The Realm of Nature 



CHAP. 



correct hypothesis. The law of gravitation explains that two 

 revolving bodies circle round the centre of gravity of the 

 pair. In the case of the Earth and Moon the centre of 

 gravity of the system lies within the Earth, hence the Moon 



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FIG. 15. Problem of the Earth and Sun. Showing how observation of the Sun's 

 place amongst the stars cannot tell whether the Earth (E) goes round the 

 Sun (S), or the Sun goes round the Earth. 



appears to revolve round it. It remains to inquire where 

 the centre of gravity of the Earth and Sun lies ; in other 

 words, whether, and by how much, the Earth or the Sun is 

 the greater body. 



107. The Sun's Distance and Mass. The horizontal 

 parallax ( i oo) of the Sun is not quite 9" ; and being so 

 minute it is not easily measured accurately. Since the Sun's 

 parallax is about ^i^ of the Moon's ( 100), it follows that 

 the Sun must be about 380 times more distant from the 

 Earth than is the Moon. Accurate determinations give 

 the average distance as 92,700,000 miles. Since the Sun 

 subtends as large an angle to our eye (about 30') as the 

 Moon does, it follows that the Sun, being 380 times as 

 distant, must have a diameter 380 times as great as that of 

 the Moon, that is to say, about 800,000 miles. The Sun's 

 volume is thus more than 1,200,000 times that of the Earth. 

 By the attractive force it exerts the Sun's mass is proved to 



