<P> On Earth, astronomers often use Julian Dates--a simple sequential count of days--for timekeeping purposes . "For historical utility with respect to the Earth - based atmospheric, visual mapping, and polar - cap observations of Mars..., a sequential count of sol - numbers" the Mars Sol Date (MSD) was proposed starting "prior to the 1877 perihelic opposition ." This Mars Sol Date (MSD), is a running count of sols since December 29, 1873 (the birth date of astronomer Carl Otto Lampland). Another proposal suggests a start date (or epoch) in the year 1608 (invention of the telescope). Either choice is intended to ensure that all historically recorded events related to Mars occur after it . The Mars Sol Date is defined mathematically as MSD = (Julian Date using International Atomic Time - 2451549.5 + k) / 1.02749125 + 44796.0, where k is a small correction of approximately 0.00014 d (or 12 s) due to uncertainty in the exact geographical position of the prime meridian at Airy - 0 crater . </P> <P> The word "yestersol" was coined by the NASA Mars operations team early during the MER mission to refer to the previous sol (the Mars version of "yesterday"), and came into fairly wide use within that organization during the Mars Exploration Rover Mission of 2003 . It was eventually picked up and used by the press . Other neologisms include "tosol" (for "today" on Mars), as well as one of three Mars versions of "tomorrow": "nextersol", "morrowsol", or "solmorrow". NASA planners coined the term "soliday" at least as far back as 2012 to refer to days off due to time phasing or the synching of planetary schedules . </P> <P> The length of time for Mars to complete one orbit around the Sun is its sidereal year, and is about 686.98 Earth solar days, 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 L 0 to L 90 (northern - hemisphere spring / southern - hemisphere autumn) is the longest season lasting 194 Martian sols, and L 180 to L 270 (northern hemisphere autumn / southern - hemisphere spring) is the shortest season, lasting only 142 Martian sols . One commonly used system in the scientific literature denotes year number relative to Mars Year 1 (MY1) beginning with the northern Spring equinox of April 11, 1955 . </P> <P> As on Earth, the sidereal year is not the quantity that is needed for calendar purposes . Rather, the tropical year would be likely to 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 . </P>

What is the length of year on mars