Martian New Year: Mars Year 39

Mars has no official calendar, but planetary scientists have kept one count for a quarter of a century: the Mars Year. Each one begins at Ls 0, the moment the Sun crosses the Martian equator heading north, which is the start of spring in the northern hemisphere. Ls, the solar longitude, is the angle Mars has travelled around the Sun since that equinox, so Ls 90 is the northern summer solstice, Ls 180 the autumn equinox and Ls 270 the winter solstice.

A Mars year lasts 686.97 Earth days, or 668.59 sols (a sol is a Martian solar day, 24 hours 39 minutes 35 seconds). Its seasons are far from equal, because the orbit of Mars is about five and a half times more eccentric than Earth's: the planet races through the part of its orbit near the Sun and dawdles through the far part. Northern spring lasts 193.3 sols and northern autumn only 142.7.

Why does it matter? Mars weather follows its own seasons, which fall on different Earth dates every Mars year. The polar caps grow and shrink, and the great dust storms favour the southern spring and summer, when Mars is closest to the Sun. Comparing one storm season with another, or one orbiter's decade with another's, needs a count that follows the planet. That is what the Mars Year is for.

In 2000, R. Todd Clancy and colleagues proposed that Mars Year 1 begin at the northern spring equinox of 11 April 1955. The starting point is a convention, chosen because it puts the great global dust storm of 1956 inside Mars Year 1. The count caught on, and in 2015 Sylvain Piqueux and colleagues extended it backwards to the first telescopic observations of Mars, so that even nineteenth-century sightings have a Mars Year and a season.

Counting forward from 1955 at 686.97 days a year gives Mars Year 39 for the equinox of 30 September 2026.