Is Earth a sphere?

[Janx]

A sphere is a good enough description for conversational language. Depends on the context of the discussion.

I would assume that a circle means that any given point on the perimeter is exactly the same distance from the center (radius) as any other point on the perimeter.

A sphere is pretty much the same thing in 3d. Any point on its surface is exactly the same distance from the center as any other point on its surface.

Any object that is a sphere probably cannot retain that shape when in the ongoing presence of other forces (gravity, impact). I suppose I can't prove that, but I don't anybody can prove an astronomical body is perfectly round either.

It is obvious to me that the first meteor strike on a body is going to leave a dent and a ridge, that both create uneven distances from the center.

Therefore, a sphere as in "perfect" cannot exist for practical purposes.

Therefore, the probability someone means perfect sphere, outside of a theoretical geometry lesson is also improbable.

The earth isn't shaped like a potato. Nor is it flat like a record. Is it a sphere? I guess it depends on who we're talking to.

 

[Nytmare]

Smoothness and roundness are two kinda different things.

I'm fairly certain (at least if I'm remembering all those episodes of Nova correctly) that when a scientist says that the Earth is not a perfect sphere, they are not talking about it's topology or imperfections due to hills, valleys, cities, and cows. They're talking about the fact that the distance around the planet is about 100 miles shorter if you go vertically, than if you go horizontally.

It's only squished a little, yes, but squished enough that you can only call it "almost" a sphere.

 

[Bullgrit]

Discussing this subject has brought up interesting thoughts, and I'm having fun looking these up through Google. Correct me if I'm wrong, but apparently the atmosphere is not considered in the diameter of Earth. But in other cases, the atmosphere is considered just as much a part of Earth as the molten core. The diameter of the gas giant planets includes their atmospheres. Interesting double standard.

Bullgrit

 

[Nytmare]

Two interesting conversations -

http://possiblywrong.wordpress.com/2011/01/03/is-the-earth-like-a-billiard-ball-or-not/

http://www2.ljworld.com/news/2003/jul/27/holy_hotcakes_study/


Also Bullgrit, again I might be wrong, but I'm fairly certain that the gas giants don't have a solid surface like the Earth does.

[EDIT] http://www.universetoday.com/47354/jupiters-surface/

http://cosmoquest.org/forum/showthread.php?129200-What-is-the-surface-like-on-a-gas-giant

 

[MarkB]

Discussing this subject has brought up interesting thoughts, and I'm having fun looking these up through Google. Correct me if I'm wrong, but apparently the atmosphere is not considered in the diameter of Earth. But in other cases, the atmosphere is considered just as much a part of Earth as the molten core. The diameter of the gas giant planets includes their atmospheres. Interesting double standard.

Bullgrit

Atmosphere is a tricky thing to measure for such purposes, as it attenuates gradually - you'd have to decide on a cut-off point where it becomes thin enough to be considered just 'space'.

With the gas giants, a measurement of diameter that does not include the atmosphere would be little more than guesswork, as we can't directly view their liquid surface, and the current scientific models of their internal structure contain a fair margin of error.

 

[Umbran]

Interesting double standard.

Well, here's the thing: apples and oranges. It isn't a double standard. It is more like two standards, used for two objects with significantly different properties.

We usually don't concern ourselves overmuch about the atmosphere when discussing the size of Earth, because the planet has a mean radius of 6,371.0 km, but 75% of the mass of the atmosphere lies in a thin envelope only about 11 km thick. And, being a gas, the mass of the atmosphere doesn't amount to much, compared to the mass of the planet. If you thought the difference between the polar and equatorial radius was too small to worry about, the difference between including the atmosphere and not including it is even smaller.

 

[tomBitonti]

The thing is, to be terribly overprecise, there are no spheres. None, zippo, nada, except as a mathematical construct.

No object that exists as a real physical object is a precise, perfect, sphere.

Since the question didn't say, "is the earth a sphere to within 1% of precision", which is how it would be worded for scientific or engineering precision, and since there is no context to supply that addition, we must fall back on everyday meanings.

Within everyday meanings, the earth is a sphere.

The real problem here is that we aren't given a context in which to decide if the everyday meaning is good enough. Absent that context, there is just a little too much room for different people to supply their own context and arrive at different answers.

As a nit: Instead of saying "oblate spheroid" you can say "slightly flattened at the poles", which is 100% everyday English.

Thx!

TomB

 

[tomBitonti]

Discussing this subject has brought up interesting thoughts, and I'm having fun looking these up through Google. Correct me if I'm wrong, but apparently the atmosphere is not considered in the diameter of Earth. But in other cases, the atmosphere is considered just as much a part of Earth as the molten core. The diameter of the gas giant planets includes their atmospheres. Interesting double standard.

Bullgrit

Well, gas giants are, for the majority of their radius, all atmosphere. That is quite distinct from a rocky planet, such as the earth, which has a rather thin and tenuous atmosphere in comparison. Practically, measuring the radius of a gas giant without including its atmosphere wouldn't be very useful. A gas giant such as Jupiter does have deeply buried liquid (ish - liquid might not fully and accurately describe the state) and solid layers. I don't think we have accurate measurements of the radii of those layers.

In this space, gas giants are significant in that they have very high rates of rotation. (Tickles me that we should talk about gas giants in this space.) Because of the high rates of rotation, the planets have high oblateness.

See:

http://lasp.colorado.edu/education/outerplanets/giantplanets_whatandwhere.php

http://www.hs.uni-hamburg.de/DE/Ins/Per/vonEssen/img/iau282_highres.pdf

The figures for oblateness for the solar system are:

Jupiter: 6.5%
Saturn: 9.8%
Uranus: 2.3%
Neptune: 1.7%

With O = (R_equator - R_Pole)/R_equator

From that second link, even with it's near 10% oblateness, Saturn still looks spherical.

Thx!

TomB

 

[Umbran]

The thing is, to be terribly overprecise, there are no spheres. None, zippo, nada, except as a mathematical construct.

No object that exists as a real physical object is a precise, perfect, sphere.

Probably true, in a practical sense. But note there's a difference between "almost a sphere, with random variations", and "almost a sphere, with rather specific variations arising from specific known causes". The first is often not worth comment, while the second may more often seem worth comment.

Since the question didn't say, "is the earth a sphere to within 1% of precision", which is how it would be worded for scientific or engineering precision, and since there is no context to supply that addition, we must fall back on everyday meanings.

We may fall back. There is no "must" about it.

 

[Leif]

As a nit: Instead of saying "oblate spheroid" you can say "slightly flattened at the poles", which is 100% everyday English.

But I object! "Oblate" is a common word in my vocabulary, most often used to describe the tendency of my physical form as I age. I find that I grow more oblate with each passing week. Moreso if the previous week happened to include Thanksgiving or Christmas. Someday, I may be disc-shaped.