[DailyMedieval is on semi-hiatus for the holidays, and I am re-cycling some older posts. For Christmas I received A Short History of Nearly Everything by Bill Bryson. I have just read the section on the brilliant Lord Kelvin, who estimated the age of the Earth at as high as 400 million years. Interestingly, he kept revising his estimate, from 400 million to 100 million to 50 million and, finally, to 24 million. The difficulty in adhering to his longer estimates, for him and others in the burgeoning field of geology, was not that they could not imagine the Earth being older, but that for the Earth to be that old, the Sun would have to be around—and their best estimates of how the Sun worked could not imagine the nuclear forces that would allow it to produce heat continuously for hundreds of millions of years. This reminded me of the post for 10 June 2012.]
How Does the Sun Work?
He first explained the three methods of heat generation:
- An object that is hot
- The scattering of rays
He decided that Method 2 was also insufficient to explain the heat, because the motion that creates heat is caused by two substances moving in opposite directions—for instance, rubbing your hands together to warm them up—and the sun's circular motion does not act upon a second substance moving in an opposite direction: everything up there moved from east to west.
Method 3, he decided, must pertain. He reminds his reader that Euclid explained how a concave mirror can focus the sun's rays to cause a fire. He stated that the sun's rays falling upon the earth are scattered, but reflection by a mirror or refraction by a (clear) spherical body can change the direction of the rays, focusing them via the medium of the dense air and generating heat. For him, this had much to do with the denseness of the medium: he stated that the same amount of light falls on a mountaintop and scattering can be observed there, but the thinness of the medium of air disallows the generation of heat.