The Gregorian Calendar: Why Are The Months And Days The Way They Are?

By Karen Harris
(Biblioteca del Vaticano/Wikimedia Commons)

January 1 marks the start of a new year, at least according to the Gregorian calendar. We've been using this calendar system for more than 400 years, ever since Pope Gregory XIII set up a system to correct the errors in Julius Caesar's previous calendar system. The switch from the Julian to Gregorian was not without its hiccups, however, and even today, not all parts of the world count their days by it.

The Julian Calendar

Humanity has used numerous different systems for tracking the days, but around 40 B.C.E., an astronomer named Sosigenes collaborated with Roman Emperor Julius Caesar to develop a revolutionary new calendar. See what we did there? Because years are measured by revolutions around the Sun? Get it?

Anyway, Sosigenes and Caesar divided the year, which Sosigenes had calculated at 365.25 days, into 12 months, but they were off by 11 minutes and 14 seconds per year. By the 1500s, those 11 minutes and 14 seconds had added up, and the calendar had become out of sync with the solar year. The solstices and equinoxes were all off, which meant that important religious days, most importantly Easter, were also out of whack.