184 



EARTH AND HEAVENLY BODIES 



influence of the sun's gravitation, but they differ from 

 planets in a number of ways. 



When a comet is visible to the unaided eye it has 

 a head and a tail. As it moves toward the sun the tail 



FIG. 302. ORBIT OF A COMET 



is back of the head, but as it moves away the tail is 

 directed away from the sun. Most comets are not visi- 

 ble to the naked eye and many do not have a bright 

 head or a tail but appear as faint, hazy patches in the 

 sky. Halley's comet (Fig. 303), named after the Eng- 

 lish astronomer Halley, is perhaps the most famous 

 comet of all. It is visible in the sky every seventyTsix 

 years. It appeared last in 1910 and will return in 1986. 

 Some comets appear only once to us and then dis- 

 appear, perhaps forever, because their orbits are not 

 closed like the orbits of the planets. 



Comets are the largest bodies of the solar system. 

 The head of the great comet of 1811 was 1,150,000 

 miles in diameter. The tails of some comets have ex- 

 ceeded 100,000,000 miles in length, and a comet that 

 appeared in 1843 had a tail over 200,000,000 miles long. 

 Some of the light of comets is reflected sunlight and 

 some of it comes from self-luminous material that 

 they contain. 



The matter in a comet is very thin. The tail and 

 the outer edge of the head seem to be composed of 

 very small gaseous particles and possibly some finely 

 divided dust. The center of the head is composed of 

 denser material, probably solid particles widely sepa- 

 rated from each other. One astronomer estimates that 

 in a cubic mile there is, on the average, probably not 

 more than a half ounce of solid material. 



Exercise. For centuries many people have been afraid 



of comets because they think that one might collide with 

 the earth and destroy It. Do you think these fears are 

 justified? Comment on the facts about comets which 

 are known to science. 



Meteors and meteorites. If one observes the sky on 

 a clear, moonless night he may see occasional streaks 

 of light in the sky that last a second or two and are 

 gone. These are meteors, but they are often improperly 

 called "shooting stars." Although the meteors seem to 

 dart out from among the stars, they are not stars at 



Courtesy Yerkes Observatory 



TIG. 303. HALLEY'S COMET 



all. They are bits of dust or metal, perhaps like grains 

 of sand, which belong to our solar system. They are 

 members of ,the sun's family, moving swiftly through 

 space, that come into the atmosphere of the earth. 

 Rushing through the air with great speed, the friction 

 they produce with the atmosphere heats them to white 

 heat and destroys them. 



Swarms of meteors travel about the sun. If their or- 

 bits cross the earth's orbit, a meteor shower is seen 

 each year at the time when the earth reaches the inter- 

 section of the two orbits. Perhaps the best known 

 meteor shower is the Leonids. Each year a few of 

 the Leonids are seen in November, but a great dis- 

 play takes place every thirty-three years, that being 



