v The Earth a Planet 71 



be more than 300,000 times that of the Earth. The centre 

 of gravity of the Earth-Sun System must, indeed, lie within 

 the Sun, and it is therefore as certain that the Earth goes 

 round the Sun as that a weight of 50 Ibs. will cause i grain 

 to fly up if the two are placed in the opposite scales of a 

 balance. 



1 08. Proof of Revolution. If a man, sitting in a dog- 

 cart on a dead-calm day while a steady downpour of rain 

 is falling, finds the raindrops driving against his face instead 

 of falling straight upon his hat, he concludes correctly that 

 this aberration or wandering of the raindrops from their 

 normal path is due to the fact that the dogcart is not at 

 rest but in rapid motion. By estimating the angle at which 

 the rain strikes he may even calculate the rate at which 

 he is being carried along. The astronomer, sitting in his 

 observatory, detects a similar aberration in the light -rays 

 from each of the stars. He finds the light reach him at a 

 different angle at various times of the year, so that each star 

 traces out a minute annual curve on the sky the greatest 

 radius of which is about 20". No other cause can account 

 for this aberration of the starlight except the fact that the 

 observatory and the Earth itself are rushing with tremendous 

 velocity through space in a closed curve which takes one year 

 to complete. The rate of motion can be calculated from the 

 angle of aberration, when the velocity of light is known. 



109. The Earth's Orbit. The regular change in the 

 angular diameter of the Sun seen from the Earth ( 105) 



.proves that the annual orbit is not a circle, as the two 

 bodies are sometimes nearer and at other times farther apart. 

 The form is an ellipse (Fig. 16), of which the Sun occupies 

 one focus (S) ; but the ellipse is very like a circle, the 

 ratio of the longest to the shortest diameters being as 

 100,000 to 100,014. Indeed, if a circle 3 inches diameter 

 were drawn with a very sharp pencil making a line 5 ^ 

 of an inch thick, it would represent the orbit correctly, 

 the difference between the ellipse and the circle being con- 

 cealed by the thickness of the line. The place of the Sun 

 would, however, require to be represented ^ of an inch 

 from the centre of the circle. Certain slow changes take 



