The Realm of Nature 



CHAP. 



place in the form of the orbit on account of the per- 

 turbation of the Earth by other planets. The eccentricity, 

 or distance of the Sun from the centre, increases to a very 

 marked degree, diminishes until the orbit becomes almost 

 a circle, and then begins to increase again. The time 

 elapsing between successive maxima of eccentricity is 

 about half a million years. The Earth moves round 

 this orbit with varying speed, moving fastest when nearest 

 the Sun (or in perihelion, ^), and slowest when most remote 

 (or in aphelion, a) ; the average velocity is about i8j miles 



FIG. 16. Ellipse, representing the Earth's orbit enormously exaggerated in 

 ellipticity and eccentricity. S, the sun ; a, aphelion ; p, perihelion. 



per second or 66,000 miles an hour. Before Newton 

 proved that the power of gravity would produce precisely 

 this effect, Kepler had discovered the nature of the motion, 

 and expressed ' it in his " Second Law " thus : The radius 

 vector, or line joining the centres of the Earth and 

 Sun, sweeps through equal areas in equal times. 

 In Fig. 16 the figure SAB is equal in area to the 

 triangle SCD ; S being the sun, SA, SB, SC, SD, being 

 successive positions of the Earth's radius vector. Hence, 

 since the radius vector sweeps through the angle SAB in 

 the same time as it takes to sweep through SCD, the Earth 

 traverses the long part of its orbit from A to B through 

 perihelion in the same time as it traverses the much shorter 

 distance from C to D through aphelion. 



no. The -Year. The period in which the Earth 



