As people gained a deeper understanding of how mathematics and astronomy could be used to determine time, they were able to create more-accurate calendars. Some of the earliest calendars, such as those employed by the Sumerians and Egyptians, were based on the phases of the moon. In Western Europe a solar calendar, the Julian Calendar, first introduced by the Romans, was in use until the late 15.00s. Under this particular system, the year is too long by 11 minutes, 14 seconds, an excess that amounts to almost 1.5 days every two centuries and seven days every one thousand years. As a result, by 1545 the Vernal equinox, which marked the first day of Spring (21 March) and was used in determining Easter, had moved ten days from its proper date.

In the sixteenth century Pope Gregory VIII decided to rectify the problem. He assembled church, civic...