The Commemoration of the 400th Anniversary of the Gregorian Reform of the Calendar was not simply a commemorative occasion, but intended to serve as a stimulus for further reflection upon the calendar. What is the role of the calendar in our daily activities? How many of us pay any attention to how and why the calendar is structured the way it is? Yet the calendar forms the basis for the rhythm of our various daily activities. Do we reflect, for instance, that the duration of the hour and the length of the week have no basis in natural astronomical phenomena, even though both are of very long usage? On the other hand, the three “natural” periods are the day, the month and the year, arising from the relative motions of the earth, moon and sun; the incommensurability of these periods is the fundamental reason for the long and continuing history of calendar reform.
The Gregorian reform took place during the time when the long-standing geocentric models of the solar system were being increasingly challenged. While the practical urgencies of the reform did not require taking sides in what we might summarily call the Ptolemaic-Copernican controversy, there were some interesting aspects of the interplay between contested astronomical models and the calendar reform.