482 SCIENTIFIC RECREATIONS. 



seen, corresponds to the daily revolution of a star or stars. Solar time is 

 regulated by the sun, and this is the astronomical time generally observed, 

 except for sidereal investigations. But the sun is not always regular; the 

 orbit of the earth causes this irregularity partly. The earth moves faster in 

 winter than in summer, so the sun is sometimes a little fast and sometimes 

 a little slow. Astronomers therefore strike an average, and calculate upon a 

 MEAN SUN, or uniform timekeeper. Mean time and true (apparent) time 

 are at some periods the same viz., on the I5th of April, on the I4th of 

 June, on the 3ist of August, and on the 24th of December. Twice it is 

 after, and twice before it. The time occupied by this "mean" sun passing 



Fig. 521. Fusee and spring. 



from the meridian and its return to it, is a mean solar day, and clocks and 

 chronometers are adjusted by it. 



Twenty-four hours represents a complete revolution of the heavenly 

 bodies. The mean solar time is 23 h 56' 4*09 i", while twenty- four hours of 

 mean time are equal to 24 h 3' 56*5 5" of sidereal time. The difference 

 between the times is given by Dr. Newcomb as follows, and is called the 

 Equation of Time : 



DIFFERENCES BETWEEN MEAN AND APPARENT TIME. 



February loth . . . True Sun 15 minutes slow. 



April 1 5th . . , . Correct. 



May I4th * . . . . 4 minutes fast. 



June I4th Correct. 



July 25th . . . . 6 minutes slow. 



August 3 ist . . . . Correct. 



November 2nd . . . 16 minutes fast. 



December 24th Correct. 



MEASUREMENT OF DISTANCES. 



Before passing to consider the planetary system we must say a few words 

 respecting the manner of ascertaining the distances of inaccessible objects, 

 and by so doing, we shall arrive at an idea how the immense distances 

 between the sun (and the planets) and the earth have been so accurately 

 arrived at. To do this we must speak of parallax, a very unmeaning word 

 to the general reader. 



