446 



PHYSICAL SCIENCES. 



Fit*. -25. 



satellites. Let S, fig. 25, be the sun, J Jupiter, and a B b his shadow. 



Let the earth be moving in its orbit, in the direction EARTH, and the 

 third satellite in the direction abmn. 

 When the earth is at E, the satellite, in 

 moving through the arc a 6, will vanish 

 at a, and reappear at b, on the same side 

 of Jupiter. If the earth be in R, Jupiter 

 will be in opposition ; and then the satel- 

 lite, in moving through the arc a 6, will 

 vanish close to the disc of the planet, 

 and will reappear on the other side of it. 

 But, if the satellite be moving through 

 the arc m n, it will appear to pass over 

 the disc, and eclipse the planet. 



NOTE 95, pp. 30, 43. Meridian. A 

 terrestrial meridian is a line passing 

 round the earth and through both poles. 

 In every part of it noon happens at the 

 same instant. In figures 1 and 3, the 

 lines N Q S and N G S are meridians, C 

 being the centre of the earth, and N S 

 its axis of rotation. The meridian pass- 

 ing through the Observatory at Green- 

 wich is assumed by the British as a fixed 

 origin from whence terrestrial longi- 

 tudes are measured. And as each point 

 on the surface of the earth passes through 

 360, or a complete circle, in twenty- 

 four hours, at the rate of 15 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. 31. Conjunction. Let S be the sun, fig. 24, E the earth, 

 Fig. 26. and J J ' C ' the orbit of Jupiter. Then the eclipses 



which happen when Jupiter is in are seen 16 m 26* 

 sooner than those which take place when the planet is 

 in C. Jupiter is in conjunction when at C, and in 

 opposition when in 0. 



NOTE 97, p. 31. In the diagonal, 8,-c. Were the 

 line A S, fig. 26, 100,000 times longer than A B, 

 Jupiter's true place would be in the direction A S', 

 the diagonal of the figure A B S' S, which is, of 

 course, out of proportion. 



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

 tial bodies are so distant that the rays of light 

 coming from them may be reckoned parallel. There- 

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



