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The European Space Agency’s (ESA) Hipparcos satellite made accurate measurements of the distance between Earth and 100,000 different stars, and moderately accurate measurements of the distance between Earth and 1 million other stars, from 1989 to 1993. The ESA released the data to the scientific community in 1997, and the measurements soon began affecting cosmological theories.



For example, the measurements changed the accepted distances to some globular clusters (clusters of stars outside the main disk of the Milky Way Galaxy) and led to revisions of calculations of the ages of these clusters. Before the Hipparcos data, some of these clusters appeared to be older than the universe (as predicted by Hubble’s constant), but the revised distance measurements give the clusters an age within cosmologists’ estimates of the age of the universe.


 

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