262 Trees, Stars, and Birds 



The orbits of comets. Comets disappear in the 

 distance, but after years of absence some of them visit 

 us again. As the same comet changes its appearance 

 so much from time to time, it is not easy to tell whether 

 a comet we see this year is the same -comet that was seen 

 years ago. The paths comets follow through the sky 

 have been carefully studied by astronomers, who in this 

 way have made out many different ones. Some have 

 been found after a few years to return along the same 

 path. Halley's comet returns only after a period of 

 about 77 years, so that the same person is not likely 

 to observe it twice. It was seen in 1681, 1759, 1835, 

 1910, and probably will appear again in 1986. An 

 interesting point about the orbits of comets is that they 

 move across the heavens in any direction, while the 

 orbits of the planets lie only in an east-and-west plane. 



The mass of comets. Although comets are large, 

 we know that there is not much substance to them, for 

 when they pass near a planet they never pull the planet 

 out of its path, but the planet pulls them quite out of 

 their course. Even the faint stars can be seen through 

 the tail of the comet and through all but the densest 

 part of the head. They are probably made up of nu- 

 merous small bodies separated by relatively wide spaces 

 that are filled with certain light gases. Probably the 

 solid bodies are meteoric stones. 



The great planets, like Jupiter and Saturn, not only 

 alter the courses of those comets which happen to be 

 passing near them but they sometimes separate the mem- 

 bers of a swarm of small bodies so widely that they no 

 longer appear as a comet. The swarm then continues 



