Mars Exploration Rover "Spirit" self-portrait, NASA/JPL-Caltech/Cornell
“Do you realize it’s ell ess one-seventy already?” Phyllis said one night. “Didn’t we land at ell ess seven?”. So they had been on Mars for half a Martian year. Phyllis was using the calendar devised by planetary scientists; among the colonists it was becoming more common than the Terran system. Mars’s year was 668.6 local days long, and to tell where they were in this long year it took the Ls calendar. This system declared the line between the sun and Mars at its northern spring equinox to be 0 degrees, and then the year was divided into 360 degrees, so that Ls = 0 – 90 degrees was the northern spring, 90 – 180 degrees the northern summer, 180 – 270 degrees the fall, and 270 – 360 (or 0) degrees the winter.