Man's physical universe



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.


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


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?

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