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A year on Mars is defined the same way it is on any other planet, including Earth. A year is the time it takes the planet to orbit the Sun once.
Mars is further from the Sun than Earth, however, and so its orbit is longer. This means that a year on Mars is about 687 Earth days long (88% longer than an Earth year).
Planetary scientists track Mars years starting with Mars Year 1 (MY1) which began on April 11, 1955. This date is arbitrary; it was chosen by scientist R. Todd Clancy in a publication describing seasonal temperatures around a great dust storm in 1956, but it has since become convention by the scientific community.
Months and Weeks
Both months and weeks are used on Earth by humans to organize time. They are arbitrary (even though they may have roots in natural processes like the lunar cycle)
Since no humans live on Mars (yet), there isn't a use for breaking up time into work weeks or similar. As of today, there is no such thing as a Martian week or month.
Solar Longitude (Ls)
Solar Longitude measures the point where Mars is in its orbit around the sun. It's a numerical representation of seasons, since we don't track months on Mars.
Solar Longitude (Ls, said out loud ell sub ess) starts at 0° (the beginning of Spring, or Vernal Equinox, in the Northern Hemisphere) and goes to 360° degrees one year later.
Here are some Ls numbers and their cooresponding seasons.
|Ls||Northern Hemisphere||Approx. Earth date|
|0°||First day of Spring||Mar.|
|90°||First day of Summer||Jun.|
|180°||First day of Fall||Sep.|
|270°||First day of Winter||Dec.|
Sols: the Martian Day
A day on Mars is defined the same way as it is on any other planet. A day is the time it takes for a planet to spin around its axis once.
Mars spins slightly slower than Earth does, however. A solar day on Mars is around 40 minutes longer than Earth's.
Here is how the Martian day compares to Earth's:
|Solar||24h||24h 39m 35s|
|Sidereal||23h 56m 4s||24h 37m 23s|
In general, on Earth and on Mars, we use solar days to measure time. Learn more about the difference between a solar day and a sidereal day here.
The second (s) is the Standard International (SI) unit of time. When measuring duration, seconds on Mars are the same as seconds on Earth or anywhere else.
Minutes and hours are derived from seconds (60 seconds to 1 minute and 60 minutes to 1 hour), and so they are also the same on Mars.
Measuring the duration of time (ie. "How long did I sleep?") is different from marking time during a day on Mars. When NASA landed their first Mars surface mission in 1976 (the Viking lander), they adopted a convention of a 24 hour clock just like Earth.
This practice splits the Martian day up into 24 hours, with traditional sexagesimal divisions (60 minutes and 60 seconds). This means that the duration of time between 1PM and 2PM is actually longer than an hour. But this is useful for humans, because now 2PM is now the same place in a day on Mars as it is on Earth.
Plus, we don't need to have an awkward 39 minute hour at the end of the day!
Mean vs True Solar Time
On any planet, "True" solar time is what could be measured on a sundial - it uses the actual position of the Sun to determine the time of day.
However, just like Earth, Mars' days are not actually constant in duration. The rotation speed of the planet and the eccentricity of its orbit cause daily and seasonal changes to the duration. This is inconvenient for timekeeping.
So, just like on Earth, we adopt another type of solar time. "Mean" solar time averages out the daily and seasonal variations over a year in order to get a day duration that is constant.
Mean solar time is what you use every day on Earth. It's also what engineers operating space missions on Mars surface use, and it's what this site displays in its clocks.
Timezones and Local Time
Just like Earth, Mars is round. At any given time, parts of the planet are in daylight, facing the Sun, while others are in darkness, facing away from the Sun. Local time varies depending on your longitude, which is inconvenient if you need to coordinate with someone in a different longitude.
On Earth, we've solved this problem using timezones. But timezones are a compromise, because the position of the sun changes every step you take. Imagine having to change your clock by 10 seconds because you commuted a few kilometres to the East or West! Having timezones generally 1 hour wide keeps the meaning of 2PM the same everywhere, but allows proximate communities to all work on a shared time.
On Mars there are no flight schedules to sync up, and no communities to keep on shared time. And so there are no timezones (yet). Instead, we can calculate local time using math depending on your precise longitude.
On Mars we define longitude 0 at a place called Airy crater, named after George Biddell Airy, an English mathemetician and astronomer who contributed to establishing the Greenwich Observatory as Earth's prime meridian. The time at longitude 0 is called Mars Coordinated Time (MTC), just like Earth's UTC.
All of the mission clocks displayed above calculate the Local Mean Solar Time of the mission based on their longitude. For NASA rovers, it is updated using data from NASA's Jet Propulsion Laboratory as the vehicles traverse the Martian surface.
Want to learn more about Martian time? Here are some great resources.
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