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A Martian year is approximately 668.6 sols, equivalent to approximately 687 Earth days [1] or 1.88 Earth years. The sol was adopted in 1976 during the Viking Lander missions and is a measure of time mainly used by NASA when, for example, scheduling the use of a Mars rover .
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 day (22 s) due to uncertainty in the exact geographical position of the prime meridian at Airy-0 crater.
On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the Sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the Sun is overhead again (1→3 = one solar day). More simply, 1→2 is a complete ...
A mean solar day (what we normally measure as a "day") is the average time between local solar noons ("average" since this varies slightly over a year). Earth makes one rotation around its axis each sidereal day; during that time it moves a short distance (about 1°) along its orbit around the Sun.
It is based on the product of the count of Earth days past and the ratio of the length of Earth's solar day length (86400 seconds) with a Mars solar day (88775.24409 seconds), rounded to the nearest sol. That ratio comes from technical notes on time keeping on Mars from NASA's Goddard Spaceflight Center. [1] To use, type:
Mars's average distance from the Sun is roughly 230 million km (143 million mi), and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. [185] A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours. [2]
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The Mars Julian sol count is analogous to the Julian Day count on Earth, in that it is a continuous numerical counting of days from an epoch. The Mars Julian sol epoch is the same as for the Darian calendar, thus Mars Julian sol 0 is 1 Sagittarius 0.