How big objects look in the night sky is often described by their angular size. But what exactly does this mean and how is it measured?
The angular size of an object tells us how big something looks in the night sky. If you have two objects which are identical but at different distances away from you, the one which is furthest away will have a smaller angular size. Bigger objects will have a larger angular size than smaller objects at the same distance away.
Luckily most celestial objects are roughly spherical so they look like a circle in the night sky. This means 1 angular size is enough to describe the size of the object. Otherwise we would need one for the object’s width and one for its height!
You are probably familiar with angle being measured in degrees, with 360 degrees to make a full a circle. In astronomy they work in the same way, with the night sky being 180 degrees - that’s with completely clear horizons.
If we divide each degree up into 60 equal slices, each slice represents an angle that is 1 arcminute across. If we then divide an arcminute up into 60 yet smaller evenly spaced slices, those slices each represent an angle that is 1 arcsecond across.
We can measure angles by specifying the number of degrees, arcminutes, and arcseconds that they span. An arcsecond is an extremely tiny angle, it’s 1/3,600th of a degree!
Here is a handy trick that you can use to estimate the angular size of something. All you need to know is that if you hold your hand at arm's length, the distance across the end of your pinky finger spans an angle of about 1 degree.
Have a go by looking at the moon. You should gets an angular size of about 30 arc minutes.