The Mars dust storm season begins just after perihelion at around Ls = 260°. Mars is at aphelion (its greatest distance from the Sun, 249 million kilometers, where it moves most slowly) at Ls = 70°, near the northern summer solstice, and at perihelion (least distance from the Sun, 207 million kilometers, where it moves fastest) at Ls = 250°, near the southern summer solstice. By contrast, Mars' elliptical orbit makes its distance from the Sun change with time, and also makes it speed up and slow down in its orbit.
On Earth, spring, summer, autumn, and winter are all similar in length, because Earth's orbit is nearly circular, so it moves at nearly constant speed around the Sun. Ls is 0° at the vernal equinox (beginning of northern spring), 90° at summer solstice, 180° at autumnal equinox, and 270° at winter solstice. The way that scientists mark the time of Mars year is to use solar longitude, abbreviated Ls (read "ell sub ess"). Unlike on Earth, there is no leisurely-orbiting moon to give Mars "months," and while there have been many imaginative calendars suggested for Mars, none is in common use. The time of day, Local Solar Time, depends upon the lander's longitude on Mars. Each landed Mars mission keeps track of "Local Solar Time," or LST, at its landing site, because Local Solar Time relates directly to the position of the Sun in the sky and thus the angle from which camera views are illuminated. For convenience, sols are divided into a 24-hour clock.
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