And everything in space spins too. The fact that the Earth spins, the Sun spins and we and every other planet spin around the Sun is something that most of us just take for granted, but, like everything else in our universe, there’s a reason for it. There is a technical expression for this, but I’ll leave that for later because I don’t want to stop you reading and you don’t need a technical term to understand why this happens.
Let’s go back in time, around 5 billion years ago gravity had already pulled together lots of molecules of mostly hydrogen gas into a vast sphere where the heat and pressure made those join, or fuse together and create our star, the Sun, that’s hydrogen fusion. There is a massive cloud of other stuff around the Sun, bits of rock and dust, then over time small bits are drawn together by gravity to become bigger bits and the bigger bits have a little more gravity so they pull more and more bits together until one day you’ve got some planets. This is a massive over simplification of the process, but in essence it’s correct.
Now then. What about the spinning? In what I’ve just described everything is already spinning. Picture the time before all the Sun and planets, just lots of atoms, dust et all in one place and moving about, now imagine two bits of space dust or maybe two pebbles floating together and hitting each other. The chances are that they won’t hit dead on but will clip each other. This clipping make each of the two bits spin in much the same way as a footballer can make a ball swerve around other players into the goal. What both the footballer and the rocks are doing is imparting angular momentum.
Once there are lots of bits of rock spinning, they pass near other debris and the gravitational kick they give starts them spinning too. If you’re thinking that some would spin one way and some the other way you’d be correct but there are always more going one way than the other and over an immense time whatever direction most are going in wins the battle. Yes, the direction of spin in the solar system is the result of pure chance decided a very long time ago.
It’s probably obvious that the larger or more massive something is, the more angular momentum it can pass on to other things near it, so the Sun’s spin will have a vast effect on anything nearby, like the planets which is why they all orbit in the same direction. If you like a bit of simple maths, there’s a formula for this, L= mvr. L is the angular momentum, m=mass v=velocity and r=the radius. I don’t normally mention maths, but this bit is so beautiful because it works with a speck of dust or our whole solar system, and the rest of the known universe.
Of course there are what seems like exceptions. For example whilst all the planets orbit same way and most spin in the same direction there are two oddities. Venus spins on its axis the opposite direction to all the rest and nobody is certain why, but there are some interesting ideas. Lastly Neptune moon Triton is the only large moon in the solar system that orbits backwards, a retrograde orbit as we call it.
Next month I’ll talk a bit more about that.
Charles Oates, Vega Baja Astronomy Group.
● To find out more about observing and astronomy why not join our group, email us at vegabaja.astronomygroup@gmail.com to find out more.
