The rotation is irrelevant to the argument. I might as well imagine the Earth to be standing still in this problem.
If you didn't know that calculating fluxes required a cosine term, if you didn't know this was a flux in the first place, how would you argue for a cosine term when trying to...
I am trying to compute the amount of energy received by the Earth from the sun, by integrating over the Earth's surface. I keep reading that the formula should be:
E = e-max * cos @ * area
e-max is the solar insolation,
@ is the angle between the sun and the perpendicular to the land...