Happy birthday to German scientist Petrus Apianus, aka Peter Apian, born in Saxony on April 16, 1495. A mathematician, astronomer, and cartographer, he was a favorite of Charles V of the Holy Roman Empire. In 1540, Apian created Astronomicum Caesareum and dedicated it to his imperial benefactor. It was a sumptuous Renaissance instructive manual that explained, in part, how to use an astrolabe to calculate the altitude of the stars and planets. (Bibliotheque Nationale de France)