426 ASTRONOMY. 



and with this the situations of the orbits of all the 

 other planets are compared. 



The planes of the orbits of all the other planets 

 must necessarily pass through the centre of the 

 sun j but if extended as far as the fixed stars, they 

 form circles different from one another, as- also 

 from the ecliptic ; one part of each orbit being on 

 the north, and the other on the south side of the 

 ecliptic. Therefore the orbit of each planet cuts 

 the ecliptic in two opposite points, which are called 

 the nodes of that particular planet ; and the nodes 

 of one planet cut the ecliptic in planes different 

 from the nodes of another planet. A line passing 

 from one node of a planet to the opposite node, or 

 the line in which the plane of the orbits cuts the 

 ecliptic, is called the line of nodes. That node 

 where the planet passes from the south to the north 

 side of the ecliptic, is called the ascending node ; 

 and the other is the descending node. The angle 

 which the plane of a planet's orbit makes with the 

 plane of the ecliptic, is called the inclination of 

 that planet's orbit. Thus (Fig. 2.), where F re- 

 presents the sun, the points A and B represent the 

 nodes, and the line A B the line of nodes formed 

 by the intersection of the planes of the orbits C and 

 D. The angle E F G, is the angle of inclination 

 of the planes of the two orbits to each other. A 

 line drawn from the lower focus of a planet's orbit 

 (viz. where the sun is) to either end of the conju- 

 gate axis of its orbit (which line is equal to half 

 the transverse axis), is called the mean distance of 

 the planet from the sun. But according to some, 

 the mean distance is a mean proportional between 

 the two axes of that planet's orbit. The distance 

 of either focus from the centre of the orbit is called 

 its eccentricity. 



