How long does Mars take to orbit the Sun?

A complete orbit of Mars around the Sun takes 686.980 Earth days according to NASA's planetary fact sheet, or 1.881 Earth years. Counted in Martian days, the year is 669.6 sols, equivalent to those same 687 Earth days used in the agency's outreach fact sheets. The figure in sols is lower than in Earth days because the Martian sol is somewhat longer: the planet completes one rotation every 24.6 hours.

The reason lies in the size of the orbit. Mars orbits at an average distance of 228 million kilometers (142 million miles) from the Sun, with a semi-major axis of 227.956 million kilometers, and covers that path at an average orbital speed of 24.08 km/s. A planet farther out has more distance to cover and also travels it more slowly than Earth, so the year is lengthened by both factors at once.

For anyone operating there, the long year isn't just an academic detail. Martian seasons last far longer than Earth's, and they exist because the planet's axis is tilted 25 degrees relative to its orbital plane. On top of that, there's an orbital eccentricity of 0.0935, high for an inner planet: the distance to the Sun changes noticeably between the closest and farthest points, affecting temperature, large dust storms, and the power available to surface vehicles. That's why missions date their observations in Martian years as well as the Earth calendar.