February 29, “leap day”, is an oddity on the calendar that occurs every four years in an attempt to sync up the Gregorian calendar (the calendar most of the world uses to keep time) with Earth’s rotation around the sun.
One calendar year on Earth is 365 days long; however, the Earth actually takes 365.2422 days to lap the sun. That puts the planet roughly a quarter of a day behind at the end of each year.
To maintain consistency and ensure the seasons line up each year, an extra day was added to the already short month of February — a modification that happens every four years. But even this doesn’t fully solve the problem; additional tweaks are needed.
For instance, if a year is divisible by 100, there’s no extra day — unless the year is divisible by 400. That means that 1700, 1800 and 1900 did not have a leap day, but 2000 did. This adjustment ensures that Earth is as close as possible to the same point in its orbit in consecutive calendar years and keeps our seasons inline.
Unsurprisingly, Earth is not the only planet that needs leap days. The phenomenon could occur on other planets in our solar system as well as those around other stars. That’s because you can’t fit an exact number of spins into one trip around the sun for any planet. There’s usually going to be something left over. Leap days on other worlds, such as Mars, could be more complicated than those here on Earth.
One year on Mars lasts for approximately 668.6 Martian days. (A Martian day is called a sols and equals 24 hours, 39 minutes, and 35.244 seconds.) Future inhabitants might decide that a year on the red planet will be described as 668 days. How do we adjust?
Over the decades, many different ideas for the Martian calendar have been proposed. The most popular one, called the Darian calendar, was created in 1985 by Tomas Gangale.
According to Gangale, the Martian calendar would feature 24 months, each named for the Latin and Sanskrit words for the constellation of the zodiac, like Sagittarius and Dhanus, and so forth. The first five months in each quarter would have 28 Martian days (or sols), with the sixth having only 27. Even-numbered years would total 668 days and odd-numbered years would have 669. The exception to that rule: even-numbered years that were divisible by 10.
Another option was proposed by Michael Allison, a retired NASA scientist. In his version, the Martian calendar would have 668-days divided into 22 months, each totaling 30 or 31 days, similar in fashion to Earth. To make sure the seasons lined up, all years divisible by five will have three leap days, bringing the total to 671.
So which of these calendars do scientists use to keep track of time on Mars? Currently, none.
Instead, they use two systems to keep track of time: one counts the number of Martian days since the start of a mission, and the other keeps track of where Mars was in its orbit at that time.
Right now scientists don’t care if the seasons line up perfectly with the calendar, but that may change when you add humans into the mix. NASA and other space agencies around the world have their sights set on Mars. Once humans land on the red planet and spend significant time there, we are going to need a common calendar to keep track of seasons.