332 FERGUSON'S LECTURES. 



LECT. the sun. she rises when the sun sets (or soon after, and 



Y 



^^-^ shines all the night. 



In southern latitudes, Virgo and Libra rise at as small 

 angles with the horizon, as Pisces and Aries do in the 

 northern ; and as our spring is at the time of their har- 

 vest, it is plain their harvest full moons must be in Virgo 

 and Libra / and will therefore rise with as little differ- 

 ence of time, as ours do in Pisces and Aries. 



For a fuller account of this matter, I must refer the 

 reader to my Astronomy, in which it is described at 

 large. 



PROBLEM XI. 



To explain the equation of time, or difference of time between 

 well regulated clocks and true sun-dials. 



The earth's motion on its axis being perfectly equable, 

 and thereby causing an apparent equable motion of the 

 starry heaven round the same axis, produced to the 

 poles of the heaven ; it is plain that equal portions of 

 the celestial equator pass over the meridian in equal 

 parts of time, because the axis of the world is perpen- 

 dicular to the plane of the equator. And therefore, if 

 the sun kept his annual course in the celestial equator, 

 he would always revolve from the meridian to the meri- 

 dian again in twenty-four hours exactly, as shewn by a 

 well-regulated clock. 



But as the sun moves in the ecliptic, which is oblique 

 both to the plane of the equator and axis of the world, 

 he cannot always revolve from the meridian to the 

 meridian again in 24 equal hours ; but sometimes a 

 little sooner, and at other times a little later, because 

 equal portions of the ecliptic pass over the meridian in 

 unequal parts of time, on account of its obliquity. And 

 this difference is the same in all latitudes. 



To shew this by a globe, make chalk-marks all 

 around the equator and ecliptic, at equal distances from 



