ASTRONOMY. ¥2J 



The two points in a planet's orbit which are far- 

 thest and nearest to the body round which it moves, 

 are called the apses, or apsides; the former of 

 which is called the higher apsis, or aphelion ; the 

 latter is called the lower apsis, or perihelion. The 

 diameter which joins these two points is called the 

 line of the apsides. When the sun and moon are 

 nearest to the earth, they are said to be in perigee ; 

 when at their greatest distance from the earth, they 

 are said to be in apogee. 



When a planet is situated so as to be between 

 the sun and the earth, or so that the sun is be- 

 tween the earth and the planet, then that planet is 

 said to be in conjunction with the sun. When the 

 earth is between the sun and any planet, then that 

 planet is said to be in opposition. It is evident 

 that the two inferior planets must have two con- 

 junctions with the sun, and the superior planets 

 can have only one, because they can never come 

 between the earth and the sun. When a planet 

 comes directly between us and the sun, it appears 

 to pass over the sun's disc, or surface, and this is 

 called the transit of the planet. When a planet 

 moves from west to east, viz. according to the order 

 of the signs, it is is said to have direct motion, or 

 to be in consequentia. Its retrograde motion, or 

 motion in antecedentia, is when it appears to move 

 from east to west, viz. contrary to the order of the 

 signs. 



The place that any planet appears to occupy in 

 the celestial hemisphere, when seen by an observer 

 supposed to be placed in the sun, is called its helio- 

 centric place. The place it occupies, when seen 

 from the earth, is called its geocentric place. 



The planets do not move with equal velocity in 

 every part; of their orbits ; but they move taster 



