Martian Christmas

Someone on Twitter said “Merry Martian Christmas”, and I immediately thought, “Wait a moment – Mars has a longer year than Earth, so it’s highly unlikely that an anniversary would coincide on both planets.”

So I did some back of the napkin math to calculate the true date of Christmas on Mars.

I began with the assumption that Jesus Christ was born on the putative date of 25 December 1 B.C. There’s lots of debate about what year or time of year Christ might really have been born, but no one is rescheduling their Christmas parties over it.

To calculate the length of time since the birth of Christ seemed like a tricky proposition, what with all the leap years, and the change from Julian to Gregorian calendar.

Fortunately, astronomers have given every day since 1 January 4713 B.C. a consecutive number to simplify things. Thus today, 28 December 2010, is Julian Day number 2455559.

Thanks to this convenient site, it was easy to determine that 25 December 1 B.C. was Julian Day number 1721417. Subtract one from the other and we see that 734142 days have passed since the birth of Christ.

The next step is converting that from Earth days to sols (sol is the term for a Martian day). There are 1.027491 days per sol so 714499.689 sols have passed. A Martian year is 668.5991 sols, so 1068.65189 Martian years have passed.

A Martian Dionysius Exiguus might say that on Mars it’s A.D. 1068!*

The .65189 Martian years is the time since the 1068th Martian Christmas. It works out to 435.853067 sols, or 447.835104 days – which subtracted from today’s Julian Day number gives Julian Day number 2455111.16, or 6 October 2009 as the last Martian Christmas.

We can also see that there are 0.34811 Martian years until the next Martian Christmas. That’s 232.746033 sols, or 239.144454 days. Added to today’s Julian Day number gives us Julian Day number 2455798.14, or 24 August 2011.

Here are the next ten dates of Martian Christmas:

24 August 2011

11 July 2013

29 May 2015

15 April 2017

3 March 2019

18 January 2021

6 December 2022

23 October 2024

9 September 2026

27 July 2028

Future Martian Christians might simply choose to observe Christmas at the same time as terrestrial Christians, having a floating liturgical calendar running parallel to whatever secular calendar ends up being adopted on the red planet. There are pragmatic reasons for this: to maintain solidarity with the larger body of the church remaining on Earth, plus two Earth years is a long time between Christmases!

There would be drift, of course: the difference between the day and the sol would mean Martian Christians would worship on the first day of the Earth week, and not always the first day of the Martian week.

If, on the other hand, Martian Christians do modify their liturgical year to synchronise with the Martian calendar, the date of Christmas can be used to start to fill in the gaps: the season of Advent, for instance, starts on the fourth Sunday before Christmas. The season of Christmas starts on Christmas Day, of course.

A more difficult question is when Martian Easter would be: Easter and the holidays which relate to it are movable feasts whose date is determined based on criteria particularly related to Earth: Easter is the first Sunday after the full moon following the northern hemisphere of Earth’s vernal equinox.

So does Martian Easter coincide with Earth’s, because it needs to be based on the Earth-originating criteria? That would make it potentially overlap Martian Christmas. Creating a new Easter based on the Martian year would be a big deal: the dating of Easter has been set since the Council of Nicaea – it might take another such council to approve changes.

Perhaps it’s easier for Martian Christians simply to retain the Earth liturgical calendar after all, and have it overlapping the secular Martian calendar?

Mars is probably the only body in the Solar System for which the calendar question arises. Colonists on any other body are more likely to retain the Earth calendar because they will live in artificial environments or because the length of the day isn’t close enough for humans to attune to (the Venusian day is longer than its year, for instance).

*Depending on when Martian New Year is.