For future Martian explorers and settlers, a year will mean something different than a year does to us on Earth. How explorers keep time on the planet Mars will be an important factor in structuring days, work assignments, free time, and communication with Earth. 

How long is a day on Mars? 

The rotational period of Mars is officially 24.6229 hours or 24 hours, 37 minutes long. You might have read somewhere that the Earth's rotational period is actually 23 hours 56 minutes long. So where are the other four minutes each day? 

The difference is that the 23 hours 56 minutes number is a sidereal day; that is, the Earth's rotation measured from the point of view of a fixed reference angle in space. Here is a nice little animation to explain it to you.

But as Earth turns once on its axis, it also moves along its orbit around the Sun, and so the direction from the Earth to the Sun changes slightly. It takes Earth an extra four minutes to rotate through this additional angle, and so Earth's solar day, measured from the point of view of the Sun, is 24 hours. (Remember that the breaking up of the Earth day into 24 segments called hours is a human convention, unrelated to astronomical cycles.) 

This same principle applies to Mars. Although the sidereal day is 24 hours, 37 minutes, the solar day (known as sol) is 24 hours, 39 minutes, 35.2 seconds. 

Click here to read about Martian Standard Time. 
 

Many people have thought about ways of devising a Martian clock. Since a Martian sol is only 3% longer than an Earth day, one approach is to stretch Earth's units of time into slightly longer hours, minutes, and seconds (this is the system currently used by NASA). Another idea is to add 39 minutes, 35.2 seconds at the end of 24 hours. Still another idea would be to make a metric clock based on the powers of ten.  One of the first Martian clocks , developed in 1954 was designed to compare Earth and Mars time.  The device, which includes about 400 working mechanical parts, was built by the Hamilton Watch Company. 

In 1993, Robert Zubrin wrote an article called A Case for Mars , in which he recommended adopting a Martian second that lasts 1.0275 Earth seconds. Sixty such seconds would equal a Martian minute; 60 Martian minutes would equal a Martian hour; and 24 Martian hours would equal a Martian day. Multiplying Martian days, hours, minutes, and seconds by 1.0275 yields the Earth equivalent.  Here are some examples of Martian clocks and calendars:

• Martian Clock
• Martian Standard Time (a sundial)
• Martian Darian Calendar
• The Darian Calendar

Would an Earth calendar work on Mars? 
 

No. First of all the Martian 'sol' is nearly 40 minutes longer than an Earth day and, over the period of one month, that amounts to nearly an entire day.  There are other factors such as the length of the year and the difference in the length of seasons. If we tried to use an Earth calendar on Mars, the months that have 31 days on Earth would have only 30 sols on Mars, and most of our months that have 30 days would be only 29 sols long on Mars.  In order to maintain the same relationship between the date of the month and the day of the week between the two worlds, you would have to lose a day of the week at the end of a month that had to be shortened by a day (in the Martian calendar)!

Mars reaches its farthest point from the Sun or aphelion when it is spring in its northern hemisphere. This has the effect of making spring the longest season (194 sols), with summer the second longest season (177 sols). Mars reaches its closest point to the Sun or perihelion when it is autumn in its northern hemisphere; thus, autumn is the shortest season (142 sols), and winter is the second shortest season (156 sols). Visit this site to see the current sunlit region on Mars and the time at a selected location on Mars. 

Although several dozen calendars have been proposed for Mars since 1880, there has been no pressing need to adopt a standard one. As we begin to plan for a human Mars mission and establishing a colony or base on the planet, however, we will need a standard Martian calendar. 

What time is it on Mars now?

Questions to think about:

• How would you design a Martian clock?

• How many months would you have in a Martian year?

• What would you call the months on Mars?