Happy New Year! If you are a Martian!
If it’s Sunday, February 7, 2021 on Earth, it’s January 1, 36 (yes, only 36) on Mars – the first day of a new year.
As it turns out, this is based on science, but is also somewhat arbitrary. I know that sounds like an oxymoron, but be patient. This is fun.
Mars is the fourth planet from the sun. On average, it orbits about 228 million kilometers, compared to 150 million for Earth. Therefore, it rotates 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 Mars year is all fine and good, but when do you do that get started that measurement? Which day do you choose as day 1?
In many countries (but not all), New Year’s Day is January 1, which is quite arbitrary; Julius Caesar chose it in honor of the god Janus, the god of the beginning, after whom January is named. There is no science behind it, no natural event to tag this with. He could just as easily have chosen the first day of any other month*.
But with Mars, we have a chance to do this right, and so scientists did. They decided to base the Martian calendar year after the tropical year and start on the northern spring equinox.
The tropical year is the time measured from vernal equinox to vernal equinox. For Earth, that’s the 365.24 days you’re probably used to hearing (and that’s what the calendar year is based on). For Mars, that’s about 686.6 Earth days.
The equinoctes (yes, that’s the plural for “equinox”) and solstices are related to the axis of a planet’s rotation. A planet’s axis of rotation tends to point in the same direction in space, even if the planet orbits the sun. The summer solstice is the day when the pole is tilted most towards the sun (note that this happens in June for the Northern Hemisphere and December for the Southern Hemisphere). The winter solstice is the day when the axis is tilted most away from the sun. So the equinoctes are when the pole is pointed 90 ° away from the sun.
The Earth’s axis is tilted about 23 ° from the plane of its orbit. Coincidentally, Mars is tilted about 25 °. That means it has seasons similar to Earth’s!
That gives us a good date to choose from for the beginning of the Martian year: the spring equinox in the northern hemisphere. It could have been one of three other seasonal markers, but astronomers use the spring equinox for historical reasons.
The great thing about this is that the Martian Spring Equinox is a physical event that we can measure to determine this date. So, unlike our own calendar, any calendar that could (and possibly ever will) be used on Mars will have a physical basis for the beginning. Cool.
Because Mars has a longer year, and it is not even a multiple of the Earth year, Mars New Year’s Day falls on a different Earth calendar day each time. In 2021 it will fall on our 7th of February.
Mars revolves around the sun on an ellipse that deviates noticeably from a circle. This makes things a bit complicated. When Mars is in perihelion (closest to the sun in its orbit), it moves a bit faster than when it is at aphelion (farthest point). This means that the seasons are not all the same length and that they differ considerably. Northern Spring occurs when Mars is furthest from the Sun, so it is the longest season (about 199 Earth days) and Northern Autumn the shortest (145 Earth days).
As an aside, you might think it makes sense to start the year when a planet is in perihelion, and in some ways … but the time and date of perihelion can change. On Earth, the gravity of the other planets pulls the Earth and changes the shape of its orbit, and the Moon exerts a considerable force on the Earth, swirling it around and changing the exact time of perihelion every year. It’s usually around January 4th (so close!) But that can change by a few days from year to year.
The same is true of the equinox, which comes at a different time each year, so it’s not a good anchor for the start of the year. Worse still, the number of days in a year isn’t even a multiple, that’s why we have leap years, and that makes things worse. Finally I pointed to a day and said “Yes, this one one is the beginning of the year “works about as well and is simpler in many ways.
So the length of the year is scientific. But the year number – in this case Mars year 36 – is a bit arbitrary. Year 1 was chosen to start on April 11, 1955, based on a paper published in (Earth year) 2000 on seasonal changes in Mars. Coincidentally, there was a large dust storm in 1956 that was heavily studied from Earth, and this means that it fell in Mars Year 1. Good enough and good enough to start a calendar.
Later, Mars Year 0 was defined to begin on May 24, 1953 – this was to allow for the use of negative years, which is helpful for scientists. That means human Martians will argue sometime in the future about when the ages of Mars will end, but so be it.
Which reminds me: the day of Mars is about 39 minutes longer than a day on Earth. We call a Mars day a “sol” to distinguish them, and I know a few people who work with Mars rovers who let sols determine their schedule when they are on duty. After a few days I can imagine that that can get tricky, if I want to have breakfast in the afternoon and so on.
Ultimately, humans will live on Mars and will have to deal with time, day, and even year conversions talking to humans on Earth. Several calendars have been released, of course (and it’s fun figuring them out), and one day one will have to be chosen as the official one. That may have to be done soon.
That will certainly be an interesting sol.
*You might think he would have picked July 1, since that month was named after him, but was named in his honor after he died (it was called Quintilis before then), so there you go.