Before Einstein, special relativity, and E = mc2, what was the prevailing theory on the Sun's seemingly eternal energy?

Known as Kelvin-Helmholtz clouds, they form when a faster stream of air moves above rising air below. According to the Met Office website, Kelvin-Helmholtz clouds are "an extremely rare phenomenon".

But few are fortunate enough to see Kelvin-Helmholtz clouds, which look like a pattern of ocean waves in the sky. Kelvin-Helmholtz clouds, also known as fluctus clouds, is a rare phenomenon that's ...