92 GREAT MEN OF SCIENCE 



the common centre of gravity of earth and moon. Further- 

 more, the effect of the sun has also to be taken into account, 

 which Newton also did. But he goes further, inasmuch 

 as he did not merely give a correct and complete qualitative 

 explanation of the tides, but carried out calculations which 

 enabled him to find from the known heights of the tide the 

 actual relationship between the mass of the moon and that 

 of the earth. 



It was thus possible not merely to ascribe masses to the 

 heavenly bodies according to a well-defined concept, which 

 had never previously been imagined, but even to determine 

 the magnitude of these masses. The calculation of the mass 

 always takes place according to the law of gravitation; it is 

 only possible when the body in question produced sensible 

 motion in another body, as when the moon acts on the earth's 

 water. The earth, Jupiter, and Saturn, move their moons; 

 thus Newton was able to calculate the mass of these planets. 

 The sun moves all the planets, and he is thus also able to 

 deduce its mass. Knowing the mass of all these heavenly 

 bodies, sun, planets, and moon, he is accordingly able, since 

 their diameters are known, to calculate the specific gravity 

 of the material of which they are made, in which calculation 

 the earth always serves as the standard of comparison. He 

 finds for example that the moon consists of specifically 

 heavier material, Jupiter and the sun of lighter material, 

 than the earth. By then estimating the mean specific 

 gravity of the earth quite correctly as between 5 and 6,^ 

 he is able to give the specific gravity of the other heavenly 

 bodies in ordinary measure. He was even able to state the 

 strength of gravity on the surface of the moon, Jupiter and 

 the sun. What a difference as compared with past 

 times in our knowledge of the unapproachable heavenly 

 bodies! Through Newton we almost feel at home upon 

 them. 



1 Principia, lib. 3, prop. 10. 



