Henrietta Leavitt found the ruler for measuring the universe by looking at stars whose distance didn’t matter. In 1912 she was cataloguing variable stars in the Small Magellanic Cloud, and because the whole cloud is so far away, every star in it is, near enough, the same distance from us. So she could compare their brightness straight, and saw that the slower a Cepheid pulses, the brighter it truly is. Distance was the one thing nobody knew how to measure, and she found the rule in the one place where it cancelled out.