Time on Mars

How years are counted on Mars

A Mars year is one full orbit of the Sun: about 668.6 sols, or 687 Earth days. A sol, the Martian solar day, lasts 24 h 39 min 35 s. Mars has no calendar months in practical use, so the position in the year is given by the solar longitude L_s: the angle the planet has travelled along its orbit, measured from the spring equinox of the northern hemisphere. L_s = 0° opens the year, 90° is the northern summer solstice, 180° the autumn equinox, 270° the winter solstice.

The orbit of Mars is fairly eccentric: its eccentricity is about 0.093, against 0.017 for Earth. Its distance from the Sun ranges from about 207 to 249 million km (for Earth, from 147 to 152 million), so at perihelion Mars receives about 45% more sunlight than at aphelion; on Earth the difference is about 7%.

The eccentricity is also why the seasons differ in length. Mars passes aphelion at L_s ≈ 71°, when it moves slowest, and perihelion at L_s ≈ 251°. That is why northern spring lasts about 193 sols and autumn only 143. The south has short, hot summers and long, cold winters.

Year 1 begins on 11 April 1955

The numbering in use today comes from a climatology paper: Clancy and colleagues, Journal of Geophysical Research 105 (E4), 2000, pages 9553–9571. The authors were comparing temperatures of the Martian atmosphere measured in different eras (Mariner 9, Viking, millimetre-wave observations from Kitt Peak, the TES spectrometer on Mars Global Surveyor) and needed labels for the years in order to discuss how one year differs from the next. They defined the year as the interval L_s 0°–360° and set the start of year 1 on 11 April 1955.

In this scheme, Mariner 9 falls in years 9–10, Viking in 12–15, the Phobos probe in 19–20 and Pathfinder in year 23. In the paper's main figure (“Plate 1”), the millimetre measurements are coloured by year, 21 to 24, to show how the same season repeats (or not) from one year to the next.

The paper gives no reason for the chosen date and even calls the convention arbitrary. The Planetary Society notes that year 1 was chosen to include the global dust storm of 1956, widely observed from Earth; the storm began on 20 August 1956, close to perihelion, and the calculation confirms that this date falls in MY 1. Whatever the reason, the label stuck: today people routinely speak of the global dust storm of MY 28 (2007) or of MY 34 (2018), the one that ended the Opportunity rover's mission.

Before 1955: year zero and negative years

Piqueux and colleagues (Icarus, 2015) extended the count back in time: year 0 begins on 24 May 1953, and the years before it get negative numbers. Unlike the Gregorian calendar, this count has a year zero. Their table of equinoxes goes back to about year −184, the early 17th century, when telescopic observation of the planet began.

The prime meridian runs through a crater half a kilometre wide

As on Earth, the prime meridian of Mars is a convention. In the 1830s, Wilhelm Beer and Johann Heinrich Mädler, who drew the first maps of the planet, chose a small dark spot as the origin of longitude; it was later named Sinus Meridiani, the “Meridian Bay”. In 1972, from the images of the Mariner 9 probe, the mission's mapping team fixed the meridian at the centre of a crater about 500 m wide in that region, Airy-0. The larger crater that holds it is named after George Airy, the astronomer who built the Greenwich instrument through which Earth's prime meridian passes. A crater this small is hard to locate precisely, so since 2018 the International Astronomical Union has used the Viking 1 lander as the reference, placed by definition at 47.95137° W, a value chosen so that Airy-0 stays at 0°. MTC is the mean solar time on this meridian; Mars24 also calls it Airy Mean Time, after Greenwich Mean Time.

Mission sols are another clock

Each mission counts its days separately. Sol 0 is the day of landing, reckoned in local mean solar time at the landing site. Curiosity touched down in Gale crater on 6 August 2012, in MY 31, at L_s ≈ 151°, at about 15:03 local time; Perseverance reached Jezero crater on 18 February 2021, in MY 36, at L_s ≈ 6°, at about 15:53. Mars has no time zones: local time differs from MTC by one hour for every 15° of longitude, and each mission keeps its own clock, set by the longitude of its landing site.

Local mean solar time (LMST) runs evenly. Local true solar time (LTST) follows the real Sun, and the difference between them, the equation of time, ranges on Mars from about −51 to +40 minutes, against about −14 to +16 minutes on Earth.