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Concept
Timekeeping on Mars
Summary
Though no standard exists, numerous calendars and other timekeeping approaches have been proposed for the planet Mars. The most commonly seen in the scientific literature denotes the time of year as the number of degrees on its orbit from the northward equinox, and increasingly there is use of numbering the Martian years beginning at the equinox that occurred April 11, 1955.
Mars has an axial tilt and a rotation period similar to those of Earth. Thus, it experiences seasons of spring, summer, autumn and winter much like Earth. Mars' orbital eccentricity is considerably larger, which causes its seasons to vary significantly in length. A sol, or Martian day, is not that different from an Earth day: less than an hour longer. However, a Mars year is almost twice as long as an Earth year.
Mars sol
The average length of a Martian sidereal day is 24 h 37 m 22.663 s (88,642.663 seconds based on SI units), and the length of its solar day is 24 h 39 m 35.244 s (88,775.244 seconds). The corresponding values for Earth are currently 23 h 56 m 4.0916 s and 24 h 00 m 00.002 s, respectively, which yields a conversion factor of 1.0274912517 Earth days/sol: thus, Mars's solar day is only about 2.75% longer than Earth's.
The term "sol" is used by planetary scientists to refer to the duration of a solar day on Mars. The term was adopted during NASA's Viking project (1976) in order to avoid confusion with an Earth "day". By inference, Mars' "solar hour" is of a sol (1 hr 1 min 39 sec), a "solar minute" of a solar hour (61.65 sec), and a "solar second" of a solar minute (1.0275 sec).
When accounting solar days on Earth, astronomers often use Julian dates—a simple sequential count of days—for timekeeping purposes. An analogous system for Mars has been proposed "[f]or historical utility with respect to the Earth-based atmospheric, visual mapping, and polar-cap observations of Mars, ... a sequential count of sol-numbers". This Mars Sol Date (MSD) starts "prior to the 1877 perihelic opposition.
Official source
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