a hen can either sit or set on her eggs

A Britlish hen never sets on her eggs.

Funny, I'd never associated sunset with sitting. Are you sure that's the derivation, Fal?

"The Sun rises in the East and sets in the West"

If I said the sun "sits in the West" it would mean something completely different - merely that the sun was positioned in the western part of the sky at the time I spoke.