406 



NOTES. 



NOTE 92, p. 29. Synodic motion of a satellite. Its motion during the 

 interval between two of its consecutive eclipses. 



NOTE 93, p. 29. Opposition. A body is said \n be in opposition when 

 its longitude differs from that of the sun by 18(P. If S, fig. 24, be the 



Fig. 24. 



sun, and E the earth, then Jupiter is in opposition when at O, and in 

 conjunction when at C. In these positions the three bodies are in the 

 same straight line. 



NOTE 94, p. 29. Eclipses of the 

 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 b, 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 satellite, in moving 

 through the arc a b, will vanish close 

 to the disc of the planet, and will re- 

 appear 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, 42. Meridian. A 

 terrestrial meridian is a line passing 

 round the earth and through both 

 poles. In every part of it noon hap- 

 pens at the same instant. In figures 

 1 and 3, the lines N Q S and N G S 

 are meridians, C being the center of 

 the earth, and N S its axis of rotation. 

 The meridian passing through the 

 Observatory at Greenwich is assumed , 

 by the British as a fixed origin from / 

 whence terrestrial longitudes are mea- i1 ,' 

 eured. And as each point on the sur- 

 face of the earth passes through 300, 

 or a complete circle in twenty-four 



