Why are all the planets spinning in the Solar System?

Earth and Moon

“I live on a big round ball
I never do dream I may fall
And even one day if I do
Well I'll jump up and smile back at you.

I don't even know where we are
They tell me we're circling a star
Well I'll take their word I don't know
But I'm dizzy so it may be so.”

Lyrics of “Defying Gravity” by Jimmy Buffett


We have been watching the skies for over 10,000 years and though our smarter ancestors could figure out the rhythms of the Sun, Moon and stars, they often though up some inventive reasons why things rose, moved across the sky and set below the horizon. Eventually, we figured out that we lived on a planet which both spun around, and moved in an orbit around the Sun. It was all a bit unsettling at first but over time we adapted to that bit of knowledge. Then, just as we thought we had it all figured out, astronomers, and astrophysicists figured out that everything in the universe was spinning and revolving around things. Rather than tackle the whole universe at once, we'll tackle why the Sun and planets in our Solar System are spinning.

Now, don't get too worked up (or in Jimmy Buffett's case, dizzy,) over this spinning business, because Earth's rotation makes our world a nice place to live. Rotation helps keep Earth's temperatures at a level we can live with, heating by the Sun during the day and cooling the temperatures at night. How fast does Earth spin? Surprisingly fast – over 1609 km/hr (about 1000 mi/hr) at the equator. That might sound pretty dizzying in itself, but keep in mind the Earth is pretty big, and even at that speed it still takes about 24 hours for us to make one complete turn on our axis.

Jupiter

We said that all the planets are spinning along with the Sun, so it's interesting to make a brief comparison with some of the other planets. Jupiter is the largest planet in the Solar System and is just over eleven times the diameter of the Earth. Not only is Jupiter the biggest planet in the Solar System, but it also has the shortest day, just over 10 hours! That works out to an average rotational speed of about 43,000 km/hr, (28,273 mi/hr). Next, look at Venus, almost exactly the same size as Earth, but spinning so slow it takes poor Venus 244 days just to make one rotation. Can you imagine a single day on Venus lasting 244 of our Earth days? In addition, Venus spins the opposite way of Earth.

So we've got planets spinning at different speeds and in different directions – but why do they spin at all? To understand this, we have to look back in time to when the Solar System first formed almost five billion years ago. Our Sun, planets and moons all formed from an immense cloud of gas and dust around a young Sun. The Sun's gravitational orbit caused the cloud to collapse and spin. The clumps of dust which began to collect in the cloud were also set spinning.

As the cloud began to flatten out, it began to spin faster as gravity made it smaller. The clumps also began to collapse on themselves, making them spin faster and faster. The law of physics which explains this is called the conservation of angular momentum. Another example of how this law works is an ice skater who spins faster as she pulls her arms in closer to her body.

For quite awhile, it was a really violent place in this early Solar System. The clumps of dust grew larger as they collided and gravity caused them to pull inward on themselves from all directions, which is how planets and some moons became round. Side-swipe collisions between objects would have made an object spin faster or slower and could have even changed an object's direction of spin. We think that a large object collided with the Earth making it spin even faster, making a day on the early Earth as short as six hours!

So now we know how the planets and other objects got spinning, but why do they continue spinning after almost five billion years? That's another physics thing – inertia. This law states that an object in motion will stay in motion unless something causes it to slow down. The planets keep spinning because in most cases there's very little to slow them down. Now Mercury is so close to the Sun, that the Sun's gravity is slowing little Mercury down.

We know that Earth's rotation is slowing at a rate of about one millisecond per year. That's not much, but about 70 million years ago, when the dinosaurs roamed Earth, a day was about 22 hours long. And millions of years in Earth's future, our day will be longer, around 26 hours per day, finally granting that wish to those folks who plead for more hours in a day! But it's not the Sun slowing Earth's spin down, but rather the interaction between Earth and Moon.

The Moon's gravity causes the ocean's tides to rise and fall The Sun also affects the tides, but because it is so much farther from us than the Moon, the Sun has less of an effect. While the Moon's gravity is pulling at us, it causes friction between the changing tides and the spinning Earth. The result is that the Earth's rotation slows down ever so much.

An interesting side note to this spin business is that the Moon's tidal pull, besides slowing Earth's rotation, is causing the Moon to move away from the Earth at a rate of one millimeter per year. In Earth's past – the Moon was closer and therefore appeared much larger in our skies, and far into our future the Moon will be much smaller in our skies. How nice that we live in a time when the Moon appears the same size as the Sun in our skies, making for great solar eclipses!

The thought of Earth's rotation slowing down begs the question of what will happen in the really far future of the Earth and Moon. Well billions of years from now the Moon will have moved much farther away and that constant pull of the Sun and Moon on the Earth will have ceased it's affect on the Earth's spin. It's quite likely that the Earth's rotation will slow to one rotation every 365 days, a condition we call synchronous rotation with the Sun. When this happens each half of the Earth will have permanent day or night all year.

If the Earth stopped spinning altogether, each half of the Earth would have half a year of daytime and half a year of night. This six months of day or night would cause temperatures to be far hotter or colder than they are now. Wind circulation would also change, circulating from the equator towards the poles rather than parallel to the equator. That might not sound too horrible, but the worst effect is the Van Allen radiation belts would likely disappear and that would mean no more protection from cosmic rays and other particles. That effect would be a dire threat to life on Earth.

Now of course, and thanks to the movie “The Day the Earth Stood Still” you might be curious as to what would really happen if the Earth really did suddenly stop spinning instead of slowing down gradually. Remember that our planet is spinning at about 1000 miles per hour. If our planet suddenly stopped rotating, the atmosphere would still be in motion at that speed. The winds would be worse than any hurricane you could imagine, sweeping away most everything on land, rocks, buildings and life forms. It's a very scary thought!

The ancient Egyptians saw a direct connection between life on Earth and the rhythmic order of the motions of the Sun, Moon and stars. We've applied physics and explained all those motions and we understand why planets spin, but there's eloquence in the ancient astronomer's basic ideas too.

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Kathy Miles, Author, and Chuck Peters, Systems Administrator
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