# How are mars years counted?

In this diagram about the methane concentration in the martian atmosphere, there are data points labeled "Mars year 32", "Mars year 33", and "Mars year 34". How are Mars years counted? Is there a "Mars year 0"? What is the event that defines the start of the time scale?

The specific year numbering is an arbitrary convention begun on April 11, 1955 (the northern spring equinox). The solar longitude system is applied to Mars; it's a geometric way of describing where in its orbit a planet is. The solar longitude is denoted $L_2$; on April 11, 1955, $L_s=0^\circ$; it increases to $L_s=360^\circ$, at which point a new Martian year begins.