Sols on Mars
Published on May 27, 2008 at 12:48 am.
9 Comments.
Filed under Mars, time.
The Phoenix has been on Mars for one sol. Its nominal lifetime is 90 sols. So, what’s this “sol” thing that we keep talking about when we talk about Mars and the landers on its surface?
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.
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 powering 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.
-Astroprof
Ed Davies on May 27, 2008 at 7:29 am: 1
“One sol on Mars corresponds to about 365.5° of rotation.”
Oops, I think you mean 360.5°. Thinking of Earth years, perhaps.
Astroprof on May 27, 2008 at 8:31 am: 2
Thanks, Ed, for catching my typo! I fixed it.
Mark Snow on May 27, 2008 at 12:09 pm: 3
Is there a convenient, and visual way, that students can keep track of the sidereal and solar days for a planet? What would you suggest?
Smiles.
HP on May 27, 2008 at 5:20 pm: 4
Do astronomers divide a planet’s sol into subunits? And if so, do you follow the 24:60:60 pattern, or go decimal?
In other words, do you work with a Martian hour of 1/24 sol, or 1.029 terrestrial hours, or would you express that as 0.04 Sol? (I think my math is right…)
Astroprof on May 27, 2008 at 6:35 pm: 5
Mark, there might be something out there, but I don’t know what. I am also not sure just how visual you’d like. Would a simple table or chart do? It might be better (and certainly more exciting) to have a planet rotating and slowly going through its diurnal day/night cycle, too. That would involve a bit of code writing, though. You could come up with something a visual representation for students using Starry Night or some other planetarium software, as well. You could have them measure the time it takes for a star to return to the same part of the sky and the time that it takes the Sun to do the same thing.
Astroprof on May 27, 2008 at 6:39 pm: 6
HP, it is done both ways. Sometimes it is simply a digital value: 2.3342 sols. And, sometimes it is 4 sols 14 hours 33 minutes. In the latter case, it is often conventional to divide one sol into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. That means that one mean sol is 86400 planetary seconds. Since a Martial sol is a bit longer than one Earth day, that means that Martian hours, minutes, and seconds are correspondingly longer than Earth hours, minutes, and seconds.
Lifeboat Foundation Blog » Carnival Of The Space Geeks (Mars, Stars And Life From Afar?) on May 29, 2008 at 1:47 am: 7
[…] For those of you wondering why scientists are “jumping for joy” over the dusty polygons, the Martian Chronicles will enlighten you, while the mysterious Professor known as AstroProf gives a short lesson about Martian days. […]
Anthony on January 29, 2010 at 2:22 pm: 8
Hi all scientists!
I would like to know, whether the
day of landing on Mars must be called
“sol 0″ or “sol 1″ !
Both denominations can be found in
science books, but which one is right?
Please answer to my this address:
cybercat@abacho.de
T h a n k s !
Yours
Anthony
Boomtown Rat on February 10, 2010 at 10:48 am: 9
Naturally the right denomination is:
sol 0