Here on Earth, we have a 24 hour day. That is, if the sun were directly overhead, it would take 24 hours to come back to the same point (known as a ‘solar day,’ a ‘sidereal day’ is slightly shorter). The day the amount of time it takes our planet to rotate once on its axis. The rate of spin is a result of literally billions of years of formation and evolution. Our Earth most likely started out as a small chunk of debris in a large dust field around a relatively young proto-star. Over time, it amassed more material using gravity. Spinning objects react to changes in mass distribution. Think of a figure skater who pulls their arms in to spin faster (you can also have a lot of fun doing this on an office rolley char!).
As the planet grew in the early stages of the solar system, more material fell onto Earth, constantly changing the mass and distribution thereof. The 24 hour day we have now didn’t happen over night. And to complicate the matter further, our day is not a constant number. The Moon has a strong influence on the Earth, at this very moment it is gradually slowing us down. Our day is getting longer! But not very quickly. Earthquakes even have an effect, changing the distribution of mass on Earth, and thus changing its rate of spin.
One can see there are a multitude of different variables that would go into the rate of spin of a planet: density of debris it evolved in, type of material present, amount of impacts in ‘modern’ times, presence of a large moon, amount of tectonic activity, …to name a few. What are the chances, then, that Earth and Mars have a day almost exactly the same length!
Earth day = 24 hours
Martian day = 24 hours and 40 min
Crazy! Mars is about half the size of us, has no large moons, very little active plate tectonics (though it does occur, and definitely HAS occurred in the past), and it has a density of 3.93 g/cm^3 (compared to Earth’s 5.5 g/cm^3). All these differences that could potentially play a large role in the length of day on Mars and yet…only 40 min off (2% different). Interesting.
All the above spawned from me enjoying the Mac app ‘Mars24 Sunclock app‘ that I found while reading Alan Cross’s tweets (@alancross). It is an app that gives you an actual mars clock! telling you what part of the planet is day and what part is night, what the local time is for various places (for instance Curiosity)!It’s 11:09pm EDT for me, but 09:45am for Curiosity. The Mars Exploration Rover Opportunity is in the middle of night at the moment, at 11:35p.