May 12, 2014
Ask The Scientist – Why Does The Earth Rotate?
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
This article is the latest installment in a new series where redOrbit's in-house experts will answer questions submitted by you, the reader. Got a science or space question that's stumping you? Each week we'll select a handful of the wiliest questions you can whip up to tease the brains of our resident gurus (we like call them ‘geeks’).
“Why does earth rotate? And if it is for conserving angular momentum, then why doesn't its speed slow down with time?” – Anjum N.
This is a common question, one that I get nearly every semester from my astronomy students. And, of course, our everyday experiences tell us that in order to cause something to rotate, a force must be applied – like spinning a nickel on a table. Also, over time – in most cases very quickly – the object slows down and comes to rest.
So, how is it that the Earth began to rotate? And, perhaps more puzzling, why is the Earth still rotating billions of years later?
The simple answer to this quandary, as our question proposer guessed, is a concept in physics known as Conservation of Angular Momentum. Imagine an ice skater spinning on the ice with her arms extended. As she then pulls her arms into her side, the rate of her rotation increases. This is the conservation law in action.
Now, let’s imagine a large interstellar gas cloud. We see these all over our galaxy, some isolated and small. Others extend hundreds of light-years across or more.
For our purposes, we will imagine one a few light-years in diameter, mainly composed of hydrogen and helium and trace amounts of other elements. The individual molecules of gas are moving around in a nearly random fashion. If we add up this motion, we will find that the cloud, overall, has a non-zero momentum and angular momentum. While it is possible that the system is completely at rest, this is highly unlikely in general.
Now, if this cloud is suddenly disturbed – say by a nearby supernova event or passing star – the cloud may start to collapse. As it does the cloud begins to shrink, becoming more dense. Consequently, the cloud begins to rotate more rapidly, just as our ice skater did when she pulled her arms in.
Even if the initial rotation speed of the gas cloud was low, the immense compression as the cloud begins forming into a solar system is enough to install a considerable rotation. Over hundreds of millions of years, the central part of the cloud forms the central star – our Sun – while the rest of the disk supplies the material that will form the planets. And, as one would expect, the co-rotation of the disk is why all of the planets orbit in the same direction around the Sun.
As we focus in on the individual planets, like Earth, we notice that all of the planets orbit to at least some degree. Again, this is due to the overall angular momentum of the system being conserved. Every piece of matter that went into forming the Earth each had its own particular angular momentum, and as they all became joined together they contributed to the “spin” of Earth.
In the early days of formation there were other things that could have added to the rotation we see today: impacts from large planetoids, and even asteroids or comets. Today, the spin of the Earth is more consistent, since there is little to affect its motion.
The reason a spinning top slows down and eventually stops is because of friction and drag, sapping the rotational energy of the top and bringing it to rest. But the Earth does not experience significant drag from the solar system.
Of course, there are things like the Moon’s gravity, the drag of the magnetic field through interstellar space and the solar wind, which can slow the Earth down. But these effects are small in comparison to the rotational energy of Earth.
So there you have it: the reason that the Earth orbits the Sun and spins on its axis is due to conservation of angular momentum. And while there exist natural ways in which this motion can be altered, the Earth won’t stop spinning anytime soon.