>>Shannon Hall, The New York Times
Published: 2020-02-29 16:57:54 BdST
You probably know why: The time it takes Earth to rotate on its axis is called a day — but it doesn’t take an even number of days to complete a single loop around the sun, or one orbit. Instead it takes a messy 365.2422 spins. And yet the calendar year runs out after 365 days. That means that when the clock strikes midnight on New Year’s Eve, Earth hasn’t quite circled all the way back to its starting point.
“It’s like being a quarter of a day behind at the end of every workday,” said Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology. “After four days, you would need one full day to catch up on all your work. It’s the same for the Earth’s orbit and the calendar.”
So every four years, the month of February has 29 days instead of 28. But even that solution isn’t perfect, because the year is not exactly 365.25 days. We have to make additional tweaks. If a year is divisible by 100, for example, there’s no extra day — unless the year is divisible by 400. In other words, the year 2000 was a leap year, but 1900 was not, nor will the year 2100 be one (its nearest leap years will be 2096 and 2104).
These contortions are awkward, but they’re fairly straightforward compared with the adjustments that would need to be made to the calendars of alien civilisations if they existed elsewhere in our solar system.
On Mars, a year lasts 668.6 Martian days. Should the calendar year include only 668 days, it would quickly fall out of alignment with the Martian seasons. Luckily, astronomers, science-fiction writers and enthusiastic hobbyists have presented several proposals for Martian calendars.
“I daresay there have been more different proposals for Mars calendars than there are different calendars for the Earth,” said Michael Allison, a retired NASA scientist.
One of the most popular — the Darian calendar — was created in 1985 by Thomas Gangale, a space law expert. It breaks up the lengthy year into 24 months of 27 and 28 Martian days — each of which alternates between Latin and Sanskrit names for constellations of the zodiac, like Virgo and its Sanskrit equivalent, Kanya.
To keep the calendar in harmony with the Martian seasons, Gangale proposed that even-numbered years have 668 Martian days (except those divisible by 10), and odd-numbered years have 669 Martian days. That works out to an average of 668.6 — the length of a Martian year.
But it isn’t the only way to reach that average. Allison has proposed another calendar — “a whimsical exercise,” he said.
He thought it was important to maintain similarities to Earth’s calendar, just in case future Martians wanted to celebrate major holidays. So he retained the 12 months we know and love, then added 10 extra months (each is 30 or 31 days) and named them after Johannes Kepler, Ray Bradbury and other famous astronomers, mathematicians and science-fiction writers.
In his calendar, Allison proposed that years divisible by five would have three leap days, for a total of 671 days. But all other years would have 668 days.
While there have been many imaginative calendars suggested for Mars, none is in common use.
A photo provided by NASA, a sunset on Mars on the 145th Martian day, or sol, of the Insight lander’s mission. On Mars, a year lasts 668.6 Martian days. The New York Times
So scientists and engineers who work on surface missions on the red planet use two systems. One counts the number of Martian days that have elapsed since the start of a particular mission and the other marks the location of Mars within its orbit (and thus allows NASA to note the season). The two systems allow scientists to sidestep the complications that arise in trying to sync the two.
“There may come a time — if you have cultural civilisations that are living in this environment and you want to preserve the seasonal significance of a particular date on the calendar — that you would likely introduce some sort of a leap system,” Callas said.
With so many proposals, we’re certainly prepared.
Calendars for other worlds in our solar system get exceedingly difficult to calculate.
“On Jupiter, it would be hopeless,” Binzel said. “It’s a gas planet and different latitudes have different rotation periods. I think the Jovians would find themselves very confused.”
Then there’s Venus where a single rotation of the planet takes longer than its entire year (it also spins upside down). No matter how you work the problem, that’s never going to come out nice and even.
Luckily, Venus doesn’t have noticeable seasons, so you need not worry if your calendar doesn’t sync up with the year.
But there is one planet where the calendar would need zero finessing: Mercury. The small planet revolves exactly three times, or days, over the course of two years — allowing its calendar to naturally align every other year.
In that way, it may be like many other planets orbiting stars throughout our galaxy. Astronomers suspect that plenty of closer exoplanets revolve exactly once every year. These planets show only one face to their star, leaving the other side in perpetual darkness. And while that might make life on those worlds difficult, their calendars would always be in sync.
But even if by some wild cosmic coincidence, a planet’s orbit could be evenly divided into days, it likely wouldn’t stay that way for long.
Earth’s spin, for example, is slowing over time. Eventually one year won’t last 365.2422 days but precisely 365 days, allowing us to drop the leap year — at least temporarily.
“You have to enjoy the leap year while it’s here,” said Konstantin Batygin, a professor of planetary science at the California Institute of Technology. “Because in millions of years, maybe tens of millions of years, it’s just not going to be around anymore.”
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