Article October 2, 1902 Issue

The Sun's Heat

Henry Norris Russell

Scientific American

As the sun moves southward, and the shortening days bring the coolness of autumn, it seems appropriate to inquire: What is the nature of this immense supply of heat? By actual measurement (allowing for the heat absorbed by Our atmosphere) it is found that the heat received by the earth under a vertical sun is enough, if all of it could be utilized, to run a one horse power engine for every four square feet of exposed surface. On account, however, of the absorption of heat by the air and the great mechanical losses in any form of heat engine, not ten per cent of this power can be put to practical use. Nevertheless the energy is there, though we cannot harness it.

Now the cross-section of the earth, as seen from the sun, is about twelve hundred million million square feet. So the rate at which the sun expends energy in warming the earth amounts to about three hundred million million horse power.

But this is not all. If there were a number more of planets, each as large as the earth and at the same distance from the sun, they would form a close-packed spherical shell inclosing the sun completely, and intercepting all its radiation. This sphere would be 186,000,000 miles in diameter and would contain over 2,000,000,000 planets. The total radiation of the sun must then be 2,000,000,000 times what the earth receives. The corresponding number of horse power is about 600,000,000,000,000,000,000,000—a number quite beyond our power to grasp. It amounts to over 10,000 horse power for every square foot of the sun's whole surface.

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