NOTES. 



465 



NOTE 46, p. 11. The equinox. 

 Fig. 11 represents the celestial 

 sphere, and C its centre, where the 

 earth is supposed to be. q <Y> Q 

 is the equinoctial or great circle, 

 traced in the starry heavens by an 

 imaginary extension of the plane 

 of the terrestrial equator, and 

 E <Y> e s is the ecliptic, or appa- 

 rent path of the sun round the 

 earth. <Y"* - rt - the intersection of 

 these two planes, is the line of the 

 equinoxes; <Y* is the vernal equi- 

 nox, and -L. the autumnal. When 

 the sun is in these points, the 

 days and nights are equal. They 

 are distant from one another by a 

 semicircle, or two right angles. 

 The points E and e are the solstices, where the sun is at his greatest distance 

 from the equinoctial. The equinoctial is everywhere ninety degrees distant 

 from its poles N and S, which are two points diametrically opposite to one 

 another, where the axis of the earth's rotation, if prolonged, would meet the 

 heavens. The northern celestial pole N is within 1 24' of the pole star. As 

 the latitude of any place on the surface of the earth is equal to the height of 

 the pole above the horizon, it is easily determined by observation. The eclip- 

 tic E <V e ^ is also everywhere ninety degrees distant from its poles P and p. 

 The angle P C N, between the poles P and N of the equinoctial and ecliptic, 

 is equal to the angle e C Q, called the obliquity of the ecliptic. 



NOTE 47, p. 11. Longitude. The vernal equinox, <Y>, fig. 11, is the zero 

 point in the heavens whence celestial longitudes, or the angular motions of the 

 celestial bodies, are estimated from west to east, the direction in which they all 

 revolve. The vernal equinox is generally called the first point of Aries, though 

 these two points have not coincided since the early ages of astronomy, about 

 2233 years ago, on account of a motion in the equinoctial points, to be ex- 

 plained hereafter. If S <Y\ fig- 10, be the line of the equinoxes, and cp the 

 vernal equinox, the true longitude of a planet p is the angle <Y* S p, and its 

 mean longitude is the angle ^ Cm, the sun being in S. Celestial longitude 

 is the angular distance of a heavenly body from the vernal equinox ; whereas 

 terrestrial longitude is the angular distance of a place on the surface of the earth 

 from a meridian arbitrarily chosen, as that of Greenwich. 



NOTE 48, pp. 1 1 , 66. Equation of the centre. The difference between <Y> C m 

 and <Y> S p, fig. 10 ; that is, the difference between the true and mean longitudes 

 of a planet or satellite. The true and mean places only coincide in the points 

 P and A ; in every other point of the orbit, the true place is either before or 

 behind the mean place. In moving from A through the arc A Q P, the true 

 place p is behind the mean place TO ; and through the arc PDA the true place 

 is before the mean place. At its maximum, the equation of the centre measures 

 C S, the excentricity of the orbit, since it is the difference between the motion 



H H 



