Geology 



Earth, and possibly the Sun, have been slowly evolved 

 from a parent nebula. 



Let us now ascertain the position and relative im- 

 portance of the Earth in the family of bodies which we 

 call the Solar System. 



In measuring the distances between the stars we 

 adopted the light-year as our unit, but this is obviously 

 too large for measurements within the Solar System, so 

 it is customary to use the Earth's radius as the unit in 

 this case. It is also convenient when comparing the 

 weights of the different planets to use the Earth's mass 

 as unity. 



The Solar System is made up of a great central 

 body, the Sun which is intensely hot and is about 

 330,000 times as heavy as the Earth 8 major planets, 

 of which the Earth is one, and a large number of minor 

 planets or asteroids. Several of the planets have 

 satellites like our Moon, which move round them, and 

 one, Saturn, has a remarkable system of rings, of which 

 more on a later page. 



As our present business is with the Earth, we will 

 now turn our attention to its place in the system, and to 

 its movements. The planets in order, commencing with 

 that nearest the Sun, are Mercury, Venus, Earth, Mars, 

 Jupiter, Saturn, Uranus, and Neptune. 



The mean or average distance of the Earth from the 

 Sun is roughly 93 millions of miles, but the distance is 

 different at different times of the year, and this has some 

 considerable effect upon our seasons, though it is not, of 

 course, their cause. The orbit, or path of the Earth 

 round the Sun, is not a circle but an ellipse, the Sun 

 being at one of the foci. When the Earth is at its 



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