NOTES. 



481 



NOTE 127, p. 56. A pendulum is that part of a clock which swings to and 

 fro. 



NOTE 128, p. 59. Parallax. The angle a S b, fig. 29, under which we view 

 an object a b : it therefore diminishes as the distance increases. The parallax 

 of a celestial object is the angle which the radius of the earth would be seen 

 under, if viewed from that object. Let E, fig. 32, be the centre of the earth 



Fig. 32. 



E H its radius, and m H O the horizon of an observer at H. Then H m E is 

 the parallax of a body m, the moon for example. As m rises higher and higher 

 in the heavens to the points m', m", &c., the parallax H m' E, H m" E, &c., 

 decreases. At Z, the zenith, or point immediately above the head of the ob- 

 server, it is zero; and at m, where the body is in the horizon, the angle H m E 

 is the greatest possible, and is called the horizontal parallax. It is clear that 

 with regard to celestial bodies the whole effect of parallax is in the vertical, or 

 in the direction mm' Z', and as a person at H sees m' in the direction H m' A, 

 when it really is in the direction E m' B, it makes celestial objects appear to be 

 lower than they really are. The distance of the moon from the earth has been 

 determined from her horizontal parallax. The angle E m H can be measured. 

 E H m is a right angle, and E H, the radius of the earth, is known in miles; 

 whence the distance of the moon E m is easily found. Annual parallax is the 

 angle under which the diameter of the earth's orbit would be seen, if viewed 

 from a star. 



NOTE 129, p. 59. The radii n B, n G, &c., fig. 3, are equal in any one 

 parallel of latitude, A a B G; therefore a change in the parallax observed in 

 that parallel can only arise from a change in the moon's distance from the 

 earth: and when the moon is at her mean distance, which is a constant 

 quantity equal to half the major axis of her orbit, a change in the parallax 

 observed in different latitudes, G and E, must arise from the difference in the 

 lengths of the radii n G and C E. 



NOTE 130, p. 60. When Venus is in her nodes* She must be in the line 

 N S n where her orbit P N A n cuts the plane of the ecliptic E N e n, fig. 12. 



I I 



