438 



PHYSICAL SCIENCES. 



Fig. 12. 



direction Njo. Let E N m e n be the shadow or projection of the orbit on 

 the plane of the ecliptic, then N S n is the intersection of these two planes, 



for the orbit rises above the 

 plane of the ecliptic towards 

 N p, and sinks below it at N P. 

 The angle p N m, which these 

 A two planes make with one an- 

 other, is the inclination of the 

 orbit PNpA to the plane of 

 the ecliptic. 



NOTE 54, p. 9. Latitude 

 of a planet. The angle p S m, 

 fig. 12, or the height of the 

 planet p above the ecliptic E N m. 



In this case the latitude is north. Thus, celestial latitude is the angular 

 distance of a celestial body from the plane of the ecliptic, whereas terrestrial 

 latitude is the angular distance of a place on the surface of the earth from 

 the equator. 



NOTE 55, p. 9. Nodes. The two points N and n, fig. 12, in which the 

 orbit N A n P of a planet or comet intersects the plane of the ecliptic e N 

 E n. The part N A n of the orbit lies above the plane of the ecliptic, and 

 the part n P N below it. The ascending node N is the point through 

 which the body pa.sses in rising above the plane of the ecliptic, and the 

 descending node n LS the point in which the body sinks below it. The nodes 

 of a satellite's orbit are the points in which it intersects the plane of the 

 orbit of the planet. 



NOTE 56, p. 10. Distance from the sun. Sp in fig. 12. If cyD be the 

 vernal equinox, then cyo Sp is the longitude of the planet p, m Sp is its 

 latitude, and Sp its distance from the sun. When these three quantities 

 are known, the place of the planet p is determined in space. 



NOTE 57, pp. 10, 59. Elements of an orbit. Of these there are seven. 

 Let PNAw, fig. 12, be the elliptical orbit of a planet, C its centre, S the 

 sun in one of the foci, cyD the point of Aries, and E N e n the plane of the 

 ecliptic. The elements are the major axis A P ; the excentricity C S ; 

 the periodic time, that is, the time of a complete revolution of the body in 

 its orbit ; and the fourth is the longitude of the body at any given instant 

 for example, that at which it passes through the perihelion P, the point of 

 its orbit nearest to the sun. That instant is assumed as the origin of time, 

 whence all preceding and succeeding periods are estimated. These four 

 quantities are sufficient to determine the form of the orbit, and the motion 

 of the body in it. Three other elements are requisite for determining the 

 position of the orbit in space. These are, the angle CyD S P, the longitude 

 of the perihelion ; the angle A N e, which is the inclination of the orbit to 

 the plane of the ecliptic ; and, lastly, the angle cyD S N, the longitude of 

 N the ascending node. 



NOTE 58, p. 10. Whose planes, $c. The planes of the orbits, as 

 PNAn, fig. 12, in which the planets move, are inclined or make small 

 angles e N A with the plane of the ecliptic E N e n, and cut it in straight 

 lines, N S n passing through S, the centre of the sun. 



