How long is one year on Mars, measured in Earth days?

One year on Mars lasts about 687 Earth days. Mars takes this long to complete one orbit around the Sun because it travels on a larger orbit than Earth and moves at a different average speed.

This orbital period is also about 669.6 Martian sols, or solar days. A Martian year is therefore nearly twice as long as an Earth year, although a Martian day is only a little longer than an Earth day.

Mars has seasons because its rotational axis is tilted by about 25.2 degrees, close to Earth’s axial tilt. However, the seasons are unequal in length because Mars has a noticeably elliptical orbit. Southern-hemisphere summers occur near Mars’s closest approach to the Sun and are shorter and warmer than northern summers.

The 687-day figure is an average orbital period, not a calendar used by current human settlements. Future Mars calendars would need to account for both the planet’s long year and its approximately 24-hour-39-minute day.