Happy Birthday, Christopher
Christopher Clavius was born today in 1538 or maybe 1537, depending on when you count the year beginning, which was not Jan 1 back then. That's not the only thing we don't know for sure about this astronomer and mathematician - his surname may have been Klau, Clau, or even Schlüssel.
But he gave the world the Gregorian calendar - named after the pope who used it, not the man who created it. So goes the world, doesn't it?