The Romance of the Heavens 11 



of years to traverse, we reach the stars or rather, a star, for the 

 distances between stars are as great as the distance between the 

 nearest of them and our Sun. The Earth, the planet on which 

 we live, is a mighty globe bounded by a crust of rock many miles 

 in thickness ; the great volumes of water which we call our oceans 

 lie in the deeper hollows of the crust. Above the surface an 

 ocean of invisible gas, the atmosphere, rises to a height of 

 about three hundred miles, getting thinner and thinner as it 

 ascends. 



Except when the winds rise to a high speed, we seem to live 

 in a very tranquil world. At night, when the glare of the sun 

 passes out of our atmosphere, the stars and planets seem .to 

 move across the heavens with a stately and solemn slowness. It 

 was one of the first discoveries of modern astronomy that this 

 movement is only apparent. The apparent creeping of the stars 

 across the heavens at night is accounted for by the fact that the 

 earth turns upon its axis once in every twenty-four hours. When 

 we remember the size of the earth we see that this implies a 

 prodigious speed. 



In addition to this the earth revolves round the sun at a speed 

 of more than a thousand miles a minute. Its path round the sun, 

 year in year out, measures about 580,000,000 miles. The earth 

 is held closely to this path by the gravitational pull of the sun, 

 which has a mass 333,432 times that of the earth. If at any 

 moment the sun ceased to exert this pull the earth would instantly 

 fly off into space straight in the direction in which it was moving 

 at the time, that is to say, at a tangent. This tendency to fly off 

 at a tangent is continuous. It is the balance between it and the 

 sun's pull which keeps the earth to her almost circular orbit. 

 In the same way the seven other planets are held to their 

 orbits. 



Circling round the earth, in the same way as the earth 

 circles round the sun, is our moon. Sometimes the moon passes 

 directly between us and the sun, and cuts off the light from us. 



