91) THE UNIVERSE A VAST SYSTEM OF PARTS that the sun's energy is Hberated in a series of reactions between hydrogen and carbon in which the carbon is regenerated but the hydrogen is converted into heHum. The rate of this reaction corresponds closely with the actual rate of radiation observed for the sun. Eventually, however, the sun seems doomed ; and as it becomes colder, life on earth must cease to exist. Perhaps, in the future, man may discover the sun's secret and be able to produce energy by subatomic changes. For the present, however, man finds that the problems of harnessing and controlling the energy of the sun present more likely prospects. At present man utilizes the sun's energy after it has been converted into the potential energy of water stored above sea level, or into the kinetic energy of the winds, or into the chemical energy stored up in the forms of food, wood, coal, gas, and oil. A few small solar heaters and engines have been devised, by which the sun's energy is transformed directly into heat, but practical developments along this line are not yet in sight. STUDY QUESTIONS 1. How long does it take for light to travel from the sun to the earth? 2. How far is the sun from the earth? 3. Is the sun gaseous, liquid, or solid? Give reasons for your answer. 4. Discuss the temperature of the sun. 5. Compare the sun and the earth as to density. 6. To what extent does the earth's atmosphere diminish the amount of heat received by the earth from the sun? 7. Give some data to show how much energy is received from the sun by the earth. 8. What is a possible source of the sun's energ>? 9. What is thought to be the nature of the sunspots? 10. How do the sunspots influence the earth? 11. What are the indirect ways of using the sun's energy? 12. How would you construct a solar heater?
UNIT II SECTION 6 NEWTON'S LAWS ARE UNIVERSAL IN APPLICATION THEIR Introduction. It will be recalled that Kepler stated his observations of the motions of the planets in the form of three laws ; but it remained for one of the greatest scientists the world has ever produced, Sir Isaac Newton, to state the universal laws of motion which apply to all objects, small or large. Newton's Law of Universal Gravitation Is Useful in Describing and Predicting the Motion of Planets. Isaac Newton s (1642-1727) law of universal gravitation is most simply stated as follows: Every body of matter in the universe attracts every other body along the straight line that joins them, with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers. The important idea that occurred to Newton was not that objects was common knowledge — but are attracted to the earth — this fact that every object in the universe exercises an attraction for every other object in the universe. In developing his law of universal gravitation, Newton set out to see whether or not the motion of the moon corresponded with his predictions which were based on the law. He met with difficulties at once because different parts of the earth and the moon were not equally distant from the center of each. Newton finally solved this problem by using a new kind of mathematics, calculus, which he had to invent for the purpose. Calculus has been of inestimable value to modern science in solving a wide variety of problems. The Companion of Sirius. Sirius, the "dog star," is the brightest star in northern latitudes visible above the southern horizon in winter months. It is only 8.6 lightyears distant. For many years it was used as a clock star as a standard for time, but it was found unsatisfactory because of its shifting position. 91