Timekeeping on Mars

Various schemes have been used or proposed for timekeeping on the planet Mars independently of Earth time and calendars. Various schemes have been used or proposed for timekeeping on the planet Mars independently of Earth time and calendars. 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, and its day is about the same length. Its year is almost twice as long as Earth's, and its orbital eccentricity is considerably larger, which means among other things that the lengths of various Martian seasons differ considerably, and sundial time can diverge from clock time more than on Earth. The length of a Martian day is close to that of an Earth day, leading to the use of analogous time units. 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 35.244147 s (88,775.244147 seconds). The corresponding values for Earth are currently 23 h 56 m 4.0916 s and 24 h 00 m 00.002 s, respectively. This yields a conversion factor of 1.02749125170 days/sol. Thus Mars's solar day is only about 2.7% longer than Earth's. The term sol is used by planetary scientists to refer to the duration of a solar day on Mars. The term was adopted during the Viking project in order to avoid confusion with an Earth day. By inference, Mars' 'solar hour' is 1/24th of a sol, and a solar minute 1/60th of a solar hour. 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 2.7% longer than their standard (Earth) durations. 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 the Earth's thick atmosphere and oceans that soften such fluctuations. Consensus as has recently been gained in the scientific community studying Mars to similarly define martian local hours as 1/24th 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 50 minutes slower or 40 minutes faster than a Martian clock (on Earth, the corresponding figures are 14m 22s slower and 16m 23s faster). Mars has a prime meridian, defined as passing through the small crater Airy-0. 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 and one minute apart.) Note that the most widely used standard for specifying locations on Mars uses 'planetocentric coordinates', which measure longitude 0°–360° East and latitude angles from the center of Mars. An alternative used in some scientific literature may use planetographic coordinates, which measure longitudes as 0°–360° West and determined latitudes as mapped onto the surface.