Science in the Bluegrass — The longest Advent possible

Chris Graney

Advent is four full weeks this year, the longest Advent possible. Christmas falls on a Sunday. So does the Solemnity of Mary, the Holy Mother of God (New Year’s Day). Priests and others involved in liturgies have a light load this year, with far fewer Masses than there would be if these days fell at mid-week.

We won’t see this happen again for a while.

The Earth takes about 365.25 days to orbit the sun. That’s the astronomical basis for a year. Every 365.25 days the sun returns to the same spot in the zodiac, as we look across Earth’s orbit to see the sun against the distant stars. At midnight this Dec. 25, the sun will appear to be right next to the star SAO 186478 in the zodiac constellation Sagittarius; it will return to that spot next year on Dec. 25 at 6 a.m.

A year on our calendars consists of 365 days: the 31 days each of January, March, May, July, August, October and December; plus the 30 days each of April, June, September and November; plus the 28 days of February. But in terms of weeks, the calendar year is 52 weeks of seven days each. That is 364 days, not 365. It takes one more day to get 365.

Because of that “one more day,” each year the day of the week on which a given date falls changes by one day. In 2020, Christmas fell on Friday. In 2021 it fell on Saturday. This year it falls on Sunday.

With the day changing by one each year, and with seven days in a week, you might expect that Christmas would fall on Sunday once every seven years. But astronomically, a year is 365 — and a quarter — days.  Because of that quarter of a day, we have a leap year once every four years.

On leap years, there are 29 days in February, and the calendar year consists of 366 days. That means that in terms of seven-day weeks, we have those 52 weeks of seven days giving us 364 days … plus two more days to get 366. In 2019, Christmas fell on Wednesday. 2020 was a leap year. So in 2020, because of Feb. 29, Christmas jumped two weekdays, to Friday.

So, you may be saying, with leap years we get a Sunday Christmas every six years, right? Sorry, but there’s more.

You can fit two leap years in. In 2011 Christmas was on Sunday, and we had two February 29ths (2012 and 2016) before we got another Sunday Christmas, in 2016. So there were only five years between one Sunday Christmas and the next.

And lastly, the leap year can cause the calendar to leap right over Sunday Christmas. That’s what is coming now. Christmas this year is on Sunday; in 2023 it will fall on Monday; in 2024, on Wednesday (because 2024 will be a leap year); in 2025, on Thursday; in 2026, on Friday; in 2027, on Saturday. But 2028 is a leap year, meaning Sunday is skipped over, and so in 2028 Christmas will fall on Monday.

The process then continues on, with another leap year jump in 2032. The result is that the next year in which Christmas falls on Sunday will be 2033. That is 11 years in the future! There is actually a repeating pattern of years between Sunday Christmases: six years, then five, then six, then eleven.

So enjoy this long Advent. You will not see another until the year 2033, thanks to the timing of Earth’s orbit around the sun.

Chris Graney is an astronomer and historian of science with the Vatican’s astronomical observatory (www.vaticanobservatory.org). He lives in Louisville.

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