Anyone who can help me out with the answer to why planets are round will have me eternal gratitude. cheers.
Not to be nit-picky, but arent they really oval due to their spinning on their axis? I think they are slightly flattened on the poles and somewhat buldging at the equators ( very slight)
You’re right starguard. The shape is called an oblate spheroid and has led to an awful political joke.
The Earth isn’t round, it’s an oblate spheroid. It’s flattened at the polls, just like (insert recently defeated politician’s name here).
Technically, the shape of the Earth is a “geoid”.
Alright, but then technically, I’m not human-shaped; I’m a Jurphoid pseudo-cylinder.
Bodies of over a certain mass follow the process outlined at Ice Wolf’s link. Smaller objects are not isostatically or gravitationally forced into quasi-spheroidal shape: Phobos, as most people are aware, is roughly potato-shaped. I recall seeing reference to one of the earth-grazing asteroids as being roughly “rectangular” – i.e., more or less brick-shaped.
I have no clue what the transitional figure is – worth noting that observed objects of 100 KM diameter and above are roughly spheroidal, while observed objects of 10 KM long axis and smaller are “free form.” So we have some limits on the range.
The mechanical strength of the object is important. You’d expect a rocky object to be able to maintain a funky shape than any icy body, for example. Also, asteroids are beleived to be “rubble piles.” Any given object is probably made up of lots and lots of rocks, some big, most small, but the whole thing is covered by a layer of crushed rock from imacts, which is called regolith. The size of the blocks underneath is the subject of great interest to folks who study asteroids that we have good close-up images of, like Ida, Gaspra, Eros, etc.
Isn’t that saying that the earth is “earth-shaped”?
On its face, yes, but the fact is that there’s a rigorous definition of a geoid defined to model the normal surface to the Newtonian “gravitational vector field”. I did provide a link…
I don’t have a cite but in one of his F&SF essays Isaac Asimov commented that if the Earth were shrunk to the size of a billiard ball, it would be rounder and smoother than a billiard ball. Round enough for me.
DD
That’s only because all the billiard balls in the universe are on the Earth itself, so if the Earth was shrunk to the size of a billiard ball, an actual billiard ball would be so incomprehensibly small that for all practical purposes it would be a dimensionless (and necessarily shapeless) point.
Even still, it would be more substantial than this post, which is entirely pointless.
Vaguely related.
Why is it said that some of the planets rotate “backwards”?
Is it based on them not turning the same direction as the majority of planets in the solar system, or is there some physical reason that it should theoretically be spinning the other direction?
A geoid isn’t any specific shape, though, as it’s just a visualization of the strength of gravitational acceleration. Arguably, only sufficiently large bodies are “geoid” shaped, because with smaller gravitational fields, mechanical properties can trump gravity.
Also, I think the “shrunk down to a billiard ball” thought experiment is to demonstrate how unspeakably tiny things like the Himalayas are on the scale of the planet. It would be smoother than a billiard ball but it might be visibily out-of-round.
To answer my own question…
In the beginning, there was an accretion disc.
Objects closer to the center of the solar system orbit faster than those on the edges. When there were no planets, there was a pretty smooth gradation of dust swirling around, the stuff close to the protosun whipping about and the stuff further out just cruising along watching the scenery. As planets began to gorm and attract this dust, the material attracted from within it’s orbit would have a slightly higher momentum in the direction of orbit than material attracted from outside the orbit. Net affect is that planets should rotate in the opposite direction that they orbit.
Yes planets or moons that rotate (or revolve) in the opposite direction from the other main objects in the solar system are said to rotate (or revolve) backwards.
The technical term for this is retrograde rotation (or revolution). But be careful retrograde also refers to the apparent backwards motion of planets farther from the sun than the Earth when the Earth swings past at a faster speed. (Much like a car you’re passing migh tlook like it’s going backwards.)
If you took a spaceship straight up from the North Pole of Earth and looked down, the Earth woudl seem to rotate in a counterclockwise direction and also revolve around the sun in a counterclockwise direction. All of the major planets revolve around the sun in the same direction. And all of the large moons revolve around their planets in the same counterclockwise direction. Many (and I believe almost all) of the smaller moons of Jupiter and further planets revolve in the opposite direction. It is believed that these moons were captured by their planets after the solar system was formed and it is energetically easier to be captured with a retrograde revolution. The larger moons are believed to have formed with the planets (or perhaps to have been ejected from them)
Most of the planets rotate this same way as well. Uranus does not. Uranus’ axis of rotation is tipped slightly more than 90 degrees (I think 98 degrees) so it technically rotates retrograde, though it’s really more like it’s “rolling” on it’s side as it goes around the sun. Venus also has a retrograde rotation. But its rotation is very slow. It rotates once in around 240 days while it revolves around the sun once every 225 days or so (these figures are from memory, but I know the rotation period is longer than the revolution period). I believe it’s recently been determined that Pluto also has a retrograde rotation.
Not “a geoid”, the geoid. There is only one.
Not acceleration. The geoid is an equipotential surface, generally aligned with “sea level” and as such is a time-average. The magnitude and direction of acceleration varies across the geoid. In general, the acceleration is not towards the center of the Earth, sometimes missing by tens of kilometers.
Depends upon which standards you use. Ordinary billiard balls can be less round than the Earth, better ones are not. The Earth is smoother than either, in scale.
I don’t think that necessarily follows.
The usual reference is straight up from the plane of the ecliptic–the Earth is tilted by over 23 degrees from that.
Uranus’s rotation is either prograde, tipped 98 degrees, or retrograde, tipped 82 degrees. The two are sometimes combined, so it is said to be retrograde, tipped 98 degrees. At least, I think that is what is happening. 
Venus’s axis is said to be tipped 177 degrees.
Oh? And there is no comparably defined “geoid” for, say, Venus?
Tipped 177 degrees from what? If you define the angle between the angular momentum vectors of the planet’s rotation and its orbit (yes, I’m assuming space is flat so there are no parallel transport issues), then there’s no meaningful definition of “retrograde”. If you define the acute angle between the lines these vectors generate, then “retrograde” means that the vectors orient their lines oppositely. Either Venus is tipped 177 degrees or it’s tipped 3 degrees and retrograde.
The tipping is either measured relative to a line orthogonal to the ecliptic or to a line orthogonal to the palnet’s own plane of rotation. I forget which for sure, but believe it’s the latter. It’s not the acute angle that’s measured but the angle between the vectors defining angular momentum. (That is, the vector that points towards you if when you look at the object it appears to be rotating counterclockwise.) This angle can be anythign from 0 to 180 degrees. If the angle is more than 90 degrees, the rotation is said to be retrograde. That is just a clasification. I’ve never heard it said that Uranus has a retrograde tilt of 82 degrees.
This is simply like saying. “I’m thinking of a negative number: -5.” In some sense the minus sign there is redundant. We could say “I’m thinking of a negative number whose magnitude is 5.” but we tend not to use this awkward phrase except when teaching about magnitudes or absolute values.
To be utterly pedantic, no. For instance, the corresponding concept with Mars is the areoid.
Pedantry does have its limits and “geoid” is commonly, and perfectly reasonably, used for other planets.
Interesting position, but I think the writers there are due for a close shave with Occam’s Razor. While every planet in the solar system has a mythological background, this cannot be assumed to hold forever. To generalize the prefix “geo-” to refer to a planet in general is far from abusive. There may not always be a natural prefix for “-graphy” or “-oid”.
And for Pete’s sake, what the hell am I supposed to call myself if not a geometer? I certainly don’t measure the damned Earth.