If you’re not an Orthodox Christian, you probably don’t understand why January 7 is Orthodox Christmas. The reason is found, at least partly, in astronomy.
In 46 BC, Julius Caesar set up the calendar system which the Roman Empire would use for centuries and European countries would inherit. The Julian calendar added an extra day every four years to make up for the regular calendar having 365 days in a year but the earth’s orbit actually being about a quarter day longer. That kept the spring equinox at about March 21 every year–for some time.
Adding an extra day every four years is actually an overcorrection. It assumes Earth’s orbit is exactly 365 days, 6 hours long. But it’s really 365 days, 5 hours, 48 minutes, and 46 seconds. Simply adding that extra leap year day every four years without fail adds up to a discrepancy of a full day after 128 years. The equinox happens a day earlier.
That wasn’t a big deal at first. An equinox on March 20 instead of March 21 wasn’t something most people noticed. By the time the equinox crept back to March 19, the people who remembered it having been on March 21 ...