SECT. II. INCLINATION OF DEBITS OF PLANETS. y 



slower than the circular or mean motion (N. 45). As it is ne- 

 cessary to have some fixed point in the heavens from whence to 

 estimate these motions, the vernal equinox (N. 46) at a given 

 epoch has been chosen. The equinoctial, which is a great circle 

 traced in the starry heavens by the imaginary extension of the 

 plane of the terrestrial equator, is intersected by the ecliptic, or 

 apparent path of the sun, in two points diametrically opposite to 

 one another, called the vernal and autumnal equinoxes. The 

 vernal equinox is the point through which the sun passes in 

 going from the southern to the northern hemisphere ; and the 

 autumnal, that in which he crosses from the northern to the 

 southern. The mean or circular motion of a body, estimated 

 from the vernal equinox, is its mean longitude ; and its elliptical, 

 or true motion, reckoned from that point, is its true longitude 

 (N. 47) : both being estimated from west to east, the direction 

 in which the bodies move. The difference between the two is 

 called the equation of the centre (N\ 48); which consequently 

 vanishes at the apsides (N. 49), or extremities of the major axis, 

 and is at its maximum ninety degrees (N. 50) distant from these 

 points, or in quadratures (N. 51), where it measures the excen- 

 tricity (N. 52) of the orbit ; so that the place of the planet in 

 its elliptical orbit is obtained by adding or subtracting the equa- 

 tion of the centre to or from its mean longitude. 



The orbits of the principal planets have a very small obliquity 

 or inclination (N. 53) to the plane of the ecliptic in which the 

 earth moves ; and, on that account, astronomers refer their mo- 

 tions to this plane at a given epoch as a known and fixed position. 

 The angular distance of a planet from the plane of the ecliptic is 

 its latitude (N. 54), which is south or north according as the 

 planet is south or north of that plane. When the planet is in 

 the plane of the ecliptic, its latitude is zero ; it is then said to be 

 in its nodes (N. 55). The ascending node is that point in the 

 ecliptic through which the planet passes in going from the 

 southern to the northern hemisphere. The descending node is 

 a corresponding point in the plane of the ecliptic diametrically 

 opposite to the other, through which the planet descends in going 

 from the northern to the southern hemisphere. The longitude 

 and latitude of a planet cannot be obtained by direct observation, 

 but are deduced from observations made at the surface of the 



B 3 



