Happy New Year! If you’re a Marsman!
If it’s Earth on Sunday, February 7, 2021, it’s January 1, 36 (yes, only 36) on Mars: the first day of a new year.
It seems to be science-based, but also somewhat arbitrary. I know it sounds like an oxymoron, but stick with me. This is fun.
Mars is the fourth planet from the sun. On average, it orbits about 228 million kilometers, compared to 150 million for the earth. Therefore, it orbits more slowly and has a longer path to cover, so the year is longer than ours: about 687 (Earth) days compared to 365 and change for our planet.
Measuring the length of the March year is all well and good, but when do you do it? start that measurement? Which day do you choose as Day 1?
In many countries (but not all at all) New Year’s Day is January 1st, and it’s pretty arbitrary; Julius Caesar chose to worship the god Janus, the god of the beginning, after whom January is named. There is no science behind this, no natural event to mark it. He could just as easily have chosen the first day of any other month*.
But with Mars, we have the chance to do it right, and scientists have done it too. They decided to base the Mars calendar year on the tropical year and start it on the northern spring equinox.
The tropical year is the time measured from spring equinox to spring equinox. For Earth, this is the 365.24 days you are probably used to (and this is what the calendar year is based on). For Mars, it is about 686.6 Earth Days.
The equinoxes (yes, this is the plural for “equinox”) and solar eclipses are related to the axis of rotation of a planet. The axis of rotation of a planet tends to point in the same space in space, even as the planet rotates around the sun. The summer solstice is the day on which the pole tilts most towards the Sun (note that this happens in June for the northern hemisphere of the Earth and December for the south). The winter solstice is the day on which the ash is sent away most of the sun. The equinox is therefore when the pole is directed 90 ° from the sun.
Earth’s axis is tilted approximately 23 ° from the plane of its orbit. Coincidentally, Mars is tilted by about 25 °. That means it has seasons like Earth’s!
This gives us a good date to choose at the beginning of the March year: the northern hemisphere’s spring equinox. It could have been one of the three other seasonal markers, but astronomers tend to use the historical (spring) equations for historical reasons.
The beauty of this is that the terrestrial equinox of Mars is a physical event that we can measure to determine this date. So, unlike our own calendar, one that can be used on Mars (and possibly will one day be) has a physical basis for its beginning. Cool.
Because Mars has a longer year, and it is not a multiple of Earth’s year, Mars’ New Year’s Day is on a different Earth calendar day each time. In 2021, it falls on February 7th.
As this happens, Mars orbits the sun on an ellipse that deviates markedly from a circle. It complicates things a bit. When Mars is in the perihelion (the nearest point of the sun in its orbit), it moves a little faster than when it is on the aphelion (the farthest point). This means that the seasons are not all equally long, and that they are not quite enough. The northern spring occurs when Mars is furthest from the sun, so it is the longest season (about 199 days on earth), and the northern autumn the shortest (145 days on earth).
One side: you may think it makes sense to start the year when a planet is in the perihelion, and it does in some ways … but the time and date of the perihelion may change. On earth, the gravity of the other planets pulls the earth and changes the shape of its orbit, and the moon exerts a decent force on the earth, swinging it around and changing the exact time of the perihelion each year. It’s usually around January 4th (so close!), But that can change from year to year by a few days.
Same for the equinox, which comes at a different time each year, so it is not a good anchor for the beginning of the year. Worse, the number of days in a year is not even more, so we have leap years, and that’s getting worse. Finally showed up after a day and said, “Yes, this the one is the beginning of the year, works like that too and is in many ways simpler.
The length of the year is therefore scientific. But the year – in this case March 36 – is a bit arbitrary. Year 1 was chosen to start on April 11, 1955, based on an article published in (Earth) 2000 about Mars’ seasonal temperature changes. As it happens, there was a major dust storm in 1956 that was strongly studied from the earth, and this means that it fell in the first year of March. Favorable and good enough to start a calendar with.
It was later defined that Mars Year 0 would begin on May 24, 1953 – that was to allow the use of negative years, which is useful to scientists. This means that human Martians will argue at some point in the future about when Mars will end centuries, but it is also so.
Which reminds me: Mars Day is about 39 minutes longer than Earth Day. We call a Mars Day a ‘sol’ to distinguish them, and I know a few people who work with Mars robbers, and their roster is determined by sols when they are on duty. After a few days, I imagine that it can be difficult, want to eat breakfast during lunch and so on.
Eventually, humans will live on Mars, and they will have to deal with time, day, and even year conversions to talk to humans on earth. Of course, different calendars have been offered (and it’s nice to find out), and one day one will have to be chosen as the official one. It may have to happen soon.
It will definitely be an interesting sol.
*You would think he would choose July 1, as the month was named after him, but it was named in his honor after he died (it was named Quintilis before then), so there you go.