# AP Biology

posted by
**Summer**
.

How many joules of sunlight hit Earth per day? I have looked everywhere and can't find an answer!!

The sun emits about 4*10^26 Watt. The flux of solar energy at the Earth is thus about:

F = 4*10^26 Watt/(4 pi R^2)

where R = 149.6*10^9 meters is the Sun-Earth distance:

F = 1422 Watt/m^2

Multiply this by the intercepting cross section of the Earth, which is pi times the square of the radius of the Earth (and *not* 4 pi times radius squared) to obtain the amount of sunlight in joules that hits the Earth per second.

1422 Watt/m^2 * pi (6378*10^3 m)^2 =

1.82*10^17 Joules/second

There are 24*3600 seconds in a day, so about 1.6*10^22 Joules of sunlight hit the Earth per day.