Greco-Martian Calendar
The Martian Sol
A solar day is the time it takes for a planet to rotate once about its axis so that the sun reappears in the same position in the sky (from position 1 to position 3 in the accompanying diagram). That amount of time is divided into 24 hours, each of which is divided into 60 minutes, each of which is divided into 60 seconds. On Mars, a solar day is known as a “sol”. Since Mars rotates a little slower than Earth, if we take a sol and divide it into 24 hours, an hour into 60 minutes, and a minute into 60 seconds, a Martian second turns out to be 1.0275 Earth seconds.
Okay, you say, no big deal. We all just move about 3% slower, and it’s a wash. As long as we’re not trying to keep in sync with Earth, that’s fine, but if we are, in one day we’re off about 40 minutes, in ten days 400 minutes, etc. You can’t say, for example, it’s always 10 hours earlier at a particular location on Mars. It will constantly change in relation to Earth time.
The Martian Year
A solar year is the time it takes a planet to complete one full revolution around the sun.
For Earth, this takes about 365.25 days. Breaking that time into 12 months means you get months that are roughly 30 days long. Mars, on the other hand, takes about 668.6 sols to get around the sun. Using 12 months would make each month unbearably long at about 56 sols each. The common solution is to break the Martian year into 24 months instead, resulting in each month being 27 or 28 days long.
Also, since Mars’ orbit is eccentric, the seasons are not evenly spaced like on Earth. Northern spring is the longest season, and northern winter is the shortest.
The Greco-Martian Calendar
The Greek alphabet has 24 letters in it, making it a convenient and familiar set of names to give Mars’ months. Below are the names of the months, from left to right, top to bottom, with the number of days in each month listed in parentheses:
| Alpha (28) | Beta (28) | Gamma (28) | Delta (28) | Epsilon (28) | Zeta (27) |
| Eta (28) | Theta (28) | Iota (28) | Kappa (28) | Lambda (28) | Mu (27) |
| Nu (28) | Xi (28) | Omicron (28) | Pi (28) | Rho (28) | Sigma (27) |
| Tau (28) | Upsilon (28) | Phi (28) | Chi (28) | Psi (28) | Omega (27-29) |
Since we need 668 sols in a year, not counting that pesky 0.6, the Greco-Martian calendar divides those 24 months into groups of 6 and gives the first five of each set 28 days and the sixth 27 days. The last set begins with 28 days each, but the last month, Omega, is used during leap years to adjust for that extra 0.6 sols each year.
The date is always listed as YearMonthDate, with no spaces and no extra zeros. For example, 56Tau9 is the 9th of Tau in the year 56. If it’s the current year or no specific year being discussed, the Greco-Marian calendar allows just MonthDate to be listed, as in Tau9.
The beginning of the calendar year is the Mars northern hemisphere winter solstice, which is when the sun appears at its furthest south in the sky.
The first day ever in the Greco-Martian calendar is Sunday 1Alpha1, which corresponds to 10/9/1971 on Earth. This marks the Martian winter solstice just before the first human-designed machine successfully landed on Mars (the Soviets’ Mars 3, which launched May 28, 1971 and landed December 2, 1971.)
Greco-Martian Date Finder
Mars Date:
Greco-Martian Leap Years
On Earth, we have a leap year every four years to make up for that 0.25 days left over each normal year. On Mars, a similar system is needed, but it’s a little more complicated. There’s an extra 0.6 sols every year. So, the Greco-Martian calendar uses a leap year every other year, adding one or two days to the last month of the year, Omega, to adjust the calendar to the seasons on Mars.
It works like this:
if the year ends in a zero, Omega has 29 days
else if the year ends in an even number, Omega has 28 days
else Omega has 27 days
That’s it! That’s the Greco-Martian calendar’!
Seasons on Mars
One last date-related issue with Mars – the seasons! As mentioned above, Mars’ orbit is eccentric, making the start of the four seasons unevenly spaced. The Greco-Martian calendar sets the start of each season to a specific date that comes as close as possible to the actual season changes while maintaining year-to-year consistency.
The dates are as follows:
| Winter Solstice | Alpha1 |
| Spring Equinox | Zeta15 |
| Summer Solstice | Nu15 |
| Fall Equinox | Tau26 |