So these basic astronomical quantities, the star's temperature,

the radius of a star, the distance from the star to the planet

will determine how much energy hits the planets surface.

So now let's look at the energy that comes into the Earth.

The energy in is going to equal the amount of light that hits

the Earth's surface minus the amount of light that bounces back.

So we have a flux coming into the surface F and some fraction

that we call the albedo times f, bounces back.

So the energy in is one minus the flux, and a times flux,

this quantity hits every part of the Earth's surface.

And the fraction of the surface that is exposed to the sunlight

will go as the area of the Earth seen in projection towards the sun.