Consider a planet of size R_{p} at a distance
D from the Sun. Define the **albedo (a)**
of a planet to be the fraction of incoming energy that is reflected off
the planet (albedo can be as small as 0, or as large as 1). *Question:
How does albedo relate to the amount of energy absorbed by a planet?*

With this definition, the amount of energy a planet receives from the Sun every second is given by

Now, how much energy is the planet giving off each second (not reflecting, but actually emitting)?

In thermal equilibrium, these rates are in balance:

Equating these two rates and doing some algebra, we find that

Note the dependencies, and that T_{p} ** does
not** depend on R

Let's try this for Venus. Putting in the numbers (a=0.6,
T_{sun}=5770K, R_{sun}=7x10^{5} km, D=0.72 AU)
we get **the equilibrium temperature of Venus =
260 K.**

The **surface temperature of Venus
= 740 K**. *Whoa! Where'd we mess up??*

For that matter, the equilibrium
temperature of the Earth is 255 K (or ~ -1 F). *Something's
wrong!*