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Space Topics: Mars

Mars' Calendar

Click to enlarge > Frost at the Viking 2 Lander site Credit: NASA / JPL

Time on Mars is easily divided into days based on its rotation rate and years based on its orbit.  Sols, or Martian solar days, are only 37 minutes and 22 seconds longer than Earth days, and there are 668 sols (684 Earth days) in a Martian year. 

For convenience, sols are divided into a 24-hour clock.  Each landed Mars mission keeps track of “Local Solar Time,” or LST, at its landing site, because Local Solar Time relates directly to the position of the Sun in the sky and thus the angle from which camera views are illuminated.  The time of day, Local Solar Time, depends upon the lander’s longitude on Mars.  Local Solar Time is about 12 hours different for the Spirit and Opportunity rovers on Mars.

Unlike on Earth, there is no leisurely-orbiting moon to give Mars “months,” and while there have been many imaginative calendars suggested for Mars, none is in common use.  The way that scientists mark the time of Mars year is to use solar longitude, abbreviated Ls (read “ell sub ess”).  Ls is 0° at the vernal equinox (beginning of northern spring), 90° at summer solstice, 180° at autumnal equinox, and 270° at winter solstice.

On Earth, spring, summer, autumn, and winter are all similar in length, because Earth’s orbit is nearly circular, so it moves at nearly constant speed around the Sun.  By contrast, Mars’ elliptical orbit makes it speed up and slow down in its orbit.  Mars is at aphelion (its greatest distance from the Sun, where it moves most slowly) at Ls = 70°, near the northern summer solstice, and at perihelion (least distance from the Sun, where it moves fastest) at Ls = 250°, near the southern summer solstice.  The Mars dust storm season begins just after perihelion at around Ls = 260°.

To find out what time it is at your favorite spot on Mars, download the Mars24 application from the Goddard Institute for Space Studies.