NOTES. 473 



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

 planets and satellites added together. 



NOTE 83, pp. 29, 42. Conjunction. A planet is said to be in conjunction 

 when it has the same longitude with the sun, and in opposition when its longi- 

 tude 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 O. 

 Again, suppose the earth to be in O, then an inferior planet will be in conjunc- 

 tion when at E, and in opposition when at F. 



NOTE 84, p. 30. 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. 30. 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. 31. In the same manner that the excentricity of an elliptical 

 orbit may be increased or diminished by the action of the disturbing 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. 32. 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 



Fig. 22. 



