348 PHYSIOGRAPHY. [chap. 



sun seems, in fact, to change his place in the heavens every 

 day, but the circuit of changes is completed in the course 

 of a year ; so that, next midsummer, he will be again in just 

 the same place as that which he occupied last midsummer. His 

 apparent movement is, in fact, due to the movement of our 

 earth around the sun, in a direction, like that of its rotation, 

 from west to east. And, just as the time of one rotation 

 of the earth on its axis constitutes a day, so the time of one 

 revolution round the sun makes a year. This revolution is 

 completed in about 365^ days.^ 



It is in consequence of this annual motion of the earth 

 that time, told by the stars, differs from time told by the sun. 

 It was said above (p. 342), that the sidereal day is nearly 

 four minutes shorter than the solar day. The sidereal day 

 represents the period of the earth's rotation, but the solar 

 day is due, not simply to the rotation ; but to this move- 

 ment, combined with that of the earth's progressive movement 

 through space. The subject is worth examining, because 

 it offers one of the best proofs of the earth's annual move- 

 ment. Suppose that it were possible to see the sun and, at 

 the same time, a certain star, on the meridian together, at 

 noon to-day ; then, it would be found to-morrow, that the 

 star reached the same meridian nearly four minutes before the 

 sun came there. But, it is clear, that if the earth simply 

 rotated upon its axis, the star and the sun ought to reach 

 the meridian at the same time. The delay in the sun's arrival 

 is due to his apparent journey in the heavens, which is 

 opposite to that of the diurnal rotation of the stars, so that 



1 More precisely 365 days, 6 hours, 9 minutes, 1075 seconds of mean 

 solar time. As tire calendar year contains 365 days, the extra quarter 

 of a day gives an additional day every fourth year, or leap year ; and, by 

 this addition, the correction necessary to cause the seasons to fall Ln the 

 same months of every year is nearly made. 



