Timekeeping on Mars
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.
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 , and the length of its solar day is . The corresponding values for Earth are currently Sidereal time| and, respectively, which yields a conversion factor of 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 in order to avoid confusion with an Earth "day". By inference, Mars's "solar hour" is of a sol, a "solar minute" of a solar hour, and a "solar second" of a solar minute.
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 "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 starts "prior to the 1877 perihelic opposition." Thus, the MSD is a running count of sols since 29 December 1873. Numerically, the Mars Sol Date is defined as, 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 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, 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 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 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.
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.
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.
Coordinated Mars Time
Coordinated Mars Time or Martian Coordinated Time is a proposed Mars analog to Universal Time on Earth. It is defined as the mean solar time at Mars's prime meridian. The name "MTC" is intended to parallel Earth's Coordinated Universal Time, but this is somewhat misleading: what distinguishes UTC from other forms of UT is its leap seconds, but MTC does not use any such scheme. MTC is more closely analogous to UT1.Use of the term "Martian Coordinated Time" as a planetary standard time first appeared in a journal article in 2000. The abbreviation "MTC" was used in some versions of the related Mars24 Sunclock coded by the NASA Goddard Institute for Space Studies. That application has also denoted the standard time as "Airy Mean Time", in analogy of Greenwich Mean Time. In an astronomical context, "GMT" is a deprecated name for Universal Time, or sometimes more specifically for UT1.
Neither AMT or MTC has yet been employed in mission timekeeping. This is partially attributable to uncertainty regarding the position of Airy-0, which meant that AMT could not be realized as accurately as local time at points being studied. At the start of the Mars Exploration Rover missions, the positional uncertainty of Airy-0 corresponded to roughly a 20-second uncertainty in realizing AMT. In order to refine the location of the prime meridian, it has been proposed that it be based on a specification that the Viking Lander 1 is located at 47.95137°W.
Lander mission clocks
When a NASA spacecraft lander begins operations on Mars, the passing Martian days are tracked using a simple numerical count. The two Viking mission landers, Mars Phoenix, the Mars Science Laboratory rover Curiosity, InSight, and Mars 2020 Perseverance missions all count the sol on which the lander touched down as "Sol 0". Mars Pathfinder and the two Mars Exploration Rovers instead defined touchdown as "Sol 1".Each successful lander mission so far has used its own "time zone", corresponding to some defined version of local solar time at the landing site location. Of the nine successful NASA Mars landers to date, eight employed offsets from local mean solar time for the lander site while the ninth used local true solar time.
Information as to whether China's Zhurong rover project has used a similar timekeeping system of recording the sol number and LMST has not been disseminated.
''Viking'' Landers
The "local lander time" for the two Viking mission landers were offsets from LMST at the respective lander sites. In both cases, the initial clock midnight was set to match local true midnight immediately preceding touchdown.''Pathfinder''
Mars Pathfinder used the local apparent solar time at its location of landing. Its time zone was AAT−02:13:01, where "AAT" is Airy Apparent Time, meaning apparent solar time at Airy-0. The difference between the true and mean solar time is the Martian equation of time.Pathfinder kept track of the days with a sol count starting on Sol 1, on which it landed at night at 02:56:55.
''Spirit'' and ''Opportunity''
The two Mars Exploration Rovers did not use mission clocks matched to the LMST of their landing points. For mission planning purposes, they instead defined a time scale that would approximately match the clock to the apparent solar time about halfway through the nominal 90-sol primary mission. This was referred to in mission planning as "Hybrid Local Solar Time" or as the "MER Continuous Time Algorithm". These time scales were uniform in the sense of mean solar time and were not adjusted as the rovers traveled. The HLST of Spirit is AMT+11:00:04 whereas the LMST at its landing site is AMT+11:41:55. The HLST of Opportunity is AMT−01:01:06 whereas the LMST at its landing site is AMT−00:22:06. Neither rover was likely to ever reach the longitude at which its mission time scale matches local mean time. However, for atmospheric measurements and other science purposes, Local True Solar Time is recorded.Spirit and Opportunity both started their sol counts with Sol 1 on the day of landing, corresponding to MSD 46216 and MSD 46236, respectively.