THE EARTH'S ORBIT. 21 



of their distances, measured upon the circumference of 

 the opaque body, The point of greatest illumination, 

 in that case, would not be equally distant between the 

 extremities of the axis, but nearer the continually illu- 

 minated extremity, by a portion of the circumference, 

 equal to half the angular difference already mentioned. 

 An alternation of the two last mentioned positions of 

 the earth's axis with regard to the centre of the sun, is 

 all that is necessary for producing the succession of 

 seasons, the whole varied phenomena of the year ; 

 and the contrivance by which it is brought about is the 

 most simple that can be imagined. The orbit of the 

 earth, or the path that it annually describes round the 

 sun, is an ellipse or oval, not very different from a 

 circle, but still an oval, and the sun is situated in the 

 longest diameter, in one of the foci, that is, one 

 of the centres from which the ellipse would be drawn, 

 or, as geometricians say, described. It is hardly 

 necessary to mention, that an ellipse differs from 

 a circle merely by being drawn round two centres 

 instead of one ; and that, as the one li e drawn from 

 the centre of the circle to the circumference or boun- 

 dary is always of the same length, ad is always equal 

 to half the diameter, so the two lines drawn from the 

 two centres of an ellipse to any point in the circumfe- 

 rence are always, taken together, of the same length, 

 and their sum always equal to the longest diameter. 

 Such is the form of the orbit along which the earth 

 moves every year ; and it lies in the same plane, that 

 is, if a perfectly flat surface were applied to it, it would 

 touch it in every point. 



