A “Sol” is the name given to a single solar day on Mars, a unit of time fundamental to space exploration and timekeeping on the Red Planet. The term, which is Latin for “sun,” was adopted by NASA during the Viking missions in the 1970s to avoid confusion with the terrestrial “day.” This distinct measurement is used by planetary scientists and mission control teams to synchronize activities with the Martian cycle of light and darkness. Using the Sol ensures that rovers and landers operate on a schedule consistent with the local environment rather than Earth’s time zones.
Defining the Martian Day
A Sol represents the time it takes for Mars to complete one full rotation relative to the Sun, marking the interval from one noon to the next. This is formally known as a solar day, which governs the daily cycle of sunrise, high noon, and sunset. Mars also has a sidereal day, which is the time it takes for the planet to rotate 360 degrees on its axis with respect to distant stars.
The sidereal day on Mars is approximately 24 hours and 37 minutes long. Because Mars orbits the Sun as it rotates, it must spin slightly further than 360 degrees for the Sun to return to the same position. This makes the solar day longer than the sidereal day. The Sol, or solar day, is the measurement used for all practical timekeeping on the Martian surface.
Sol vs. Earth Day: The Time Difference
The duration of a Sol is 24 hours, 39 minutes, and 35 seconds, making it about 2.75% longer than Earth’s 24-hour solar day. This slight extension occurs because Mars rotates on its axis slightly slower than Earth. This difference of roughly 39 minutes per day is significant enough to cause Earth time and Martian time to rapidly diverge over weeks.
This consistent time difference creates a challenge for human teams controlling Mars missions from Earth. Mission control staff often operate on a Martian-based schedule, sometimes called the “Sol shift,” to keep rovers synchronized with the Martian environment. This means their workday starts about 40 minutes later each Earth day, causing their sleep and work cycles to constantly shift. This daily shift ensures controllers are working during Martian daylight hours, when the rovers are most active.
Why Space Missions Use Sols
Space missions use Sols as their primary time unit because it ties operations directly to the local solar environment on Mars. This synchronization is important for the functionality of the spacecraft, especially those relying on solar power. Using Sols allows mission planners to ensure that activities requiring maximum power, such as driving or collecting data, are scheduled for the brightest part of the Martian day.
The Sol-based schedule also helps manage the rover’s thermal systems, as temperatures fluctuate sharply between day and night. Coordinating activities with the Sol ensures that sensitive instruments avoid the extreme cold of the Martian night and that onboard heaters are used efficiently. Furthermore, using a Sol count (e.g., “Sol 1037”) provides a consistent time reference for all mission logs, independent of Earth’s time zones or calendar systems.