Though no standard exists, numerous calendars and other timekeeping approaches have been proposed for the planet Mars. The most commonly seen in scientific literature denotes the time of year as the number of degrees from the northern vernal 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. A sol, or Martian day, is less than an hour longer than an Earth day. A Mars year is almost twice as long as Earth's, though its orbital eccentricity is considerably larger, which causes those seasons to vary significantly in length.
