What orbits the Earth

[Madalch]

In the latest commentary, Randi expresses depression because more people voted for "The Sun" than "The Moon" as an answer to "What orbits the Earth?" on the French version of "Who wants to be a millionaire?"

But seriously, if you see a contestant who is so stupid as to not know the answer to such a question, are you going to help him win the million bucks (Francs, Euros, whatever), or are you going to mess with the fellow's little mind by voting for Mars or Venus?

 

[Ratatoskr]

My thoughts exactly, and in the clip you can see people in the audience that are laughing, so my guess is that they wanted to mess with him.

Mars for teh win!

 

[Donn]

I took a wild moon stab, but I really agree with the last option.

 

[dogbite666]

The first two are correct. According to Keplar's laws objects in space orbit each other. The Moon orbits the Earth, however the position of the Earth is a little offset due to this motion hence the point about which the Earth and the Moon rotate is not the centre of the Earth as we might expect but a few miles off the Earth's centre. The same goes for all the other objects, including the relationship betwwen the Earth and the Sun.

The answer is not as simple as you might first expect.

Dog.

 

[Donn]

I kinda thought there might be some "not so simple" stuff coming I think in terms of the layman's (that's me!) use of the word 'orbit' we can stick to the obvious.

From what I grok of your post I picture lines left in space over time and then looking back at them we might see how the Sun gradually went around the Earth. Something like that?

 

[dogbite666]

I think I got that wrong, its nothing to do with kepler's laws. You can work out the force between two objects using:

F = G (m1*m2) / d^2

Where G is the Gravitational Universal Constant and m1 and m2 are the masses of the subjects and d is the seperation between them.

Just as Earths graviational pull effects you, pulling you down to the Earth's surface you also exert a force on the Earth. Therefore, if you jump up and down you are having an effect on the Earths position, albeit in the magnitude of some ridculously small figure. From this force you could work out the point of motion on the line of seperation of distance d.

Double stars which are about the same mass will orbit each other, ie. that point will be half way between their seperation. Even a pea orbiting the Sun will have a calcuable effect on the point of motion.

Hope that answers your question.

Dog.

 

[Donn]

Err... I get that the point of actual rotation will be somewhere along the line between the two objects, but not how that makes the sun orbit the earth. Sorry, I'm thick.

 

[H3LL]

Does AA29 also count?

An international team of astronomers has found that an asteroid discovered earlier this year follows Earth's orbit around the Sun and will, in nearly 600 years, appear to orbit the Earth. In the October issue of the journal Meteoritics and Planetary Science, the astronomers announce that the asteroid, named 2002 AA29, follows a "horseshoe orbit" that makes it come near the Earth every 95 years. It will next come close on January 8, 2003, although even then it will be much further away than the Moon and only detectable using large telescopes. The combination of Earth's and Sun's gravity works so that even as Earth pulls in the asteroid, it speeds up and moves away from the Earth. In this way Earth is protected from impact, despite the similarity of the asteroid's orbit to Earth's.

.

 

[Robaato]

Someone on another forum I frequent commented, "I guess the grip of the Catholic Church is still felt in France."

 

[Crazycowbob]

The first two are correct. According to Keplar's laws objects in space orbit each other. The Moon orbits the Earth, however the position of the Earth is a little offset due to this motion hence the point about which the Earth and the Moon rotate is not the centre of the Earth as we might expect but a few miles off the Earth's centre. The same goes for all the other objects, including the relationship betwwen the Earth and the Sun.

The answer is not as simple as you might first expect.

Dog.

This is technically correct, as the Earth and Moon orbit the center of gravity of the combined system. This point is, however, deep within the Earth due to the large difference in mass, and can basically be considered the Moon orbiting the Earth. The same goes for the Earth/Sun relationship, though this is made a bit more complicated as the Sun's movement is based around the center of gravity of the whole solar system, not just Earth's. Still the Sun is considerably more massive the the objects hurtling through space around it, and thus the center of gravity is still within the sun itself.

For all basic purposes, it can simply be said the Moon orbits the Earth, and the Earth orbits the Sun, but the opposite could not really be said, due to the center of gravity of the systems being so far from the smaller objects.

EDIT: Forgot to mention, and interesting exception to this would be Pluto/Charon, where the mass of Charon is enough to cause the center of gravity between the two to be outside of Pluto physically, laying in space between them. Even so though, this could at best be considered a binary planet system, and still not seen as Pluto orbiting Charon...

 

[shanek]

Planet X orbits Daffy Duck.

 

[Zep]

Daffy Duck orbits Warner Brothers.

 

[phyz]

The answer is not as simple as you might first expect.

Actually, the answer to the question in the context that it was asked is quite simple. It was a multiple choice question on a game show, not a debate prompt at an AAS conference.

The barycenter of the Earth-Moon system is well within the earth and the barycenter of the Earth-Sun system is well within the sun.

So it's quite alright to say the moon orbits the earth and the earth orbits the sun.

The laughers in the audience were in the minority. You can theorize that the audience conspired to boot the guy. I don't buy it. I think they really were that ignorant.

And the guy may have been astrostupid, but his sweetheart was a heavenly body. Would I trade knowledge of planetary mechanics for her affections? Let me get back to you on that.

 

[BillyJoe]

...the point of actual rotation will be somewhere along the line between the two objects...

Actually, objects rotate around an axis.

 

[Donn]

Actually, objects rotate around an axis

But isn't that axis through the point on the line between the two objects? That's what I meant. Like:
(earth________|___)____(moon)
Axis being the standy uppy thing.
Brackets being edges of planet.
Scale being entirely risible


/d

 

[BillyJoe]

Donn,
Hope this helps:

The Earth rotates on its axis once every 24 hours
The Earth revolves around the sun once every 365 days.

(nitpicky qualifiers removed )

regards,
BJ

 

[Donn]

Revolve : To move in a curved path around a center.
Okay - I sorta got that. The dictionary makes rotate and revolve synonyms really.

But, the earth's axis, is it directly from pole to pole, or is it along the line (somewhere) between the moon and the Earth because of the gravity formula in post #6 ?

If not, what is post #6 saying?

/d

 

[BillyJoe]

But, the earth's axis, is it directly from pole to pole.

Yes, the axis of the Earth's rotation goes through both poles.

The dictionary makes rotate and revolve synonyms really.

http://www.physicalgeography.net/fundamentals/6h.html

quote:
"...The term Earth rotation refers to the spinning of the Earth on its axis...One rotation takes exactly twenty-four hours...The orbit of the Earth around the sun is called Earth revolution....[one revolution] takes 365 1/4 days..."

...what is post #6 saying?

Let's consider the Sun-Earth system:
The centre of gravity of the Sun-Earth system is a point that lies very close to the centre of the Sun but 150 million km away from the centre of the Earth. The Sun-Earth system rotates around (an axis that passes through) that point.

Let's now consider the Sun and the Earth separately:
The Sun revolves around that point. But, because that point lies very close to the centre of the Sun (actually lying within the Sun), it hardly moves at all. The Earth also revolves around that point. However that point lies about 150 million kilometers away from the centre of the Earth.

Effectively, then, the Earth revolves around the Sun.

 

[Orangutan]

Elephants, Bigger than the moon.

 

[Donn]

Let's now consider the Sun and the Earth separately:
The Sun revolves around that point. But, because that point lies very close to the centre of the Sun (actually lying within the Sun), it hardly moves at all. The Earth also revolves around that point. However that point lies about 150 million kilometers away from the centre of the Earth.

Effectively, then, the Earth revolves around the Sun.

Thanks BillyJoe, I have a nice picture in mah head now

Sorry for late response, I often don't get email post reminders.