NOTES. 



485 



to advance at the rate of 0"'31 annually ; but, as this motion is much less than 

 the former, the equinoctial points recede on the plane of the ecliptic at the 

 rate of 50"'l annually. This motion is called the precession of the equinoxes. 



NOTE 144, pp. 75, 91. Let 

 q f* Q, e <Y> E, fig. 36, be the 

 planes of the equinoctial or ce- 

 lestial equator and ecliptic, and 

 p, P, their poles. Then suppose 

 p, the pole of the equator, to re- 

 volve with a tremulous or wavy 

 motion in the little ellipse p c d b 

 in about 19 years, both motions 

 being very small, while the point 

 a is carried round in the circle 

 a A B in 25,868 years. The tre- 

 mulous motion may represent the 

 half-yearly variation, the motion 

 in the ellipse gives an idea of the 

 nutation discovered by Bradley, 

 and the motion in the circle a A B 

 arises from the precession of the 

 equinoxes. The greater axis pdof 

 the small ellipse is 18" -5, its minor axis b c is 13"74. These motions are so 

 small, that they have very little effect on the parallelism of the axis of the 

 earth's rotation during its revolution round the sun, as represented in fig. 20. 

 As the stars are fixed, this real motion in the pole of the earth must cause an 

 apparent change in their places. 



NOTE 145, p. 94. Let N be the pole, fig. 11, e E the ecliptic, and Q q the 

 equator. Then, N n m S being a meridian, and at right angles to the equator, 

 the arc ^ mis less than the arc fy 1 



NOTE 146, p. 96. Heliacal rising of Sinus. When the star appears in the 

 morning, in the horizon, a little before the rising of the sun. 



NOTE, 147, P- 98. Let P ^ A =Q=, fig. 35, be the apparent orbit or path of 

 the sun, the earth being in E. Its major axis, A P, is at present situate as in 

 the figure, where the solar perigee P is between the solstice of winter and the 

 equinox of spring. So that the time of the sun 's passage through the arc <Y> A 

 .n- is greater than the time he takes to go through the arc := P fy 1 . The major 

 axis A P coincided with .n. ^Y*, the line of the equinoxes, 4000 years before the 

 Christian era; at that time P was in the point Y>. In 6468 of the Christian era, 

 the perigee P will coincide with -n-. in 1234 A.D. the major axis was perpen- 

 dicular to <Y> iCt, and then P was in the winter solstice. 



NOTE 148, p. 99. At the solstices, $c. Since the declination of a celestial 

 object is its angular distance from the equinoctial, the declination of the sun at 

 the solstice is equal to the arc Q e, fig. 11, which measures the obliquity of the 

 ecliptic, or angular distance of the plane <Y> e JT from the plane <\"> Q ^. 



NOTE 149, p. 99. Zenith distance is the angular distance of a celestial object 

 from the point immediately over the head of an observer. 



