THI'; Sl'NDIAL AND TIIIC CLOCK. 103 



oilier marks are so placed as to receive the shadow at the 

 hour they name. 



As the earth goes round the sun in its elliptical orbit Ihe 

 sun, not being- in the centre, so to speak, of the ellipse, but in 

 one of its foci, — there nuist be aphelion and perihelion, that 

 is, — certain times when the earth is farthest from the sun and 

 others when it is nearest the sun. 



When the attraction of the sun has pulled the earth back 

 from aphelion to perihelion it has imparted the additional 

 velocity without which the earth could not pass the dead point 

 and escape falling into the sun. Again, when l)}- means of 

 this additional velocity the earth has flown off to aphelion the 

 sun's attraction is able, and no more than able, to overcome 

 the earth's tendency to fly off into space. So the earth once 

 more passes the dead point and comes back. 



This variable speed of the earth in its orbit has a direct 

 influence upon our record of the flight of time. The coming 

 of the sun, namely, and not that of a star to the meridian is 

 used as the point of departure and there is a difference between 

 the two. Supposing that today the sun and a certain star 

 pass the meridian at the same time they will not do so tomor- 

 row. The sun will have moved to the eastward ; that is to 

 say, he will, owing to the earth's movement in orbit be pro- 

 jected upon the star vault at a point farther east than the day 

 before, reaching the meridian later. And the difference is of 

 course cumulative. But the velocity of the earth in orbit is 

 variable; tin. refore the sun suffers variable "retardation" in 

 coming to the meridian. 



The siui, then, conies to the meridian at intervals differing 

 in length ; and the dial correctly marks these intervals. 



It would be difficult, almost impossible to construct conve- 

 nient timekeepers which should follow such variations, nor 

 would it be desirable. 



Mean solar time then, as contradistinguished from appa- 

 rent time is a conventional record kept bv civilized man of the 

 revolution of the earth on its axis : and, as in the case of 



