An arcsecond is a very small angle

Angles are used a lot in astronomy. We can measure both the size of an object in the sky and mark its position by using angles.

However, sometimes the angles are very small - much less than a degree. Therefore, astronomers often use things called arcminutes and arcseconds.

• there are 360 degrees in a circle.
• an arcminute is 1/60^th of a degree.
• an arcsecond is 1/60^th of an arcminute.

Therefore, there are :

360 degrees × 60 arcminutes × 60 arcseconds = 1,296,000 arcseconds in a circle

That means that a five-pence coin would seem to be 1 arcsecond across when it was 3 kilometres away! (This is the distance to Liverpool Cathedral in the picture).