The conventions that we use to track the passage of time in a day, week, month, or year are clocks and calendars. People rely heavily on the clock in order to schedule their tasks, and on the calendar to plan their weekly work, meals, and exercise and recreation times. Months help people understand the passage of longer periods of time and tell us what season it is. 

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.

Click here to read about how humans have organized their lives with calendars throughout history. 

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. 
 

Next... Keeping Time, Continued (pg. 15 of 35)