The Horse-Power of the Sun

by Sir William Thomson

The Manufacturer and Builder, Volume 19, Issue 9, September 1887


From human history we know that for several thousand years the sun has been giving heat and light to the earth as at present; possibly with some considerable fluctuations, and possibly with some not very small progressive variation. The records of agriculture, and the natural history of plants and animals within the time of human history, abound with evidence that there has been no exceedingly great change in the intensity of the sun’s heat and light within the last three thousand years; but for all that there may have been variations of quite as much as 5 or 10 per cent, as we may judge from considering that the intensity of the solar radiation to the earth is six and a half per cent greater in January than in July; and neither at the equator nor in the northern or southern hemispheres has this difference been discovered by experience or general observation of any kind. But as for the mere age of the sun, irrespective of the question of uniformity, we have proof of something vastly more than three thousand years in geological history, with its irrefragible evidence of continunity of life on the earth in time past for tens of thousands, and probably millions of years.

Here, then, we have a splendid subject for contemplation and research in natural philosophy, or physics, the science of dead matter. The sun, a mere piece of matter of the moderate dimensions which e know it to have, bound all around by cold ether, has been doing work at the rate of four hundred and seventy-six thousand million million million horse-power for three thousand years and at possibly more, and certainly not much less than that for a few million years. How is this to be explained? Natural philosophy can not evade the question, and no physicist who is not engaged in trying to answer it can have any other justification that that his whole working time is occupied with work on some other subject or subjects of his province by which he has more hope of being able to advance science.

—Sir William Thomson.