NOTES. 407 



Aours, at the rate of 15 degrees in an hour, time becomes a representative 

 of angular motion. Hence if the eclipse of a satellite happens at any 

 place at eight o'clock in the evening, and the Nautical Almanac shows 

 that the same phenomenon will take place at Greenwich at nine, the 

 place of observation will be in the 15 of west longitude. 



NOTE 96, p. 30. Conjunction. Let S be the sun, fig. 24, E the earth, 

 and J OJ' C' the orbit of Jupiter. Then the eclipses which happen when 

 Jupiter is in O are seen 16m 26 sooner than those which take place when 

 the planet is inC. Jupiter is in conjunction when at C and in opposition 

 when in O. 



NOTE 97, p. 30. In the diagonal, Src. Were the line A S, fig. 26, 

 100,000 times longerthan^ A B, Jupiter's true place Fig. 26. 



would be in the direction A S', the diagonal of the , 



figure A B S' S, which is, of course, out of propor- 

 tion. 



NOTE 98, p. 31. Aberration of light. The ce- 

 lestial bodies are so distant, that the rays of light 

 coming from them may be reckoned parallel. 

 Therefore, let S A, S' B, fig. 26, be two rays of light 

 coming from the sun, or a planet, to the earth 

 moving in its orbit in the direction A B. If a tele- 

 scope be held in the direction A S, the ray S A, 

 instead of going down the tube, will impinge on its 

 side, and be lost in consequence of the telescope 

 being carried with the earth in the direction A B. 

 But if the tube be held in the position A E, so that 

 A B is to A S as the velocity of the earth to the 



velocity of light, the ray will pass through S' E A. 



The star appears to be in the direction A S, when 



it really is in the direction A S', hence the angle S A S' is the angle of 



aberration.. 



NOTE 99, p. 31. Density proportional to elasticity. The more a fluid, 

 such as atmospheric air, is reduced in dimensions by pressure, the more 

 it resists the pressure. 



NOTE 100, p. 32. Oseillation of pendulum retarded. If a clock be 

 carried from the pole to the equator, its rate will be gradually diminished, 

 that is, it will go slower and slower, because the centrifugal force which 

 increases from the pole to the-equator, diminishes the force of gravity. 



NOTE 101, p. 33. Disturbing action. The disturbing force acts here 

 in the very same manner as in note 63 ; only that the disturbing body d, 

 fig. 14, is the sun, S the earth, and p the moon. 



NOTE 102. pp. 34, 36, 81. Perigee. A Greek word signifying round 

 the earth. The perigee of the lunar orbit is the point P, fig. 6, where the 

 moon i nearest to the earth. It corresponds to the perihelion of a planet. 

 Sometimes the word is used to denote the point where the sun is nearest 

 to the earth. 



NOTE 103, p. 34. Eveetion. The evection is produced by the action of 

 the radial force in the direction S p, fig. 14, which sometimes increases 

 and sometimes diminishes the earth's attraction to the moon. It produces 

 a corresponding temporary change in the eccentricity, which varies with 

 the position of the major axis of the lunar orbit in respect of the line S d, 

 joining the centers of the earth and sun. 



NOTE 104, p. 34. Variation. The lunar perturbation called the varia- 

 tion is the alternate acceleration and retardation of the moon in longitude, 

 from the action of the tangentlnl force. She is accelerated in going from 

 quadratures in Q and D, fig. 14, to the points C and O, called syzygies, 

 BJid i retarded in going from the syzygies C and O to Q and D again. 



