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.
Orbit of Mars
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's 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.
Sols
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 per sol: thus, Mars's solar day is only about 2.75% longer than Earth's; approximately 73 sols pass for every 75 Earth days. 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's "solar hour" is 1⁄24 of a sol (1 h 1 min 39 s), a "solar minute" 1⁄60 of a solar hour (61.65 seconds), and a "solar second" 1⁄60 of a solar minute (1.0275 seconds).
Mars Sol Date 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." Thus, the MSD is a running count of sols since 29 December 1873 (coincidentally the birth date of astronomer Carl Otto Lampland). Numerically, the Mars Sol Date is defined as MSD = JD − 2451549.5/1.0274912517 + 44796.0 − 0.0009626, where JD is the Julian Date using Terrestrial Time.
Time of day
A 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 has the advantage that no handling of times greater than 23:59 is needed, so standard tools can be used. The Mars time of noon is 12:00 which is in Earth time 12 hours 20 minutes after midnight. For the Mars Pathfinder, Mars Exploration Rover (MER), Phoenix, and Mars Science Laboratory missions, the operations teams have worked on "Mars time", with a work schedule synchronized to the local time at the landing site on Mars, rather than the Earth day. This results in the crew's schedule sliding approximately 40 minutes later in Earth time each day. Wristwatches calibrated in Martian time, rather than Earth time, were used by many of the MER team members. Local solar time has a significant impact on planning the daily activities of Mars landers. Daylight is needed for the solar panels of landed spacecraft. Its temperature rises and falls rapidly at sunrise and sunset because Mars does not have Earth's thick atmosphere and oceans that soften such fluctuations. Consensus has recently been gained in the scientific community studying Mars to similarly define Martian local hours as 1⁄24 of a Mars day. As on Earth, on Mars there is also an equation of time that represents the difference between sundial time and uniform (clock) time. The equation of time is illustrated by an analemma. Because of orbital eccentricity, the length of the solar day is not quite constant. Because its orbital eccentricity is greater than that of Earth, the length of day varies from the average by a greater amount than that of Earth, and hence its equation of time shows greater variation than that of Earth: on Mars, the Sun can run 51 minutes slower or 40 minutes faster than a Martian clock (on Earth, the corresponding figures are 14 min 22 s slower and 16 min 23 s faster). Mars has a prime meridian, defined as passing through the small crater Airy-0. The prime meridian was first proposed by German astronomers Wilhelm Beer and Johann Heinrich Mädler in 1830 as marked by the fork in the albedo feature later named Sinus Meridiani by Italian astronomer Giovanni Schiaparelli. This convention was readily adopted by the astronomical community, the result being that Mars had a universally accepted prime meridian half a century before the International Meridian Conference of 1884 established one for Earth. The definition of the Martian prime meridian has since been refined on the basis of spacecraft imagery as the center of the crater Airy-0 in Terra Meridiani. However, Mars does not have time zones defined at regular intervals from the prime meridian, as on Earth. Each lander so far has used an approximation of local solar time as its frame of reference, as cities did on Earth before the introduction of standard time in the 19th century. (The two Mars Exploration Rovers happen to be approximately 12 hours 1 minute apart.) Since the late 1990s and arrival of Mars Global Surveyor at Mars, the most widely used system for specifying locations on Mars has been planetocentric coordinates, which measure longitude 0°–360° East and latitude angles from the center of Mars. An alternative system that was used before then is planetographic coordinates, which measure longitudes as 0°–360° West and determined latitudes as mapped onto the surface. However, planetographic coordinates remain in use, such as on the MAVEN orbiter project.
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