4 Astronomy. [Lecture 23. 



This is not only the case when the sun is at his 

 greatest distances from the equator, or when he 

 is in either of the tropics, but it takes plac^, 

 though in a less degree, at all his intermediate 

 distances from it, the terminator continually 

 dividing the parallels unequally, so as to compre- 

 hend a greater or a less portion of them within 

 the illuminated part of the earth, according as 

 the sun is on the same or a different side of the 

 equator from that on which the parallels lie. 

 When he is in the equator, the terminator will 

 be in the direction P A, or p a, and will manifestly 

 divide the parallels equally, so as to make day and 

 night of the same length throughout the globe. 

 This is the time of spring to the inhabitants of 

 that part of the earth who are placed on the same 

 side of the equator on which the sun is passing, 

 and autumn to those who are placed on the other 

 side of the equator from which he is retiring. 



It is easy, therefore, to see that it is the in- 

 clination of the axis of the earth to the plane of 

 the ecliptic, which occasions the change of the 

 seasons ; and the constancy of that inclination 

 their constant succession. 



The sun and moon being of all the heavenly 

 bodies the most familiar to us, and the most easy 

 of observation, have from the earliest ages fur- 

 nished us with the measure of time. Time is 

 divided into ages, years, months, weeks, days, 

 hours, minutes, and seconds : of which the month 

 originated in the motions of the moon ; the year 



