on Sol 0, corresponding to MSD 51511 (mission time zone); landing occurred around 14:23 LMST (05:14 AMT). Perseverance The Perseverance rover project specified a mission clock that matched Local Mean Solar Time at a planned landing longitude of 77.43°E. This corresponds to a mission clock of AMT+05:09:43. The actual landing site was about 0.02° (1.2 km) east of that, a difference of about 5 seconds in solar time. The local mean solar time is also affected by the rover motion; at 18.4°N, this is about 1 second of time difference for every 234 meters of displacement in the east-west direction. The date is kept using a mission clock sol count with the landing occurring on Sol 0, corresponding to MSD 52304 (mission time zone); landing occurred around 15:54 LMST (10:44 AMT). Summary Years Definition of year and seasons The length of time for Mars to complete one orbit around the Sun in respect to the stars, is its sidereal year, and is about 686.98 Earth solar days (≈ 1.88 Earth years), or 668.5991 sols. Because of the eccentricity of Mars' orbit, the seasons are not of equal length. Assuming that seasons run from equinox to solstice or vice versa, the season Ls 0 to Ls 90 (northern-hemisphere spring / southern-hemisphere autumn) is the longest season lasting 194 Martian sols, and Ls 180 to Ls 270 (northern hemisphere autumn / southern-hemisphere spring) is the shortest season, lasting only 142 Martian sols. As on Earth, the sidereal year is not the quantity that is needed for calendar purposes. Similarly, the tropical year would likely be used because it gives the best match to the progression of the seasons. It is slightly shorter than the sidereal year due to the precession of Mars' rotational axis. The precession cycle is 93,000 Martian years (175,000 Earth years), much longer than on Earth. Its length in tropical years can be computed by dividing the difference between the sidereal year and tropical year by the length of the tropical year. Tropical year length depends on the starting point of measurement, due to the effects of Kepler's second law of planetary motion and precession. There are various possible years including the March (northward) equinox year, June (northern) solstice year, the September (southward) equinox year, the December (southern) solstice year, and the tropical year based on the mean sun. (See March equinox year.) On Earth, the variation in the