On this episode of Our Island Universe: We go back to 1582, when the month of October had only 21 days.
Host: Shanil Virani, Director of the John C. Wells Planetarium Harrisonburg, VA.
Follow on Twitter as shanilv
Transcript:
Imagine waking up tomorrow and finding out that the calendar date is now 10 days later then when you went to bed! Sound likes a movie right? Truth is stranger than fiction. Europeans went to bed on Thursday, October 4, 1582 and awoke the next morning to learn that the date is now Friday, October 15, 1582. Where did 10 days go?!
We know that a year measures how long it takes the Earth to make one complete revolution around the Sun. In the 16th century, whether the Earth was at the center of the Solar System or it was the Sun, was still a controversial question. But what was known to even earlier cultures was that the pattern of stars you would see repeats every 365 days or so. We know that a year is roughly 365.25 days meaning that unless we add a leap day every 4th year (or so), our civilian calendar will slowly drift with respect to the stars. The Julian calendar, introduced by Julius Caesar in 46 BC has a regular year of 365 days divided into 12 months. Although Greek astronomers had known that the solar year was a few minutes shorter than 365.25 days, the Julian calendar did not compensate for this difference. So the calendar year gained about three days every four centuries compared to observed equinox times and the seasons.
But so what? The calculated date of Easter gradually moved out of alignment with the March equinox. By 1582, Easter was ten days out of alignment from where it supposedly had been in 325 during the Council of Nicaea. Because the celebration of Easter was tied to the spring equinox, the Roman Catholic Church considered the steady drift in the date of Easter too long to be undesirable. So the Gregorian calendar was introduced in 1582 and 10 days were removed from the calendar! Our civilian calendar is now again in synch with the stars.