5278 Astronomy. [Lecture 18. 



the morning, but that point must revolve six 

 hours more from N to n, before it has mid-day by 

 the sun: for now the angle A S D is a right 

 angle, and so is N D n ; that is, the earth has 

 advanced 90 degrees in its orbit, and must turn 

 90 degrees on its axis to carry the point N from 

 the star to the sun : for the star always comes 

 to the meridian when N m is parallel to R S A ; 

 because D S is but a point in respect to R S. 

 When the earth is at E, the star comes to the 

 meridian at 4 in the morning ; at F, at 2 in the 

 morning; and at G, the earth having gone half 

 round its orbit, N points to the star R at mid- 

 night, it being then directly opposite to the sun. 

 And therefore, by the earth's diurnal motion, 

 the star comes to the meridian 12 hours before 

 the sun. When the earth is at H, the star 

 comes to the meridian at 10 in the evening ; at 

 I it comes to the meridian at 8, that is, 16 hours 

 before the sun; at K 18 hours before him; at 

 L 20 hours ; at M 22 ; and at A equally with 

 the sun again. 



" Thus it is plain, that an absolute turn of the 

 earth on its axis (which is always completed 

 when any particular meridian comes to be parallel 

 to its situation at any time of the day before) 

 never brings the same meridian round from the 

 sun to the sun again; but that the earth re- 

 quires as much more than one turn on its axis 

 to finish a natural day, as it has gone forward 

 in that time; which, at a mean state, is a 365th 



