photographic plates

In 1902, Henrietta Swan Leavitt was hired to work at the Harvard College Observatory to count stars on photographic plates. She discovered 2,400 new variable stars - about half the total known at the time - and revolutionised our understanding of the relative brightness and variability of stars. This gave astronomers the first reliable yardstick for measuring distances in the cosmos, allowing them to calculate the distances of stars out to 10 million light-years. 

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