# How to convert arcsec to steradians

1. Dec 18, 2015

### Teenion

• Member advised to use the homework template for posts in the homework sections of PF.
If I know the angular size of the object in arcsecs lets say r, how can I convert it into steradians size? is it just that the size in steradians is r2?

2. Dec 18, 2015

### Orodruin

Staff Emeritus
You don't, not unless you know the shape of the object. An arcsecond is an angle and a steradian is a solid angle, they are different things. It is like asking how to convert a length into an area.

3. Dec 18, 2015

### Teenion

I know its spherical object (I should have mentioned it in my first post)..
Yeah, converting is not a good word for what I want to do.. Better question would be How can I obtain the size of the spherical object in steradians if I know the angular size in arcsecs.

4. Dec 18, 2015

### haruspex

Since you are working with arcsecs, I assume we're discussing very small angles. If the subtended angle (of a diameter) is theta and the distance from the observer is R, what, roughly, is the visible area? Divide that by R2 and you have the steradians.