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 way that scientists mark the time of Mars year is to use solar longitude, abbreviated Ls (read ell sub ess). Ls is 0Â° at the vernal equinox (beginning of northern spring), 90Â° at summer solstice, 180Â° at autumnal equinox, and 270Â° at winter solstice.