Mars is one of Earth’s closest planetary neighbors, a world that has captured human imagination for centuries. The rhythm of time on Mars is defined by its orbit around the Sun. Understanding the length of time it takes Mars to complete one full revolution is fundamental to planning future missions. This orbital period establishes the length of a Martian year, which is significantly longer than the year experienced on Earth.
The Length of the Martian Year
The time it takes for Mars to complete one full circuit around the Sun is approximately 687 Earth days. This duration is nearly double the 365.25 days it takes Earth to complete its own orbit, translating to roughly 1.88 Earth years. Scientists refer to this true orbital time as the sidereal period, which is the time required for Mars to return to the same position relative to the distant stars.
The extended year makes the planet’s seasons considerably longer than those on Earth. For instance, a northern hemisphere spring on Mars lasts about 194 Sols, while the shortest season, autumn, lasts around 142 Sols. This difference in seasonal length occurs because the Martian orbit is more elliptical, meaning its speed varies noticeably as it travels around the Sun.
Revolution vs. Rotation: Defining Martian Time
Planetary time is defined by revolution and rotation. Revolution refers to the planet’s movement around the Sun, which determines the length of the year. Rotation is the spin of the planet on its own axis, which determines the length of a day.
A Martian day, known as a “Sol,” is similar to an Earth day, lasting about 24 hours and 39 minutes. The diurnal cycle is only about 40 minutes longer than our 24-hour day. Because the year is 687 Earth days long, it is composed of approximately 668.6 Martian Sols. This near-match in day length allows mission controllers to operate rovers on Mars time.
Orbital Mechanics: Why Mars Moves Slower
The reason Mars takes 687 Earth days to complete its revolution is rooted in orbital mechanics, primarily its greater distance from the Sun. Mars orbits at an average distance of about 1.5 Astronomical Units (AU), roughly 1.5 times farther from the Sun than Earth. This increased distance results in a significantly longer orbital path that the planet must traverse.
The Sun’s gravitational pull decreases rapidly with distance, causing Mars to travel at a slower average orbital velocity than Earth. A planet’s orbital period is proportional to the cube of its average distance from the star, meaning greater distance leads to a disproportionately longer orbital time. Mars’s slower speed combined with its longer path means it requires nearly twice the time to circle the Sun. The planet’s velocity constantly changes throughout its elliptical orbit, speeding up when closer to the Sun and slowing down when farther away.