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. Mars Pathfinder used the local apparent solar time at its location of landing. For mission planning purposes, they defined a time scale that would approximately match the clock to the apparent solar time about halfway through the nominal sol primary mission.
The time scales are uniform in the sense of mean solar time i. The rovers traveled distances that could make a few seconds difference to local solar time. Neither rover would likely to ever reach the longitude at which its mission time scale matches local mean time. For science purposes, Local True Solar Time is used. The Mars Phoenix project specified timekeeping that matched Local Mean Solar Time at the planned landing longitude of This corresponds to a mission clock of AMT The actual landing site was at The length of time for Mars to complete one orbit around the Sun is its sidereal year , and is about Because of the eccentricity of Mars' orbit, the seasons are not of equal length.
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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, Martian years , 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.
Tropical year length depends on the starting point of measurement, due to the effects of Kepler's second law of planetary motion. It can be measured in relation to an equinox or solstice , or can be the mean of various possible years including the March northward equinox year, June northern solstice year, the September southward equinox year, the December southern solstice year, and other such years.
The Gregorian calendar uses the March equinox year. On Earth, the variation in the lengths of the tropical years is small, but on Mars it is much larger. The northward equinox year is Averaging over an entire orbital period gives a tropical year of Since, like Earth, the northern and southern hemispheres of Mars have opposite seasons, equinoxes and solstices must be labelled by hemisphere to remove ambiguity.
For purposes of enumerating Mars years and facilitating data comparisons, a system increasingly used in the scientific literature, particularly studies of Martian climate, enumerates years relative to the northern spring equinox L s 0 that occurred on April 11, , labeling it a Mars Year 1 MY1. The system was first described in a paper focused on seasonal temperature variation by R.
Todd Clancy of the Space Science Institute. This system has been extended by defining Mars Year 0 MY0 as beginning May 24, , and so allowing for negative year numbers. Seasons begin at 90 degree intervals of solar longitude Ls at equinoxes and solstices. Long before mission control teams on Earth began scheduling work shifts according to the Martian sol while operating spacecraft on the surface of Mars, it was recognized that humans probably could adapt to this slightly longer diurnal period.
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This suggested that a calendar based on the sol and the Martian year might be a useful timekeeping system for astronomers in the short term and for explorers in the future. For most day-to-day activities on Earth, people do not use Julian days , as astronomers do, but the Gregorian calendar , which despite its various complications is quite useful. It allows for easy determination of whether one date is an anniversary of another, whether a date is in winter or spring, and what is the number of years between two dates. This is much less practical with Julian days count.
For similar reasons, if it is ever necessary to schedule and co-ordinate activities on a large scale across the surface of Mars it would be necessary to agree on a calendar. American astronomer Percival Lowell expressed the time of year on Mars in terms of Mars dates that were analogous to Gregorian dates, with 20 March, 21 June, 22 September, and 21 December marking the southward equinox, southern solstice, northward equinox, and northern solstice, respectively; Lowell's focus was on the southern hemisphere of Mars because it is the hemisphere that is more easily observed from Earth during favorable oppositions.
Lowell's system was not a true calendar, since a Mars date could span nearly two entire sols; rather it was a convenient device for expressing the time of year in the southern hemisphere in lieu of heliocentric longitude, which would have been less comprehensible to a general readership.
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Italian astronomer Mentore Maggini 's book describes a calendar developed years earlier by American astronomers Andrew Ellicott Douglass and William H. Pickering , in which the first nine months contain 56 sols and the last three months contain 55 sols. Their calendar year begins with the northward equinox on 1 March, thus imitating the original Roman calendar. Other dates of astronomical significance are: northern solstice, 27 June; southward equinox, 36 September; southern solstice, 12 December; perihelion, 31 November; and aphelion, 31 May.
Pickering's inclusion of Mars dates in a report of his observations may have been the first use of a Martian calendar in an astronomical publication. In , when the calendar reform movement was at its height, American astronomer Robert G. Aitken published an article outlining a Martian calendar. In each quarter there are three months of 42 sols and a fourth month of 41 sols. The pattern of seven-day weeks repeats over a two-year cycle, i.
Whereas previous proposals for a Martian calendar had not included an epoch, American astronomer I. Levitt developed a more complete system in In fact, Ralph Mentzer, an acquaintance of Levitt's who was a watchmaker for the Hamilton Watch Company, built several clocks designed by Levitt to keep time on both Earth and Mars.
They could also be set to display the date on both planets according to Levitt's calendar and epoch the Julian day epoch of BCE. Charles F. Capen included references to Mars dates in a Jet Propulsion Laboratory technical report associated with the Mariner 4 flyby of Mars. This system stretches the Gregorian calendar to fit the longer Martian year, much as Lowell had done in , the difference being that 20 March, 21 June, 22 September, and 21 December marks the northward equinox, northern solstice, southward equinox, southern solstice, respectively.
Leovy et al. British astronomer Sir Patrick Moore described a Martian calendar of his own design in His idea was to divide up a Martian year into 18 months. Lisa Randall. Spooky Action at a Distance.
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