and track how long it took to go from minimum to maximum brightness. Using LevittΓÇÖs ΓÇ£period- brightnessΓÇ¥ relationship, the astronomer could calculate the true brightness of the star. After compar- ing the true brightness with how bright the Cepheid appeared to be, it was easy to calculate the distance to the star, or anything near it.
Shapley was interested in finding the distance to globular clusters. Since RR Lyrae stars could be seen in globular clusters, Shapley reasoned he could use them as a distance indicator, just as Levitt had used the Cepheids. But Shapley made a small mistake. Not realizing that RR Lyraes behave slightly differently from Cepheids, he overestimated the distance to the globular clusters. This error
