C:H. xii. 



KEPLER'S < THREE LAWS: 



99 



FIG. 10. 



\ 



of the planet A has the sun for one focus and the dot c for 

 the other, while the orbit of the planet B has the sun for one 

 focus and the dot d for the other, and this makes the two 

 orbits lie in a different di- 

 rection. Kepler's first law, 

 then, was faak planets move 

 in ellipses. 



Kepler's Second Law, 

 1609. His second law was 

 about the rate at which 

 planets move. He found 

 from Tycho's tables that 

 they all moved more 

 quickly when they were near the sun than when they were 

 far from it, and after an immense number of calculations 

 he found the following rule. If you could draw a line from 

 the sun to any planet on the first day of each month of 

 the year, you would enclose 

 a number of spaces, such 

 as a, b, <r, d, &c., in Fig. n, 

 and each of these spaces 

 would be the same size, 

 although not the same 

 shape. For instance, the 

 planet, when travelling from 

 i to 2 near the sun, would 

 go very qufckly and pass 

 over a number of miles, 



while when travelling from 6 to 7 it would go slowly and 

 pass over comparatively few miles. And yet the space / 

 will be exactly the same size as the space a, only it will 

 be long and thin instead of short and broad. Kepler's 



