The Sun and Solar System. 279 



part of a circle. Hence, in 365 days, the earth 

 turns 366 times round its axis; and therefore, 

 as a turn of the earth on its axis completes a 

 sidereal day, there must be one sidereal day 

 more in a year than the number of solar days, 

 be the number what it will, on the earth, or any 

 other planet, one turn being lost with respect 

 to the number of solar days in a year, by the 

 planet's going round the sun ; just as it would 

 be lost to a traveller, who, in going round the 

 earth, would lose one day by following the 

 apparent diurnal motion of the sun; and con- 

 sequently would reckon one day less at his 

 return (let him take what time he would to go 

 round the earth) than those who remained all 

 the while at the place from which he set out. So, 

 if there were two earths revolving equally on 

 their axes, and if one remained at A until the 

 other had gone round the sun from A to A 

 again, that earth which kept its place at A 

 would have its solar and sidereal days always of 

 the same length ; and so would have one solar 

 day more than the other at its return. Hence, 

 if the earth turned but once round its axis in a 

 year, and if that turn was made the same way 

 as the earth goes round the sun, there would be 

 continual day on one side of the earth, and con- 

 tinual night on the other."" 



The sun is unquestionably to us the most 

 interesting of all the heavenly bodies. The heat 

 which he diffuses animates our world, and his 



