6 Astronomy. [Lecture 23. 



In the second case (for the opposite reason) the 

 day will be in some degree shorter. 



dly, The earth's motion on its own axis is 

 always equal and regular ; and if the plane of the 

 ecliptic were parallel to the equator, there would 

 be no difference in the time marked by either of 

 these circles, for fifteen degrees of each of these 

 circles passing over any meridian would be equi- 

 valent to an hour in time. But, from the in- 

 clination of the earth's axis, as already described, 

 the ecliptic is oblique to the equator, and conse- 

 quently the earth's rotation on its axis carries 

 unequal portions of the ecliptic over the me- 

 ridian in equal times, and the difference is pro- 

 portional to the obliquity. If a sun or star were, 

 therefore, supposed to move in the circle of the 

 equator, it would always return to the same me- 

 ridian in twenty-four hours, as measured by the 

 clock ; but the sun, which moves in the ecliptic, 

 will sometimes return sooner, and sometimes later. 

 It is therefore only on four days in the year that 

 these two luminaries would come both together 

 to the same meridian, and on these days only the 

 dial and the clock will be found exactly to agree. 

 A diagram will perhaps serve better to explain 

 what I have now said. Let PI. I. fig. 2, repre- 

 sent a part of the concave sphere of the heavens. 

 Let DE be a part of the celestial equator, FG 

 a part of the ecliptic, A the intersection of the 

 two Circles at the vernal equinox, AB a degree 



