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Volume 27, 1894
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– 558 –

Art. LXIV.—On an Oversight in Croll's Mode of lengthening the Age of the Sun's Heat.

[Read before the Philosophical Institute of Canterbury, 3rd October, 1894.]

Dr. Croll has suggested that the age of the sun's heat may be indefinitely lengthened if we assume the collision of two bodies with a high velocity. He says that if two bodies, each with a velocity of 478 miles a second, were to come into collision they would develope heat enough to last fifty million years. Doubtless his figures are correct; but the explanation is not sufficient. Such an impact would result in a nebula of the character I have described as producing a temporary star. When the two bodies collide the molecular heat-motion would be approximately equal to the velocity of the masses, and, dis-regarding “selective escape,” each particle would move so fast that on its coming to the surface every one would leave on a journey never to return; and, in fact, a calculation shows, that they would have a final velocity in space of over 300 miles a second. So that this method of accounting for the sun's heat contains a fallacy. In addition to this it has two, essential elements of improbability—namely, the high initial velocity, and the improbability of complete impact.

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Practically, then, Dr. Croll has not lengthened out the age of the sun's heat beyond that given by Sir William Thomson's calculation: his suggestion would make a temporary star, not a sun.

Still, there are many ways to account for an increase of the age of solar heat. As Proctor has pointed out, the sun may have a very dense interior: this would indefinitely lengthen out its age. And the rate of solar radiation might have been much less when the sun was larger and cooler than it is at present. It is impossible to say how far an absorbent atmosphere may retard the radiation.

I have pointed out this error because it was first stated in “Climate and Time,” and after nearly a score of years it was again printed as the essential idea contained in Croll's book on stellar evolution.