Distance
Measuring distances in astronomy is difficult but crucial. We need to know how far away objects are to look at the differences between them. Two objects can look different but be very similar, with one further away than the other. This can make them challenging to study.
Brightness
Light is the main signal we receive from space. Almost everything we know about space has been found by measuring light.
One of the main ways to measure light is to look at the brightness. Measuring the brightness of something in space isn't as easy as you'd think though.
Angles
Astronomers use angles a lot to work out where things are in the sky. We can measure the size of an object in the sky and mark its position by using angles.
If we know how much of the sky an object covers (as an angle) and how far away it is - we can work out its actual size.
Solstick: How it works
To measure the size of the Earth, we can use the fact that shadows cast by the Sun are different lengths in different places at the same time. If the Earth were flat, all the shadows would be the same everywhere at a particular time, but because it is curved like a ball, some shadows are longer than others.
Solstick: Measuring the Earth
For a number of years around the Summer Solstice, the Schools' Observatory ran a national experiment to use shadow sticks to measure the size of the Earth.