# Question about intensity and area

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.