What’s this “Sol” thing that we keep talking about when we talk about Mars and the landers on its surface? So, let us discuss “What is a Sol?”
What is a Sol?
Put simply, a Sol is a Martian day. Mars rotates a bit slower than Earth, so a Martian sol is a little longer than an Earth day. Mars rotates once every 24.6229 hours (24 hours, 37 minutes, 23 seconds). So, that is a sol, right? Well, not exactly. You see Earth, itself, rotates once every 23 hours, 56 minutes, 4 seconds. But, that is not what we call a day. A day on Earth is 24 hours. Why the difference? That is because Earth moves around the Sun a bit during one rotation.
So, it takes a little more turning to make a day. This is the difference between a sidereal day (the time of one rotation) versus a solar day (the time from noon to noon). For various reasons, we set out clocks to the solar day. But, even that is a problem, because Earth’s orbit is a little elliptical. That means that it speeds up and slows down as it goes around the Sun. So, the time that it takes to line up with the Sun again varies over the course of the year. The 24 hour day is an average figure over the course of the year.
The same is true for Mars. The 24.6229 hour figure quoted above is a sidereal day. But, during that time, Mars has moved a little way around the Sun. So, that means that Mars has to rotate slightly more than one 360 degree rotation to line up with the Sun again. That means that a solar day, or a sol, is slightly longer than the sidereal day.
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One sol on Mars corresponds to about 360.5° of rotation. That extra half degree of rotation is what it takes to account for Mars’ motion about the Sun during one sol. So, a sol is about 24.697 hours (24 hours, 39 minutes, 35 seconds), or a bit more than two minutes longer than a sidereal day. But, like with Earth, that is an average figure. It is the mean Martian sol, or mean solar day on Mars. The actual solar sol (noon to noon) can be up to about a half minute shorter or longer. That is because Mars’ orbit is quite elliptical, so the distance that it moves around the Sun over a sol varies over the course of the Martian year.
When the scientists working with the landers are making their plans, they have a lot of things to keep in mind. They have to keep track of the sidereal rotation of Mars, together with Earth’s motion around the Sun, to determine when the lander will be on the side of Mars facing Earth. They also have to keep track of Martian sols, in order to know when the Sun is up to power the solar panels of the lander. And, of course, they need to keep up with Earth’s own rotation so they know which side of Earth is facing Mars.
Of course, things are somewhat easier today than it was years ago. We now have a number of orbiters circling Mars. So, if they are in the right position, they can relay signals to and from the landers even if they are on the wrong side of Mars.