Copernican system

In the fifteenth century, the reform of European astronomy was begun by the astronomer/humanist Georg Peurbach (1423-1461) andhis student Johannes Regiomontanus (1436-1476). Their efforts (like those of their colleagues in other fields) wereconcentrated on ridding astronomical texts, especially Ptolemy's, from errors by going back to the original Greek textsand providing deeper insight into the thoughts of the original authors. With their new textbook and a guide to the Almagest , Peurbach and Regiomontanus raised the level of theoretical astronomy in Europe.

Several problems were facing astronomers at the beginning of the sixteenth century. First, the tables (by means of which topredict astronomical events such as eclipses and conjunctions) were deemed not to be sufficiently accurate. Second, Portugueseand Spanish expeditions to the Far East and America sailed out of sight of land for weeks on end, and only astronomical methodscould help them in finding their locations on the high seas. Third, the calendar, instituted by Julius Caesar in 44 BCEwas no longer accurate. The equinox, which at the time of the Council of Nicea (325 CE) had fallen on the 21st, had nowslipped to the 11th. Since the date of Easter (the celebration of the defining event in Christianity) was determined withreference to the equinox, and since most of the other religious holidays through the year were counted forward or backward fromEaster, the slippage of the calendar with regard to celestial events was a very serious problem. For the solution to all threeproblems, Europeans looked to the astronomers.