Why Are Planets Round?

[SARGE]

Something been bothering me for the longest. Why are all the planets (and moons) round, in almost perfect form, rather than some odd-shaped piece of matter? And what is it that keeps all those things in motion and what started them in motion from the get-go? The earth is just the exact distance from the sun to keep us from freezing or burning up and the moon causes all the tides, etc. Everything is moving so fast yet it seems to stay the same distance with acceptable variations of course. Something started all this and was just wondering why everything seems to be round in shape, despite theories that say there were collisions, especially between the earth and moon. Weird...

[sgtspector]

I'm no physicist but I think its because the gravity of the planet or other body causes it to pull everything to the center and the orb is the smallest shape it can take when pulled toward the center. Kind of like when you make a snowball or a meatball. But I think we will have to wait to hear from all the Hawking Jr's out there before we have a decent answer.

[HAL9000]

That's pretty much it... as matter compresses, the gravitational pull keeps everything a uniform distance from the center.

As for orbits... it was for me, one of the most confusing statements to understand, but once I understood it, it made perfect sense... we'll take the moon for example... the moon is actually constantly falling towards the earth, but its forward speed keeps it from ever reaching us.

[Lespaul20]

I second the above post. I think gravity pulls from all directions in a consistent fashion so a sphere is the shape it takes.

[LeftyAce]

If you get really curious about physics, and have $60 to spare, get 'The Feynman Lectures on Physics' by Richard Feynman....3 volume set of very readable physics lectures explaining everything from gravity to quantum mechanics, etc.

And I second the statement (or third, at this point) about why the planets are round.

[RevCLB]

Write a book!

[TwoRails]

Agreed here, too, but just to stir the pot a little: why are asteroids odd shaped, like a potato for one example, when they have gravity, too? And not all orbits are close to being circular.

[sdkfz]

asteroids are broken bits of solid matter, our planet is a cooled blob on completly liquid matter that is still gooey in the middle. Gas giants are not solid except for in the middle and even there the oustide is condensed gas ranging for viscous to gooey to solid with some actual ricky core.

[LeftyAce]

Orbits are all eliptical, as per conservation of angular momentum (and Kepler's law, which describes how the planets conserve angular momentum). You can have many possible stable elliptical orbits for a given planet, but there is only 1 radius/speed combination which will keep a given mass in orbit. Circular orbits are much harder to achieve, and thus almost never occur in nature.

[mbossman2]

the Earth isn't a perfect sphere, it "bulges" along the equator and is "flattened" at the poles.

[daveyp225]

Perfect spheres or any of the "nice" shapes we like to think about for that matter do not exist in the natural world at all. For example the earth's orbit does not form an ellipse (that is, a perfect ellipse)... that would only be true in the absence of all other forces... As usual, our minds like to simplify things that encompas a range of ideas... but it is useful for our purposes to use this idea to model the planets orbits because its "almost right".

[SARGE]

All I read and see are theories about how all this works. The consensus now is that our Moon is actually a piece of Earth that got knocked off from some collision. One would think that it would be a piece of junk, yet is round. Are all planets and moons in the universe possessed of a gravitational thing that makes them all so round? Ain't arguing the sphere thing as for as being "perfectly round", yet they all seem to be close to it.

[David M]

Actually, Sarge asked why planets look "round"....he didn't say "spherical". If you look at the Earth from space from above the North or South pole...then they are perfectly round...right?

Originally Posted by mbossman2 the Earth isn't a perfect sphere, it "bulges" along the equator and is "flattened" at the poles.

That shape is called an "oblate spheroid" ...learned that one in celestial navigation.

Sight reduction tables and the Nautical Almanac have to be adjusted slightly because the Earth is not a perfect sphere. In celestial navigation, sight reductions tables use spherical trigonometry to ultimately create a line of position on a chart. Where a number of these lines cross is where you are. The brighter stars and planets can each be used to generate one line of position. A sextant is used to measure the celestial bodies height above the horizon. Taking that height and a few other bits of information you get from the Nautical Almanac, you enter that info into the sight reduction tables.

Sorry to get a little off track...just a little trivia.

http://en.wikipedia.org/wiki/Oblate

http://www.amazon.com/2007-Nautical-...3606893&sr=1-1

http://www.efalk.org/Navigation/sights.html

[Strider]

Just a little side note the History Channel is currently showing "The Universe" and it's very interesting and informative. After only seeing a couple of episodes I would have to say magnetism plays a part in planets being round amoung other factors.

[ktkendall]

There is intelligent design behind it all. The chances of it all just happening by cosmic accident are astronomical. It actually takes a much greater leap of faith to not believe in creation by intelligent design then to believe that a being much greater than us designed, created and sustains it all. I saw on the science channel recently that they did a prototype of the universe and factored in all the known forces of gravity and set it into motion, and it just spun out of control wildly, and they , (the scientific community), now realize that there are unknown, unseen, and immeasurable forces at work that are keeping the universe together. The special was called, "Part of the universe is Missing." If you get a chance, check it out!

KK

[HAL9000]

Well the title makes total sense... making a model and saying something missing and relating it to a higher being makes the assumption that they know all there is to know about the universe and physics when in reality, we're barely scratching the surface of what is out there.

[sdkfz]

Intelligent design is just a modern arguement for the existence of god which will take this thread into the locked zone.....

[blue60007]

Originally Posted by mbossman2 the Earth isn't a perfect sphere, it "bulges" along the equator and is "flattened" at the poles.

Isn't that due to the spinning of the Earth? The middles being "flung" outwards. The points farthest from the rotational axis has the greatest circular speed.

[jimmyrules712]

intelligent design is simply the belief that the universe wasn't started by completely random chance, that it is unlikely that the perfect combinations of particles came together and poof, life began.

Whether you are promoting that idea, or the idea that it was by random chance that everything started, you have to be using a bit of faith either way, because neither can be proven (at least so far).

[David M]

Originally Posted by blue60007 Isn't that due to the spinning of the Earth? The middles being "flung" outwards. The points farthest from the rotational axis has the greatest circular speed.

Yes, centripetal (outwards) force from the Earth's rotation causes the bulge at the center.

One way of thinking about why the Earth is a sphere is that the gravity generated by the Earths mass wants to pull all the mass towards the center of mass (the Earths core). So in effect, what shape would allow for the Earths mass to be closest to the center of mass? A sphere. A sphere creates an equal distance from the earths surface to its center of mass regardless of where you are on the surface.

The reason the Earth is not some odd shape like an asteroid is because other than the crust, the Earth is over 99% liquid. Therefore the Earth can shape itself according to the force of gravity. Asteroids are a solid and therefore cannot shape themselves. The strong and weak molecular forces that keep an asteroid in its odd shape far exceed any gravitational forces found on an asteroid.

[Stuey]

There used to be an orbit simulator that I used to play around with many years ago (8?) but I can't locate it anymore.

After searching for a bit, I found something similar.
http://phet.colorado.edu/simulations/orbits/orbits.swf

In a nutshell, most of what you guys said is right. I don't believe that a naturally perfect circular orbit is unobtainable though, just improbable.

[David M]

Geostationary satellites have perfect circular orbits. Not because nature put them there obviously but because man planed it that way. If they did not have perfect circular orbits then they would be wobbling from east-west and back forcing highly directional ground based parabolic antennas to have to move back and forth to maintain reception. BTW, if they did not have perfect equatorial orbits then they would be wobbling north-south.

[Lespaul20]

There used to be an orbit simulator that I used to play around with many years ago (8?) but I can't locate it anymore.

We used to play with one on the macs at school. It was just black and white though.

[David M]

I did a Google search and found a bunch of them that use Java. Some are kinda fun.

[Stuey]

Originally Posted by Lespaul20 We used to play with one on the macs at school. It was just black and white though.

For some reason, your post made me remember how I originally found the orbit simulator I was thinking of!!

It's a good thing I remembered where it was from, but a bad thing b/c I checked and it's not available online. It was embedded in the Encarta encyclopedia that we had on our first computer maybe 10 years ago. I used to play around with it and it would allow for orbits that decayed into the earth, and others with infinite parabolic trajectories. Hey, it's all I had b/c we weren't allowed to play computer games (of which we had two) too often.

[daveyp225]

Originally Posted by David M Geostationary satellites have perfect circular orbits. Not because nature put them there obviously but because man planed it that way. If they did not have perfect circular orbits then they would be wobbling from east-west and back forcing highly directional ground based parabolic antennas to have to move back and forth to maintain reception. BTW, if they did not have perfect equatorial orbits then they would be wobbling north-south.

Originally Posted by Stuey I don't believe that a naturally perfect circular orbit is unobtainable though, just improbable.

Again, "perfect" isn't the right word, as it is impossible for there to be a 0% variation. Although my definition of of the word might differ from yours as I am an anal retentive math person.

Originally Posted by jimmyrules712 intelligent design is simply the belief that the universe wasn't started by completely random chance, that it is unlikely that the perfect combinations of particles came together and poof, life began. Whether you are promoting that idea, or the idea that it was by random chance that everything started, you have to be using a bit of faith either way, because neither can be proven (at least so far).

There are always things unprovable in any system. This was proven by Carl Gauss.

I like Godel's ontological proof (earlier "Anselm's argument"), which "proves" the existence of "God-like" objects (although instantly showing there must exist just one). Although it is mathematically correct, there are obvious objections to his choice of axioms if anyone would like to research them, but I still admire his try. At the very least, it will make you think a bit.

[daveyp225]

Originally Posted by Stuey It was embedded in the Encarta encyclopedia that we had on our first computer maybe 10 years ago. I used to play around with it and it would allow for orbits that decayed into the earth, and others with infinite parabolic trajectories.

I remember that too! Had it on my first computer as well (33mhz I think). Coolest thing ever.

[David M]

Woops..double post..my apologies

[David M]

Originally Posted by daveyp225 Again, "perfect" isn't the right word, as it is impossible for there to be a 0% variation. Although my definition of of the word might differ from yours as I am an anal retentive math person.

I guess "perfect" depends on how many decimal points you want to take something described as either "perfect" or "imperfect", out to. The only thing I can think of that might be infinite is the universe itself..yet still, we really don't know that either.

[HAL9000]

I thought I had read or saw somewhere that they had a "general" shape of the universe... kinda blobby.

Planets are round cuz they would look funny square.