124 THE UNIVERSE A VAST SYSTEM OF PARTS Distances east or west of Greenwich are determined in degrees of longitude, while those north and south of the equator are estimated in degrees of latitude. To determine longitude, it is necessary to know but two things: first, the local mean time of the place whose position is to be determined; and second, the solar time of the standard meridian, which is usually taken as Greenwich, England, time. The ship's chronometer gives accurate Greenwich time, which may now be obtained also by radio. The sextant is used to measure the apparent solar time and also to measure the latitude. STUDY QUESTIONS 1. What are the four times used today? 2. What is sidereal time? 3. How does sidereal time compare with the mean solar time? 4. Why is the international date line necessary? 5. What is standard time? 6. How many standard time belts are there in the United States? Name them. 7. Is the time set forward or backward as one moves westward? Why? 8. Describe an experiment to prove that the earth rotates. 9. Explain the necessity for and the meaning of leap years. 10. What kind of time is measured by ordinary electric clocks? 11. Why is there a need for calendar reform? 12. Tell how July and August received their names. 13. What is a chronometer, and of what value is it to navigation? 14. Differentiate between apparent sun time and mean sun time. 15. Why did standard time become a necessity with the advent of the machine age? 16. How does our calendar differ from that set up by Julius Caesar? 17. When was the method of reckoning Easter decided upon? 18. What type of time is obtained with a sundial?
UNIT III CONTINUOUS CHANGES IN THE EARTHS SUR- FACE HAVE BROUGHT ABOUT CONDITIONS WHICH MAKE POSSIBLE THE LIFE OF MOD- ERN MAN INTRODUCTION TO UNIT III In speaking of the earth, geologists refer to the outer solid portion as the lithosphere, the oceans, etc. as the hydrosphere, and the gaseous envelope which surrounds the earth as the atmosphere. Inasmuch as the earth rotates daily in the path of the sun's radiant energy, it is heated unequally from day to night. This unequal heating of the earth produces changes in the densities of the hydrosphere and atmosphere, which, under the influence of gravity, result in local circulations in these spheres. A more important cause of circulation in the atmosphere and hydrosphere is the difference between the radiation received at the poles and that received at the equator. The rotation of the earth affects the direction of the above circulation. The radiant energy of the sun also brings about the evaporation of water. The water vapor so formed is transported by air currents to colder places, where it condenses as rain. Thus the energy of the sun brings about weather changes, winds, and rains, which constitute the physical bases for the changes in the earth's surface known as gradation. Gradation is the geological activity which tends to level the surface of the lithosphere; it consists of two processes: degradation, or wearingdown of the higher places, and aggradation, or building-up of the lower places. It seems probable that the earth underwent readjustments in volume, due to rearrangements of the rock materials under the stress of pressure, and that the surface therefore was thrown into wrinkles, thus forming mountain ranges. These mountain ranges were in turn torn down by various agents of erosion with a redistribution of pressure on the surface that resulted in upheavals of new mountain ranges. Some people have attributed this wrinkling process to the gradual 125