ASTRONOMY. 



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 orbit 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 1 node, and the other 

 is the descending 1 node. The angle which 

 the plane of a planet's orbit makes with the 

 plane of the ecliptic is called the inclina- 

 tion of that planet's orbit. Thus, fig-. 2. 

 Plate II. where F represents the sun, the 

 points A and B represent the nodes, and 

 the line AB 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 con- 

 jugate 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. The 

 two points in a planet's orbit, which are 

 farthest and nearest to the body round 

 which it moves, are called the apsides ; 

 the former of which is called the higher 

 apsis, or aphelion; the latter is called the 

 lower apsis, or perihelion. The diame- 

 ter 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 between 

 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 conjunc- 

 tions with the sun, and the superior pla- 

 nets can have only one, because they can 

 never come between the earth and the 

 sun. When a planet comes directly be- 

 tween us and the sun, it appears to pass 

 over the sun's disc, dr surface, and this is 

 called the transit of the planet. When a 

 planet moves from west to east, viz. ac- 

 cording to the order of the signs, it 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 tha't 

 any planet appears to occupy in the ce- 

 lestial hemisphere, when seen by an ob- 

 server supposed to be placed in the sun, 

 is called its heliocentric place. The place 

 it occupies, whe"n seen from the earth, is 

 called its geocentric place. 



The planets do not move with equal ve- 

 locity in every part of their orb its,but they 

 move faster when they are nearest to the 

 sun, and slower in the remotest part of 

 their orbits ; and they all observe this 

 remarkable law, that if a straight line be 

 drawn from the planet to the sun, and 

 this line be supposed to be carried along 

 by the periodical motion of the planet, 

 then the areas which are described by 

 this right line and the path of the planet 

 are proportional to the times of the pla- 

 net's motion. That is, the area described 

 in two days is double that which is de- 

 scribed in one day, and a third part ot 

 that which is described in six days,though 

 the arcs or portions of the orbit described 

 are not in that ratio. The planets, being 

 at different distances from the sun, per- 

 form their periodical revolutions in differ- 

 ent times : but it has been found that the 

 cubes of their mean distances are con- 

 stantly as the squares of their periodical 

 times ; viz. of the times of their perform- 

 ing their periodical revolutions. These 

 two last propositions were discovered by 

 Kepler, by observations on the planets ; 

 but Sir Isaac Newton demonstrated, that 

 it must have been so on the principle of 

 gravitation, which formed the basis of his 

 theory. This law of universal attraction, 

 or gravitation, discovered by Newton, 

 completely confirms the system of Coper- 

 nicus, and accounts for all the phenomena 

 which were inexplicable on any other 

 theory. The sun, as the largest body in 

 our system, forms the centre of attraction, 

 round which all the planets move : but it 

 must not be considered as the only body 

 endued with attractive power, for all the 

 planets also have the property of attrac- 

 tion, and act upon each other as well as 

 upon the sun. The actual point there- 

 fore about which they move will be the 

 common centre of gravity of all the bo- 

 dies which are included in our system ; 

 that is, the sun, with the primary and 

 secondary planets. But because the bulk 

 of the sun greatly exceeds that of all the 

 planets put together, this point is in the 

 body of the sun. The attraction of the 

 planets on each other also somewhat dis- 



