no THE UNIVERSE A VAST SYSTEM OF PARTS 11. Describe some of the unusual ajiKJitions of theoretical life on the moon. 12. Explain the phases of the moon. 13. How do the inner planets differ from the outer planets? 14. How is it known that Mercury has no atmosphere? 15. Which is the most distant planet? 16. E.xplain why Lowell believed in the existence of a ninth planet even before it had been discovered. 17. What causes eclipses? 18. What is the average duration of a total eclipse? 19. If I^luto and Jupiter moved with the same velocity, which would have the greater momentum, and why? 20. Would an object fall more rapidly toward the earth or toward the sun? W'hy? 21. Why does the moon appear to be larger than, or nearly the same size as, the sun? 22. How does a star differ from a planet? 23. Why can Mercury be observed only at twilight or at dawn?
UNIT II SECTION 8 COMETS AND METEORS ARE OTHER INTERESTING MEMBERS OF OUR SOLAR SYSTEM Introduction. Meteors, the so-called shooting stars, which may be seen almost any clear night, and comets, which occasionally are so large that they can be seen with the naked eye, are of great popular interest. Some of the comets and swarms of meteors travel in orbits around the sun and are, therefore, members of the solar system. There Are Many Comets of Varying Size. Several hundred comets have been observed. Most of these comets are so small that they are visible only with the aid of a telescope, but occasionally huge comets have appeared whose appearance was regarded as an omen of evil and disaster before their nature was understood. Halley's comet makes its journey around the sun once every seventyfive years, and its appearances have been recorded for some thousands of years. Comets show a much greater range in size than do the planets. The head of the great comet of 1811 was about 1,125,000 miles in diameter, larger even than the sun. Many comets have heads about the size of Jupiter, i.e., 80,000 miles in diameter. Halley's comet is 357,000 miles in diameter. Ill Fig. 29. Morehouse's comet, November 16, 1908. (Photograph from the Yerkes Observatory, reprinted by permission of the Chicago University Press.)