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He is known as the discoverer of Mira's pulsations. The next discovered Mira variable was χ Cygni, discovered by Gottfried Kirch 1686. I don't think he actually surveyed Mira's. He discovered "the" Stella Mira Ceti, AKA Mira, today known also as Omicron Ceti. Said: Rursus (☻) 18:15, 16 October 2008 (UTC)
Well yes, he's somewhat important. Tycho Brahe began debunking the idea of an eternal unchangeable heavenly sphere, and Holwarda debunked the idea of constantly bright stars, steps on the way from a geocentric cosmos with perfect planetary orbits towards a dynamic cosmos where there is no center (essentially a Demokritian/Brunoic cosmos), but there were many more steps for science to take, before essentially leaving the heliocentric system (except as regards to the planetary system).
As regards to science: he is the discoverer of star variability. Said: Rursus (☻) 18:26, 16 October 2008 (UTC)