PLANET 



4695 



PLANET 



PLAN'ET. Among the glories of the heav- 

 ens are those so-called bright stars that shine 

 with a steady light. Because of constant 

 changing of position among the stars the an- 

 cients called these bodies planets, a name 

 meaning wanderers. However, by astronomers 

 the planets are not recognized purely as stars. 

 They are bodies which, like the earth, move 

 around the sun in nearly circular paths called 

 orbits. Named in the order of their distances 

 from the sun, the planets are Mercury, Venus, 

 earth, Mars, Jupiter, Saturn, Uranus and Nep- 

 tune. Six of them, Mercury, Venus, the earth, 

 Mars, Jupiter and Saturn, were known to the 

 ancients, but they did not know that the earth 

 was a planet. Uranus was discovered by Sir 

 William Herschel in 1781, and Neptune was 

 located by mathematical calculation; two as- 



O 



Size and Distances from the Sun. The plan- 

 ets differ widely in respect to size. Mercury, 

 the smallest, is about one-eighteenth the size 

 of the earth, while it would take over 1,200 

 earths to make a planet as large as Jupiter. 

 Venus is almost as large as the earth; Mars is 

 only one-seventh the size of the earth. Saturn 

 is 770 times larger than the earth, Uranus sixty- 

 six times and Neptune sixty times larger than 

 the earth. j$ut the amount of matter in a 

 body depends upon both its volume and its 

 density, or mass, and when these factors are 

 considered we discover some very interesting 

 results. For instance, a person weighing 100 

 pounds on the earth would weigh only thirty- 

 eight pounds on Mars, but if he were to visit 

 Jupiter his weight would increase to 316 pounds. 

 On Mars he could easily jump over a ten- 



i/ 



I > 2 URANUS NEPTUNE T 



SATURN 



JUPITER 



COMPARATIVE SIZES OF THE PLANETS 



The characters following the names are the astronomical symbols of the planets. 



tronomers, Leverrier and Adams, working inde- 

 pendently, located it at the same time. 



Venus, Mars, Jupiter and Saturn are easily 

 seen with the naked eye. Mercury is seldom 

 seen because it is so near the sun. Uranus 

 may occasionally be seen under favorable at- 

 mospheric conditions, but Neptune can be seen 

 only with a good telescope. 



Planets and Stars. Before the invention of 

 flu- telescope there was no way of distinguish- 

 ing the planets from 

 the stars except by 

 brighter light. 

 Th< rrfore the ancients 

 called them stars. But 

 we now know that 

 every one can <h-nn- 

 gui.-h ;i pi.iiu-t from a 

 star with the naked 



A nfjtr silumvt 

 aiwajs 



twinkles and a plan* I > through the tele- 

 -hines with a steady 



hirhr. With a small magnifying glass (a good 

 : i glass will answer the purpose) another 

 distinction may be seen. The planet appears 

 as a full, round disk when seen through tin- 

 glass, but the star as a mere point of light 



STAR AND PLANET 



In ap- 

 (WO( M 



foot fence, but on Jupiter it would be an 

 effort for him to jump across a ditch two feet 

 wide. On Mars he could throw a ball three 

 times as far as he could at home, but when he 

 would reach Jupiter he would need a strong 

 arm to throw it a hundred feet. Since c\ 

 thing on the sun would weigh twenty-s< -\ . n 

 times what it does on the earth, a man weigh- 

 ing 150 pounds would weigh more than two 

 tons when transported to the sun. 



The distances of the planets from each other 

 increase with their distances from the sun, as 

 may be seen by making computations from the 



table cm pagr 4696. 



Classification. Humboldt classified the plan- 

 mto two groups the terrestrial and the 

 major groups. Tin- first group includes M.T- 

 cury, Venus, earth and Mars, and was given its 

 name because the earth was one of its mem- 

 bers. The group of major planets Jupiter. 

 Saturn, Uranus and Neptune was so named 

 because of their great size. But the density 

 of these large planets is much less than that 

 of the ^mailer planets, each of which has a 

 ;t\- about the same as that of tin- rarth. 

 .fu pit -r's weight proportionate to its 

 i man weighing 150 pounds on the earth 



