Asked by: Stella Rodgers, Whitley Bay
There are several ways that planetary diameters can be measured. The most common is to measure the apparent angular diameter of the planet – how big it looks against the sky – very precisely using a telescope. Combining this with a measure of its distance (deduced from its orbit around the Sun) reveals the planet’s actual size.
Another method involves studying the motion of moons as they eclipse the planet. Some accurate values of diameter, for example for Venus, come from radar observations from space probes in orbit.