 

 all the planets and satellites added together. 



NOTE 83, pp. 25, 36. Conjunction. A planet is said to be in con- 

 junction when it has the same longitude with the sun, and in opposition 

 when its longitude differs from that of the sun by 180 degrees. Thus 

 two bodies are said to be in conjunction when they are seen exactly in the 

 same part of the heavens, and in opposition when diametrically opposite to 

 one another. Mercury and Venus, which are nearer to the sun than the 

 earth, are called inferior planets ; while all the others, being farther from 

 the sun than the earth, are said to be superior planets. Suppose the earth 

 to be at E, fig. 24 ; then a superior planet will be in conjunction with the 

 sun at C, and in opposition to him when at 0. Again, suppose the earth 

 to be in 0, then an inferior planet will be in conjunction when at E, and 

 in opposition when at F. 



NOTE 84, p. 26. The periodic inequalities are computed for a given 

 time ; and consequently for a given form and position of the orbits of the 

 disturbed and disturbing bodies. Although the elements of the orbits 

 vary so slowly that no sensible effect is produced on inequalities of a short 

 period, yet, in the course of time, the secular variations of the elements 

 change the forms and relative positions of the orbits so much, that Jupiter 

 and Saturn, which would have come to the same relative positions with 

 regard to the sun and to one another after 850 years, do not arrive at the 

 same relative positions till after 918 years. 



NOTE 85, p. 26. Configuration. The relative position of the planets 

 with regard to one another, to the sun, and to the plane of the ecliptic. 



NOTE 86, p. 27. In the same manner that the excentricity of an 

 elliptical orbit may be increased or diminished by the action of the dis- 

 turbing forces, so a circular orbit may acquire less or more ellipticity from 

 the same cause. It is thus that the forms of the orbits of the first and 

 second satellites of Jupiter oscillate between circles and ellipses differing 

 very little from circles. 



NOTE 87, p. 28. The plane of Jupiter's equator is the imaginary plane 



passing through his centre 

 at right angles to his axis 

 of rotation, and corresponds 

 to the plane q E Q e, in 

 fig. 1. The satellites move 

 very nearly in the plane of 

 Jupiter's equator ; for, if J 



_ ^^gy^, >. be Jupiter, fig. 22, Pp his 



\^-y axis of rotation, eQ his 

 P " equatorial diameter, which 

 is 6000 miles longer than Pp. and if J and J E be the planes of his orbit 



