How Does the Sun Work?

Robert Grosseteste (c.1175-1235) is considered by some to be the founder of modern English intellectualism. Among other topics, he focused (pun intended) on light. One of his works seeks to explain how the sun produces heat.

He first explains the three methods of heat generation:

1. An object that is hot
2. Motion/Friction
3. The scattering of rays

He determines that Method 1 cannot apply here. For heat to transfer from a hot object, there must be a medium through which it travels, and that medium will heat up during the transfer of heat. Clearly everything between the sun and us does not heat up.

He decided that Method 2 is 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 moves from east to west.

Method 3, he decides, must pertain. He reminds his reader that Euclid explains how a concave mirror can focus the sun's rays to cause a fire. He states 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 has much to do with the denseness of the medium: he tells us 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.