DIALS AND CLOCKS. 79 



will then decrease, and it will move slowest when it passes the 

 aphelion point B. The earth is at the point A, on the 31st of 

 December, and at the point B, six months after, or July 1st, If 

 the inequality between the time indicated by the dial and that by 

 the clock was caused wholly by this change in the velocity of the 

 sun, then the dial and clock should agre% exactly when the earth 

 was in these two positions, for the earth occupies just 6 months 

 in moving from A to B, and 6 months in .returning from B to A, 

 just what it would if its orbit was a circle, and in which case the 

 dial and clock would agree. But by actual observation, the dial 

 and clock are not together twice in the year, but four times, and 

 then not when the earth is at A and B, December 31, and July 

 1st, but on December 24th, April 15th, June 16th, and August 

 31st, as we have already intimated. We must look, therefore, 

 to another source, which, united with the one we have just con? 

 sidered, will fully explain all the observed phenomena, and we 

 find it in the inclination of the sun's apparent path to the equator, 

 As the earth turns on its axis, we may suppose a rod which ex- 

 tends from the centre of the earth, and through its equator to the 

 sky, tracing out a line, or circle, in the heavens, which is called 

 the celestial equator. This circle is, as we have already shown, 

 divided into 24 parts, called hours, each hour comprehending 15, 

 and all these spaces are exactly equal. If the sun's yearly path 

 in the heavens had corresponded with the equator, or had been in 

 the same plane, then all the difference between the dial and clock 

 would have been simply what was due to his moving sometimes 

 apparently faster than at others, in consequence of the earth's 

 elliptical orbit, but this is nofthe case, the plane of the ecliptic, or 

 sun's path, is inclined to the plane of the equator. Now, on the 

 supposition that the orbit is circular, let us see what effect this 

 would have upon the sun-dial. In the next diagram, the circle 

 0, 1, 2, 3, 4, 5, &c., which are hour divisions, represents the 

 equator, and I, II, III, IV, V, VI, &c., which are also hour di- 

 visions, the ecliptic. Clock time is measured on the former, for 

 this is the circle, or others parallel to it, in which the stars, and 

 other heavenly bodies, seem to move on account of the diurnal 

 rotation of the earth. Dial time is measured on the ecliptic, and 



