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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 much more than on Earth. == Time of day == The average length of a Martian sidereal day is 24h 37m 22.663s (88,642.66300 seconds based on SI units), and the length of its solar day (often called a ''sol'') is 88,775.24409 seconds or 24h 39m 35.24409s. The corresponding values for Earth are 23h 56m 4.0916s and 24h 00m 00.002s, respectively. This yields a conversion factor of 1.0274912510 days/sol. Thus Mars' solar day is only about 2.7% longer than Earth's. A convention used by spacecraft lander projects to date has been to keep track of 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, Phoenix, and Mars Science Laboratory missions, the operations team has 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 buffer such fluctuations. Alternative clocks for Mars have been proposed, but no mission has chosen to use such. These include a metric time schema, with "millidays" and "centidays", and an extended day which uses standard units but which counts to 24hr 39m 35s before ticking over to the next 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 14min 22sec slower and 16min 23sec 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 modern standard for measuring longitude on Mars is "planetocentric longitude", which is measured from 0°–360° East and measures angles from the center of Mars. The older "planetographic longitude" was measured from 0°–360° West and used coordinates mapped onto the surface. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Timekeeping on Mars」の詳細全文を読む スポンサード リンク
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