ASTRONOMY 



441 



ASTRONOMY 



considerable accuracy. The great star known 

 as Castor has a companion with which it com- 

 pletes a revolution once in about a thousand 

 years; the two are of course immense distances 

 apart, yet to our naked eyes they seem as one 

 star. No doubt numbers of the binary sys- 

 tems, as these doubles are called, require an 

 even longer period to turn about. Many of 

 the stars have motions which are not yet well 

 un.lerstood, though it is noticed that some of 

 them seem to travel in companies. 



Besides the sun, the moon, the planets and 

 the stars there are three types of heavenly 

 bodies. The first type, including meteors and 

 meteorites, or falling stars, most of us have 

 seen. They make such a bright trail through 

 the skies that it is hard for some people to 

 believe that they are really very small, some- 

 times only grains of dust. Meteors move 



EVEN THE "FIXED STARS" MOVE 



(a) The "big dipper" and the direction in 

 which Its stars are moving. 



(b) As the "dipper" will appear in about 

 100,000 years. 



about the sun in streams, one of which is now 

 and then crossed by the earth, causing myriads 

 of the tiny bodies to be attracted by it. When 

 they reach our atmosphere, perhaps 100 miles 

 above us, they are heated to a temperature 

 of thousands of degrees, and only then do they 

 become visible. Comets are not so frequently 

 aeen as meteors. They are gaseous masses, 

 with long, feathery tails which always point 

 away from the sun. Some of them journey 

 about through space, coming within sight of 

 our earth, then passing on to far-off regions, 

 never again to be seen. Other comets have 

 been captured by our sun, and confine th< ir 

 travels to our own solar system, reappearing 

 at regular intervals. Halley's comet, the best 

 known of them, visits our skies every seventy- 

 five years; its last appearance was in 1909. 

 (See COMET.) Nebulae, unlike either meteors 

 or comets, do not come near our corner of the 

 mi i verse. They are spiral clouds of gas, some 

 white, some green, and are probably suns in 

 the making. There are thousands of nebulae 

 known, most of them in those parts of the 

 sky where the stars are least in number. The 

 *t nebula known may be seen with tin- 

 naked eye; it is at the central star in tin 



sword handle of the constellation Orion, de- 

 scribed below. 



Distances That Defy the Imagination. Sup- 

 pose you were to attempt to construct a dia- 

 gram of the position of the earth and the sun, 

 moon and stars in such a way as to show 

 their distances from one another and their 

 relative sizes. If you represented the earth's 

 diameter of 8,000 miles by a small pin hole, 

 how large a sheet of paper would you need 

 for your diagram? 



First of all you would mark the moon, a 

 mere pin prick, five-eighths of an inch from the 

 earth. The sun, you would represent by a 

 circle over two inches in diameter, placed 

 more than nineteen feet from the pinhole! 

 You have already exceeded the limits of a 

 sheet of paper, yet you have only started 

 your diagram. 



If you moved out-of-doors to get more room 

 for your work, you would continue by placing 

 Mercury seven and one-half feet and Venus 

 fourteen feet from the sun; both these planets 

 are smaller than the earth. Then would come 

 Jupiter, one hundred feet from the sun, but 

 only three-sixteenths of an inch in diameter. 

 One hundred eighty feet away from the sun 

 would be Saturn, not as large as Jupiter; a 

 little more than twice as far would come 

 Uranus, an eighth of an inch wide; nearly 700 

 feet from your starting point would be Nep- 

 tune, only a little larger than Uranus. 



Consider for a moment that the little pin- 

 hole indicates the earth, which seems quite 

 a large place if we travel about it, yet in a 

 space of 700 feet, representing nearly three 

 thousands of millions of miles, you have 

 marked only nine globes, most of them too 

 small to be seen. To these could only be 

 added the moons of other planets, and the 

 tiny asteroids. This gives you some idea of 

 the immensity of space, and yet you have only 

 dealt with a half dozen of the many, many 

 stars that are seen in the heavens. 



Beyond the planets the nearest star to our 

 sun is one which may not be seen north of 

 Cuba; after Neptune it is the closest heavenly 

 body which any one on earth can see. Nep- 

 tune was marked by 700 feet on your chart; 

 this star, called Alpha Centauris, will be 100 

 miles away from the pinhole earth! Its true 

 distance is about twenty-five millions of mil- 

 lions of miles. At this enormous distance you 

 are just beginning to peep into space. Sirius, 

 i Tinniest star in the sky, is twice as far 

 M> Alpha Centauris; the north star is prob- 



