108 THE UNIVERSE A VAST SYSTEM OF PARTS has been estimated to be not more than ten miles thick. These rings consist of small particles. According to one hypothesis, the rings came into existence as the result of tidal forces set up in a large satellite which came too near the planet. Saturn has nine moons, the largest of which. Titan, is larger than our moon, while the smallest, Phoebe, is only about 150 miles in diameter. Phoebe was the first moon of Fig. 28. Saturn. (Photograph any planet whose direction of revofrom the Yerkes Observatory, re- ^^^-^^ ^^^ observed to be counter printed by permission ot the Chicago r i i /-. t^ to that of the planet. On December 16, 1943, Saturn will be in University Press.) the best position for observation in the northern hemisphere, showing its widest opening of the rings at a time of maximum brilliancy. Uranus Was Discovered by Accident. Sir William Ilerschel (1738-1822), his sister Caroline (1750-1848), and his son John (1792-1871) made many contributions to astronomy. Sir William was a musician of unusual ability and made his living with his music. Astronomy was his hobby and he spent all of his spare time and money making telescopes and observing the sky. On the night of March 13, 1781, Herschel discovered Uranus when he happened by chance to be observing that portion of the heavens occupied by that planet. The ancients did not know of the existence of any of the planets beyond Saturn, but modern astronomers have found three other planets, the most distant one being at a distance four times as great as the distance from the sun to Saturn. It does not seem possible that either Saturn or Uranus could support life of any kind. Our chief interest in Uranus is in connection with the discovery of Neptune in the position predicted from observed discrepancies in the motion of Uranus. The Discovery of Neptune Was a Triumph of Scientific Theory. Newton's laws enable man to compute the paths of the heavenly bodies with amazing accuracy. Uranus followed its predicted path until 1831, when its deviation began to be enough to observe. By 1841 the discrepancies were so great that one of two conclusions had to be drawn; either Newton's laws were not correct, or there was an unknown body influencing the
THE PLANETS AND THEIR MOTIONS 109 path of Uranus. Two mathematicians, J. C. Adams and U. J. J. Leverrier, working independently, calculated the position of this unknown body; and in 1846 the German astronomer, Dr. J. G. Galle, whom Leverrier asked to look for it, after a half-hour's search, discovered the planet Neptune less than a degree distant from the predicted position. This was a great triumph for Newton's laws and greatly increased man's belief that his universe is one of cause and effect. Neptune is the third largest planet and has only one satellite, which rotates in a direction counter to that of the planet itself. This moon is noteworthy in that it is larger than the planet Mars. Pluto Is the Most Remote Planet. The planet Pluto was discovered at the Lowell Observatory on January 21, 1930. Lowell, who died in 1916, and other astronomers had predicted the existence of this additional planet on the basis of exceedingly minute discrepancies in the path of Uranus that even the discovery of Neptune did not completely explain. It required a long time to discover Pluto because it is no brighter than 15,000,000 stars from which it had to be distinguished. This discovery not only added to the confidence of astronomers in Newton's laws but also was a triumph for modern methods, for it had to be searched for photographically. Photography had begun to develop in Sir William Herschel's time, but comparisons of the motion pictures of today with those taken twenty years ago are sufficient to show the amazing progress which has been made in this field in our own generation. Pluto is thought to be inferior to the earth in mass and size. It is about 3,670,000,000 miles from the sun, and it is estimated that its period of revolution around the sun is about 248 years. STUDY QUESTIONS 1. Explain how the day and year on the planet Mercury can be the same. 2. Why does Mercury have such great extremes of temperature? 3. What peculiar feature is associated with the planet Saturn? 4. In what respect was the discovery of Neptune a scientific triumph? 5. What are Planetoids or Asteroids? 6. Discuss planetary moons briefly as to size, direction of rotation, number, and speed. 7. Would it be possible to see a skyscraper on the moon with a modern telescope? 8. 9. Why is it that a high jumper could jump forty feet on the moon? Why are the mountains on the moon steep and jagged? 10. What is a possible cause of the craters on the moon?