i8o ELEMENTS OF SCIENCE 



Every planet, moreover, not only describes an elliptical 

 orbit, but one whereof the sun's centre is one of the 

 foci. These laws were discovered by Kepler, who also 

 found out that the time occupied by a planet in 

 revolving round its orbit is proportional to the square 

 root of the cube of the mean (i.e., average) diameter of 

 its orbit. The times (in days) which the different planets 

 take so to revolve are : Mercury, 88 ; Venus, 225 ; the 

 Earth, 365; Mars, 687 ; Jupiter, 4333; Saturn, 10,759; 

 Uranus, 30,687; Neptune, 60,181. 



As to the relative size of the sun and planets, if the 

 Earth be represented by a pea, Venus will also be 

 so represented ; Mercury by a grain of mustard seed ; 

 Mars by a large pin's head, Jupiter by an orange, 

 Uranus by a cherry, while the Sun would need a sphere 

 4 feet in diameter to represent it. As to degrees of 

 density, Mercury is about twice as dense as the Earth, 

 which itself is about five and a half times as dense as 

 water. Jupiter, on the other hand, is but a quarter of 

 the density of our globe. 



As might be expected, though some of the planets 

 differ greatly from the earth in density and other 

 physical conditions, there is a substantial resemblance 

 between them which is sometimes carried very far. 

 Thus our nearest neighbour after the moon the planet 

 Mars, appears to be so like our earth as to have not only 

 its tracts of land and water but also caps of polar ice. 



The planets of the solar system, with their satellites, 

 move round the sun in the same direction, with the 

 exception of the satellites of Uranus and the solitary 

 attendant on Neptune. These move in a retrograde 

 direction, and the former are also very exceptional in 

 that their orbits are nearly perpendicular to the plane 

 of the earth's orbit. 



