236 



Trees, Stars, and Birds 



Two other planets were known in ancient times : 

 Mercury, which is always near the sun, most of the 



FIG. 143. Diagram showing Mars at its greatest distance and least 

 distance from the earth. 



time too near to be seen, and Saturn, which is as 

 bright as a first-magnitude star. Uranus and Neptune 

 are so far away that they never appear bright, Neptune 

 being quite invisible to the naked eye. 



In the chapters that follow, where the planets are 

 described, there is a list of dates showing when each 

 can best be seen. 



