Distances in space were largely a mystery at the start of the 20th century.
The distances of some stars had been measured using a method called 'parallax'.
Finding distances of objects far out in space needed a new discovery.
The person who made that discovery was Henrietta Leavitt.
She discovered that certain types of variable stars changed brightness in a regular way.
These pulsating stars are known as Cepheid variables, named after the star Delta Cephei.
Henrietta Leavitt looked at large numbers of these stars in a feature called the Small Magellanic Cloud, and discovered that brighter Cepheids pulsed more slowly than dimmer ones.
So astronomers could look at Cepheids and know their true brightness by measuring the time of their pulsations.
Comparing the true brightness with how bright they appear in the sky made it possible to work out the distances of the Cepheids.
Henrietta Leavitt, of course, did not have computers to do all her calculations.
In fact, her team were referred to as 'computers'!
Originally, the word applied to a person who did calculations.