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Why Are Planets Round? Side-Effects Of Earth's Slight Non-Sphericality

TheBeast17

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Forgive me for sounding dense, but why are planets round?

I know I sound like a 6yr old, but can someone provide me with an explanation as to why lumps of rock created in a vaccuum, are perfectly spherical.

Yes I know I it's a stupid question, just shut up and answer it.............
 
Planets are round because of gravity. Gravity pulls equally in all directions. If you pull equally in all directions on something, the only shape it can be is round.

However, as planets get larger, gravity gets stronger, until eventually large objects on the surface are crushed under their own weight. That's why we don't have mountains that are 50 miles high or skyscrapers that are 2,000 stories tall. Planets stay basically spherical because any large deviations get crushed, restoring that spherical symmetry.

e.g A similar process would take place if there were a tremendous crater somewhere on a planet. The sides of the crater would be under the entire weight of the crust above them, and would be pushed out into the hole, causing landslides until the hole was nearly filled.

Planets are mostly gaseous or molten, so the material flows easily into a sphere, the shape in which gravitational and pressure forces can attain balance.

Smaller bodies like asteroids are held together by chemical, not gravitational forces, and are thus not spherical at all.

:)
 
Cheers!

Up until now I had considered myself quite intelligent.

Aah well....
 
schnor said:
Gravity pulls equally in all directions. If you pull equally in all directions on something, the only shape it can be is round.

I beg to differ.:blah:
 
Am I talking arse or is the earth not actually spherical? I was told at school that it is something like 27 miles wider than it is high, as it were. Something to do with rotation, which sounds reasonable to me.
 
The radius at equator is about 20 km larger than the radius to the poles, eg. The Poles are flat and if I remember correctly, the shape is called a geoid.

The reason it's squished is indeed due to the rotation, a bit like if you spin round your arms will want to fly off. If it didn't rotate, and gravity alone shaped the earth, it would be exactly spherical.

As it's 20km out of erm, 6500 odd? Not worth worrying about really is it :D
 
Assuming a 100kilo man standing on the equator is subjected to a greater centrifugal force than a man of the same weight standing at the north pole, would the man at north pole in fact weigh less?
 
No. t'other way round. The man at the pole weighs more, just as pendulum clocks at the pole run faster.

(Or is it the other way round - it's getting late for me!)
 
Thanx Rynner, thats what I meant, but is it a fact, as I have a friend who might like to move closer to the equator with her scales.
 
It's true for 2 reasons
1) at the pole you are closer to the Earth's centre
2) at the equator the earth's rotation tries to fling you off, making you lighter on the ground.

The effect is fairly small, but measurable. I'll see if Ican find the figures later.

As for your friend's scales, it depends what type they are. A spring balance or most domestic type scales which are based on springs, will show differences in weight. However, a scientific balance, or even the traditional 'Libra' type scales, will read the same at pole and equator, because they are comparing weights. The standard 1 kg, 5 g, etc 'weights' are actually standard MASSES, and so change their weight in proportion to the gravitational field.

On the moon, 6 lb of spuds would still 'weigh' 6 lbs on a Libra balance, but on a spring balance they would only weigh 1 lb!
 
During the days of yore, the sailors used a watch based on springs and such. Because when they got near the equator gravity was lower, they would slow down. And since they used the clocks for navigation, this meant the maps became imprecise. If you look at an old map, you will find it looks like somebody has taken the Earth and twisted it towards the east.
 
Xanatic said:
During the days of yore, the sailors used a watch based on springs and such. Because when they got near the equator gravity was lower, they would slow down.
It is true that the Marine Cronometer would slow down as you neared the equator but it was not because of gravity, but because of the change in abient temperature afecting the strenght of the springs, gavity would be unlightly to these clocks as the balence that controls the rate of time keeping is always "horizontal to the world" and is governed by the length/strength of a spiral "hair spring".

Wm.:D
 
Well I thought Shnor's explanation made perfect sense.

Then all the bloody Forteans at the back started playing up.

What a shower. :p
 
James, what's happened to your sense of fun, were you replaced by a doppleganger during the boards missing days, we had some excellent replies, but a bit of fun doesn't hurt.:p
 
Now then you experts, what I want to know is, does the water really spin round the plughole the opposite way in Australia to what it does here? And why??

Carole

Help, me smilies have disappeared too, have they been caught in an anti-gravity bubble and spun off into outer space??
 
Short answer, Carole, yes, the water does go down the plughole t'other way down under. (Anticlockwise here, clockwise there, same as the weather systems.) Also climbing plants twirl in opposite directions in the two hemispheres.

For more detail than you probably want, look up Coriolis force on a search engine.

And before someone replies with "I just pulled the plug and the water went the wrong way...", let me add that the water in the bath must be left for a few hours to eliminate any swirls it might have gained from filling the bath, or indeed from any activities in the bath since it was filled!
 
This certainly does leave one wondering whether the washing machine spins in different directions in different hemisperes... :D

Actually, my question is whether water swirls down faster at the poles than it would at the equator (assuming the water didn't bother freezing at the poles...) Any answers?
 
No no, I was talking about before the Chronometer. The whole reason why the chronometer was such a good idea, was because of these earlier clocks with pendulums.
 
Xanatic said:
No no, I was talking about before the Chronometer. The whole reason why the chronometer was such a good idea, was because of these earlier clocks with pendulums.
But pendulum clocks were never in reguler use on ships because they won`t work properly due to the rocking of the ship, the efects off gravity on a pendulum controled clock are also negleable when compared with tempeature and air presure.

Wm.
 
Forgive me for sounding dense, but why are planets round?

I know I sound like a 6yr old, but can someone provide me with an explanation as to why lumps of rock created in a vaccuum, are perfectly spherical.

Yes I know I it's a stupid question, just shut up and answer it.............
They aren't perfectly spherical. They bulge a bit in the middle (photos of Jupiter tend to show this very clearly).

I read somewhere (possibly in a QI book) that if a snooker ball was enlarged to the size of a planet, small surfaces imperfections would become mountains taller than Mount Everest.
 
And the deviation from a sphere is why the earth (technically an oblate spheroid - think of a Smartie) is why the highest mountain on earth (defined as distance from the centre) is Chimborazo in Ecuador, not Everest.
 
And the deviation from a sphere is why the earth (technically an oblate spheroid - think of a Smartie) is why the highest mountain on earth (defined as distance from the centre) is Chimborazo in Ecuador, not Everest.
Good fact - never heard that one before.

According to my calculations that means there’s a 1.6 mile sea level difference between Ecuador & Himalayas.

Sounds a lot but compared to the diameter of the Earth it’s fairly minimal.
 
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