lengths of the tropical years is small, with the mean time from June solstice to June solstice being about a thousandth of a day shorter than that between two December solstices, but on Mars it is much larger because of the greater eccentricity of its orbit. The northward equinox year is 668.5907 sols, the northern solstice year is 668.5880 sols, the southward equinox year is 668.5940 sols, and the southern solstice year is 668.5958 sols (0.0078 sols more than the northern solstice year). (Since, like Earth, the northern and southern hemispheres of Mars have opposite seasons, equinoxes and solstices must be labelled by hemisphere to remove ambiguity.) Seasons begin at 90 degree intervals of solar longitude (Ls) at equinoxes and solstices. Year numbering 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 (Ls 0) that occurred on April 11, 1955, labeling that date the start of 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. Although Clancy and co-authors described the choice as "arbitrary", the great dust storm of 1956 falls in MY1. This system has been extended by defining Mars Year 0 (MY0) as beginning May 24, 1953, and so allowing for negative year numbers. Martian calendars 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. 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