AN ILL USTRA TIVE DIA GRAM. 305 



hours, or the exact duration of the earth's rotation on its axis); 

 and let us continue after this mode until the sun has accom- 

 plished, by its own proper movement from west to east, the 

 whole circuit of the heavens, traversing 360 degrees in the 

 space of a year. If we ascribe to 

 the radius oa a certain length, 

 corresponding to a definite solai 

 diameter, the lengths of all the 

 others, corresponding to the va- 

 riations of the same diameter, 

 will depend upon that of the first, 

 which, for facility of calculation, 

 we suppose to be divided into 



One thousand partS. Fl(i - 68. Diagram for Kepler's Laws. 



After having thus allotted to each straight line its approxi- 

 mate length, let us join their extremities by a curve. What do 

 we see before us ? A geometrical figure widely different from 

 a circle, for the diameters (i.e., the straight lines passing through 

 the centre) are far from being equal. The figure is an ellipse. 



If now we pass from the appearance to the reality, o will 

 be the sun, and a a' a", m m' will indicate the terrestrial 

 orbit, or the points of the curve successively occupied by the 

 earth in movement. The moveable straight lines, free at one 

 extremity, and at the other attached to the centre of the sun, 

 are called the Vector heliocentric radii. By the help of this 

 construction, you see that the point occupied by the sun is 

 beyond or without the centre ; this eccentric point is fat focus 

 of the ellipse, and the distance from this focus to the centre, 



