If at a point in space, distant from the sun by the earth’s mean distance, one square foot was exposed perpendicularly to the solar rays, then the sun heat that would fall upon it in one minute would raise one pound of water 14° Fahr.
This shows that the solar energy emitted daily amounts to
700,000,000,000 × 4 π a2 foot-pounds.
§ 7. On the Daily Contraction of the Sun Necessary to Supply the Present Expenditure of Heat.
We have seen that at the radius r the energy is
16 M (a2/r).
Hence for a change dr it is
–16 M (a2/r2) dr.
At its present size, accordingly, the energy given out by a shrinkage dr is
16 M dr.