The term “sol” is the standard unit of time used by space agencies for robotic missions exploring the Red Planet. This measurement represents the length of a single Martian day, and its use is fundamental for mission planning and communication between Earth-based teams and the hardware. Understanding the sol provides insight into the differences between Earth and Martian timekeeping.
Defining the Sol
A sol is the precise astronomical term for a mean solar day on Mars. It is defined as the interval between two successive times that the Sun returns to the same meridian in the Martian sky. This measurement accounts for both the planet’s rotation on its axis and its movement along its orbit around the Sun.
The concept of a solar day differs from a sidereal day, which is the time it takes for the planet to complete one rotation relative to distant fixed stars. Because Mars moves forward in its orbit as it rotates, it must turn slightly more than 360 degrees for the Sun to return to the same position. This additional rotation time establishes the length of the sol.
Duration in Earth Time
The length of one Martian sol is 24 hours, 39 minutes, and 35.244 seconds of Earth time. This makes a sol approximately 39 minutes and 35 seconds longer than the average 24-hour Earth day. The difference results from Mars having a slightly slower rotation period.
A Martian day is only about 2.75% longer than an Earth day. While the difference is small enough for a person to adjust, it necessitates a distinct time unit for accurate tracking. Roughly 75 Earth days pass for every 73 sols on Mars.
Context and Usage
Space agencies like NASA adopted the term “sol” during the Viking Lander missions in 1976 to eliminate confusion. Using “day” could ambiguously refer to an Earth day or a Martian day, complicating mission logs and scheduling. The word itself is borrowed from the Latin term for “sun.”
Mission calendars are tracked using a sequential numbering system, beginning with Sol 1 when a spacecraft lands on the Martian surface. Mission control teams on Earth often shift their work schedules to operate on “Mars time,” aligning with the planet’s light-dark cycle. This practice ensures that commands are sent during Martian daylight hours when solar-powered vehicles are active and communication windows are open.