ASTRONOMY 



443 



ASTRONOMY 



otherwise be. No other lens is needed than 

 that of the telescope. But there are other 

 difficulties. The movement of the stars about 

 the North Star, caused by the rotation* of the 

 earth, is of course magnified in the telescope, 

 and to prevent a photograph from becoming 

 blurred it is 'necessary to attach clockwork 

 which will keep the telescope pointed in the 

 proper direction. For very delicate work even 

 this machinery is not sufficient. Many photo- 

 graphs require several hours exposure, and all 

 this time an observer must sit with his eye to 

 a parallel telescope, making whatever minute 

 adjustments are necessary. At the end of his 

 instrument is a cross made of spider's web, 

 which is much finer and smoother than hair; 

 at the start some bright star is focused exactly 

 at the center of the cross, and if it appears at 

 any time to move the slightest distance in any 

 direction the observer brings it back by an 

 adjustment. 



The tasks assumed by modern astronomers 

 are various. Some devote themselves to spe- 

 cial researches on the sun, the moon or one of 

 the planets. Others are mapping specified sec- 

 tions of the heavens, to facilitate the discovery 

 of new stars. Many more are busy with the 

 distant stars or the nebulae, determining their 

 velocity, their distance or their composition. 

 Often what appears to the unaided eye to be 

 a single star is shown by the telescope to be 

 two or more, sometimes one behind the other 

 and having no mutual connection, at other 

 times forming a binary system. Again, what 

 appear through even the most powerful tele- 

 scopes to be single stars are proved by the 

 spectroscope to be members of binary sys- 

 tems. Much of the study of nebulae and of 

 dark stars, of which one astronomer estimates 

 there are four for every bright one, is part 

 of the attempt to discover the origin of worlds, 

 including our own. It takes so many ages for a 

 world to be formed that of course no series of 

 observations can be made of any one case, but 

 by comparing stars of all sorts, from "babies" 

 to "old men/' astronomers will no doubt m 

 tin 10 reach correct conclusions. 



The Stars and Their Names. To gain tin- 

 greatest pleasure from the stars we should 

 make them our friends. We cannot hope to 

 know them all. for there are many millions, 

 but we can. easily learn to recognize many. 

 If the stars were arranged in regular order like 

 those on many national flags it would be quite 

 a task to distinguish tin -m. hut fortunately they 

 are not. Instead, they form many irregular 



groups, which we call constellations. From 

 very early days men have given names to the 

 constellations, mostly because of some real 

 or fancied resemblance to animals or to char- 

 acters in mythology. 



The best way to become acquainted with the 

 constellations and their principal stars is to 

 compare them with the double-page maps ac- 

 companying this article, which show the best 

 known figures in the heavens of the northern 

 hemisphere at each of the four seasons, between 

 eight and nine in the evening. In using one 

 of the maps, hold it over your head, so that 

 east, west, north and south will be in their 

 proper positions. 



The Heavens in Spring. Suppose you are 

 looking at the heavens in spring. A little to 

 the north of the zenith (the point directly over 

 your head) is one group of stars which nearly 

 everyone knows the Big Dipper. On the map 

 it forms the tail and part of the body of the 

 Great Bear. From the Dipper it is always easy 

 to find the North Star, or Pole Star, for it is 

 almost directly in line with the stars Dubhe 

 and Merak, which form the front of the bowl. 

 The Pole Star is the tip of the Little Bear's 

 tail, or as others have it, the end of the handle 

 of the Little Dipper. East of the Little Dipper 

 is Draco, the Dragon, whose head is marked 

 by four stars, and whose tail winds around 

 nearly to the Big Dipper. The star between 

 Mizar of the Big Dipper and the outer star 

 of the bowl of the Little Dipper was our North 

 Star 4,000 years ago, but the constant change 

 of the angle of the earth's inclination to the 

 plane of the ecliptic (see ECLIPTIC; PRECESSION 

 OF THE EQUINOXES) has brought the earth's 

 axis gradually to its present position. After 

 12,000 years the polar star will be Vega, the 

 bright star northeast of the Dragon's head. 



Vega, it will be noticed, is much brighter 

 than the stars about it. It is called a first 

 magnitude star, for astronomers have a way 

 of classifying stars according to the amount of 

 light they give out. There are but twenty of 

 the first magnitude, and only three of them 

 is, Canopus and Alpha Centauris are 

 ) milliter than Vega. Next to Vega in bright- 

 ness is Capella, west of the Pole Star, and tin- 

 next again is Arcturus, which may be found 

 by extending the curve on the Big Dipper's 

 lie towards the east and south. Arcturus 

 is one of the largest known suns; its diameter 

 is many millions of miles. Two other first 

 magnitude stars will be found in the heavens 

 at this time, Spica in the constellation of the 



