1. Jul 30, 2008

qazxsw11111

Hi everyone. Firstly, I know that intensity= Power/Area. But this question made me my mind confused.

The question shows the sun shining upon the Earth and the soln is:

Only the hemisphere facing the Sun receives energy at any moment.
= intensity × area of Earth facing the Sun

And it shows the area of the Earth facing the sun is projected to a circle (pi R square, where R is the radius of the circle)!!!!!!! But shouldnt the area be the half the area of a Earth (sphere)????

Anyone can help me? Im really confused. Thanks.

2. Jul 30, 2008

LURCH

Yes, the area is a hemisphere but, when viewed from the Sun's perspective, it looks like a circle with the poles at the top and bottom, and the Equator across the middle.

3. Jul 30, 2008

uart

Another way to think of it is that, strictly speaking, your equation "intensity= Power/Area" is ONLY correct if the area is perpendicular to the direction of the radiation.