ASTRONOMY 



442 



ASTRONOMY 



ably a little more than ten times its distance; 

 Canopus, second in brightness, is perhaps 

 eighty times as deep in space. Most of the 

 stars are still farther away, and some that you 

 see are separated from us by perhaps 1,000 

 times the distance of Alpha Centauris; that 

 is, on your chart in which the pinhole repre- 

 iie earth's 8,000 miles, they would be 

 100,000 miles from it. Beyond this, stretch- 

 ing how far no one can guess, are countless 

 stare which we cannot see even with the 

 strongest telescopes. 



It is not easy to talk in terms of billions, 

 trillions and quadrillions of miles, so astrono- 

 mers measure their distances in light years. 

 We know that light travels so fast that in one 

 second a ray of it could pass around the earth 

 about seven and a half times, or more times 

 than you can move your finger around the end 

 of a pencil in a second. Light takes a little 

 more than a second to reach us from the 

 moon, eight minutes from the sun, and over 

 four hours from Neptune. But the rays from 

 the nearest star travel four years and five 

 months, so it is said to be at a distance of 4.4 

 light years. The rays which show you the 

 location of the north star started on their way 

 about forty-four years ago, and some of the 

 stare you seem to see may have ceased to exist 

 several thousand years ago, but the rays which 

 they gave out are still active and are just 

 reaching the earth. 



How Astronomers Work. The most striking 

 thing about all this is not that there are such 

 great distances and so many suns and worlds, 

 but that man has been able to learn so much 

 about them. There are two kinds of astrono- 

 mers, the observers and the mathematicians, 

 and it is said to be seldom that one man 

 combines in himself both characters. Some- 

 times it is the mathematicians who make dis- 

 coveries, sometimes the observers. 



This is well illustrated in the romantic story 

 of the discovery of the planet Neptune. Ac- 

 cording to Newton's law of gravity, "Every 

 particle of matter in the universe attracts 

 every other particle, with a force in the direc- 

 tion of a straight line joining the two." Thus 

 the path of each planet is influenced not by the 

 sun only, but by each of the other planets 

 irs, as well. After Herschel's discovery 

 of Uranus in 1781 astronomers set to work to 

 map its course. They soon found that it did 

 not travel in the path or at the speed which 

 the location and size of the other planets and 

 the sun would dictate, though the variation 



was so slight that it might easily have been 

 due to errors in calculation. 



In 1821 a French astronomer suggested that 

 an unknown planet was influencing Uranus, and 

 in 1843 a young mathematician at Cambridge 

 University, England, commenced to work on 

 the problem. Two years later he had finished 

 his calculations showing where the unknown 

 planet must be; these he took to the Astrono- 

 mer Royal, who laid them aside and forgot 

 about them. The next year a young French- 

 man sent a similar set of calculations to the 

 Director of the Berlin Observatory; the very 

 night of their receipt an observer commenced 

 a search for the new planet and found a small 

 star which did not appear in his maps. This 

 later proved to be the new planet, which was 

 named Neptune. In the meantime the English 

 Astronomer Royal had heard of the work 

 which the Frenchman claimed to have done; 

 he got out the papers which for a year he had 

 neglected, and, following the directions con- 

 tained in them, an English observer discovered 

 the planet just a week after his Berlin col- 

 league. So it was that the great Neptune was 

 actually found by two mathematicians who 

 themselves had no apparatus to observe the 

 stars. 



Three instruments have meant more to the 

 progress of astronomy than all others. The 

 first is the telescope, which Galileo introduced 

 to the field of astronomy; the others are the 

 camera and the spectroscope, made helpers in 

 the nineteenth century. What the spectro- 

 scope does for astronomers in the way of 

 measuring distances and velocities of stars is 



Summer 



HOW DISTANCES MAY BE DETERMINED 



In summer the earth is 186,000,000 miles from 

 its winter position. By observing a star from 

 both positions astronomers learn the angle WS 

 and by trigonometry figure its distance. But 

 inasmuch as the nearest star is more than 

 140,000 times 186,000,000 miles away, there are 

 very few stars whose distance may be found 

 exactly, even with this enormous base line. 



told in the article on that subject. Sometimes 

 all three instruments are combined into one, 

 known as a spectrograph. 



When a photograph is to be made through a 

 telescope the sensitive plate is simply inserted 

 in the place where the astronomer's eye would 



