Happy New Year! If you are marching!
When it is Sunday, February 7, 2021 on Earth, it will be January 1, 36 (yes, only 36) on Mars: the first day of a new year.
It turns out this is based on science, but it’s also a bit arbitrary. I know it sounds like an oxymoron, but be very careful. This is fun.
Mars is the fourth planet from the Sun. On average it orbits about 228 million kilometers, compared to 150 million from Earth. As a result, it orbits more slowly and has a longer way to travel, so its year is longer than ours: about 687 days (Earth) compared to 365 and changing for our planet.
Measuring the length of the year on Mars is fine, but when you do to start this measure? Which day do you choose as the first day?
In many countries (but not in any way), New Year’s Day is January 1, and this is quite arbitrary; Julius Caesar chose it to honor the god Janus, the god of beginnings, with the name of January. There is no science behind this, nor any natural event with which to label it. I could have chosen just as easily the first day of any other month*.
But with Mars we have the ability to do it right, and so did the scientists. They decided to base the year on the Martian calendar after the tropical year and begin it on the northern spring equinox.
The tropical year is the time measured from the spring equinox to the spring equinox. For Earth, these are the 365.24 days you’re probably used to hearing about (and what the calendar year is based on). For Mars, they are approximately 686.6 Earth days.
Equinoxes (yes, this is the plural of “equinox”) and solstices are related to the axis of rotation of a planet. The axis of rotation of a planet tends to point in the same direction in space even when the planet revolves around the Sun. The summer solstice is the day when the pole tilts more toward the Sun (note that this happens in June in the northern hemisphere of the Earth and in December in the south). The winter solstice is the day when the axis tilts more from the Sun. Equinoxes, then, are when the pole is oriented 90 ° from the Sun.
The Earth’s axis is inclined about 23 ° from the plane of its orbit. Coincidentally, Mars has an inclination of about 25 °. That means it has stations similar to those on Earth!
This gives us a good date to choose for the beginning of the Martian year: the spring equinox of its northern hemisphere. It could have been one of the other three seasonal markers, but astronomers tend to use the spring equinox (spring motif) for historical reasons.
The beauty of this is that the Martian spring equinox is a physical event that we can measure to determine that date. Therefore, unlike our own calendar, one that could be used (and may one day be used) on Mars will have a physical basis for its start. Wow.
Because Mars has a longer year and is not even a multiple of Earth’s year, the Martian New Year’s Day has a different Earth calendar every day. In 2021 it will fall on February 7th.
As it happens, Mars orbits the Sun in an ellipse that deviates significantly from a circle. This complicates things a bit. When Mars is in the perihelion (the point closest to the Sun in its orbit) it moves a little faster than when it is in the aphelion (the farthest point). This means that the stations do not have the same duration and that they mark strength. Northern spring occurs when Mars is farthest from the Sun, so it is the longest season (about 199 Earth days) and northern autumn the shortest (145 Earth days).
One aside: you might think that starting the year when a planet is in perihelion makes sense, and somehow it does … 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 decent force on the Earth, rotates it around, and changes the exact time of the perihelion each year. It’s usually around January 4th (so close!), But it can change several days from one year to the next.
The same goes for the equinox, which arrives each year at a different time, so it does not constitute a great anchor for the beginning of the year. Worse, the number of days in a year is not even a multiple, so we have leap years and that makes things worse. At the end, pointing to a day and saying “Yes, that one is the start of the year ”also works and in many ways is simpler.
Therefore, the length of the year is scientific. But the number of the year (in this case, the year March 36) is a bit arbitrary. Year 1 was chosen to begin on April 11, 1955, based on a paper published in (Earth Year) 2000 on seasonal temperature changes on Mars. As it happens, in 1956 there was a big dust storm that was heavily studied from Earth, and that means it fell to Mars in year 1. Auspicious and good enough to start a calendar.
Later, it was defined that the year of March 0 would begin on May 24, 1953: this would allow the use of negative years, which is useful for scientists. This means that human Martians at some point in the future will argue about when the centuries of Mars will end, but so be it.
Which reminds me: the Martian day is a little longer than an Earth day, about 39 minutes. On a Martian day we call them “sun” to distinguish them, and I know some people who work with rovers from Mars and who have the calendar set by the suns when they are on duty. After a few days, I imagine it can be tricky, wanting to have breakfast at lunchtime, and so on.
Eventually, people will live on Mars and will have to deal with conversions of time, day, and even years talking to people on Earth. Naturally, several calendars have been presented (and it’s fun to find out), and someday one will have to be chosen as the official one. This may have to happen very soon.
It will surely be an interesting one.
*One might think that he would have chosen July 1, as this month gave him the name, but he was named in his honor after his death (it was said Quintilis before), so go for it.