Numerically, the Mars Sol Date is defined as MSD = (Julian Date using International Atomic Time - 2451549.5 + k)/1.02749125 + 44796.0, where k is a small correction of approximately 1⁄ 4000 d (or 21.6 s) due to uncertainty in the exact geographical position of the prime meridian at Airy-0 crater.Ī convention used by spacecraft lander projects to date has been to enumerate local solar time using a 24-hour 'Mars clock' on which the hours, minutes and seconds are 2.75% longer than their standard (Earth) durations. This Mars Sol Date (MSD) starts 'prior to the 1877 perihelic opposition.' Thus, the MSD is a running count of sols since 29 December 1873 (coincidentally the birth date of astronomer Carl Otto Lampland). An analogous system for Mars has been proposed 'or historical utility with respect to the Earth-based atmospheric, visual mapping, and polar-cap observations of Mars., a sequential count of sol-numbers'.

When accounting solar days on Earth, astronomers often use Julian dates-a simple sequential count of days-for timekeeping purposes. By inference, Mars' 'solar hour' is 1⁄ 24 of a sol (1 hr 1 min 39 sec), and a 'solar minute' 1⁄ 60 of a solar hour (61.65 sec) 'solar second' 1⁄ 60 of a solar second (1.0275 sec). The term was adopted during NASA's Viking project (1976) in order to avoid confusion with an Earth 'day'. The term ' sol' is used by planetary scientists to refer to the duration of a solar day on Mars.

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 1.0 Earth days/sol: thus, Mars's solar day is only about 2.75% longer than Earth's. 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).

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