# Quick question

1. Dec 5, 2009

the unit of cosmic ray flux is expressed as flux /(m^2.sr.s.GeV), so what does sr stand for?
Thanks.

2. Dec 5, 2009

3. Dec 5, 2009

### blechman

steradian = unit of solid angle. $4\pi$ steradians in a sphere.

4. Dec 5, 2009

thanks.
suppose there is a piece of board in outer space, how to calculate its steradian?
what is the reference point?

5. Dec 5, 2009

### blechman

$$Area(S)=R^2 \int_S\sin\theta~d\theta d\phi$$

So it depends on your origin (the value of R, since the area is obviously invariant). The same goes for arc length in one less dimension.

6. Dec 5, 2009

### humanino

I voted for blechman in the 2009 PF member physics award.

7. Dec 5, 2009

### blechman

as to your flux calculation you mentioned: R is the distance from you to the source, so YOU are the reference point. Is that what you meant?

8. Dec 5, 2009

### blechman

Thanks humanino, I definitely appreciate your support.

9. Dec 6, 2009