How does the earth stay in orbit?
Which orbit?
There are at least two orbits:
1) orbit of Sun around the center of galaxy
2) orbit of Earth around the Sun
Both orbits are maintained by the same mechanism - the balance
of the centrifugal force and gravitational attraction, which also keeps a
satelite or shuttle (with engines off) in orbit around the Earth.
This link is not needed, since mechanism is well known and understood:
http://csep10.phys.utk.edu/astr161/lect/history/newtonkepler.html
The answer (often repeated)
"The earth is orbiting the sun and it drags the surrounding bodies with
it"
is incorrect. There is no such 'drag'
( Indeed -what would that be ? gravity of sun-earth is exactly balanced
by centrifugal force of the orbit of Earth around the Sun).
This is the correct answer:
There is another orbit, orbit of the Earth around the center of galaxy.
That is maintained by the same, familiar mechanism. Same for Earth as for
Sun. From this (cosmic view) they are like to projectiles, shot from the
two cannons in the same direction. They will travel together, falling
to Earth to which they are both attracted. In our case, Sun, Earth,
the two projectiles are not independent, (strictly speaking, the two
canon-nballs aren't either) and so we have a complex composite motion.
You do not have to study any stinking equations aero is
recommending. All you need is to grock few basic concepts of mechanics
SEARCH TERMS: Inertial Frame of reference, composite motion, relativity ..
http://www.ottisoft.com/samplact/Inertial system.htm
The basic concept (rule, advice ..) is: before you start thinking about
the motion, decide what Frame of Reference (alias Coordinate System) you
are using. You can choose any, but some are easier for certain motions.
When we say Earth moves along an elipse, we (implicitely) assume the
Frame fixed to the Sun (or to the solar system). When we talk about Sun
moving, was usually fix the system to the Galaxy.
Note: It does not end here, Our galaxy is part of the local cluster, which
also rotates
http://www-astronomy.mps.ohio-state.edu/~ryden/ast162_8/notes34.html
but let's stay within the frame of your question.
In galactic frame of reference, Earth has complex path, composite motion
of its orbit around center of galaxy, and sort of epicycles around Sun
http://www.astronomynotes.com/history/epicycle.htm
To really get it, consider this analogy: An astronaut in a shuttle holds
an apple. He let's go and the apple stays. They are both weightless.
Does the shuttle *drags* the apple along?
NO. They both orbit the Earth.
In close orbits.
Just like two cannonballs. |
Request for Answer Clarification by
crythias-ga
on
26 Jul 2004 02:27 PDT
Just to be clear, I meant to ask "How does the *earth* stay in orbit
[around the sun] if the sun moves so fast?"
I'm not satisfied with the answer so far. Though I hadn't explicitly
stated the question properly, the apple analogy only holds because it
is released by a body travelling at the same velocity as the
astronaut. The apple isn't orbiting the astronaut.
How does the earth get the angular velocity? momentum? to bring its
orbit in line with the orbit of the sun about the center of the
galaxy? Explicitly, how does the earth stay in a planar orbit about
the sun while the sun travels so fast? If we assume a "drag", wouldn't
we assume an elastic bounce? What is the angle? event? plane of the
earth's orbit about the sun with respect to the plane of the sun's
orbit around the center of the galaxy? What does it look like?
perpendicular?
I'm not really looking for an *exact* answer, but I'd like a better
description of the intersecting planes, and I'll grant that they
aren't planes, but actually wobble, but at least it's easier for this
question if we consider they are planes. Then again, the
sun/earth/moon galaxy/sun/earth parallels seem to be in tact. Why
isn't the earth pulling away from the moon enough to make the moon
exit orbit?
Does the earth/sun orbit actually affect (even enhance or perpetuate)
moon/earth? Why? Why not?
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Clarification of Answer by
hedgie-ga
on
26 Jul 2004 03:40 PDT
crythias-ga
I am not sure what to make of your RfC - and feel like
withdrawing the answer.
The answer is correct. I can, in addition, provide some links on this very
interesting topic of 'being in tact' - by which I meen the fact
that planes of planetary orbit are mostly parallel etc..
I suppose that's what you mean by:
" sun/earth/moon galaxy/sun/earth parallels seem to be in tact.."
Is that what you mean?
I am able to give you links to currrent research - formation of
solar systems - which (the research) is in a very initial state.
I cannot, for $5, carry out a research project and provide all the answers.
(and I do not invent hypoheses).
I am also not willing to deal with
" If we assume a drag "
as we know there is no drag. Are you interested in a current knowledge,
or are you pushing your own theories?
I just completed too lengthy a series of RfCs
http://answers.google.com/answers/threadview?id=377287
and, as I explained there, I am not interested to debate alt
theories.
So, please let me know if you are interested in total $5 worth
of somewhat organised links to the current state of knowledge.
You also whould have to clarify what you mean by 'why':
" why
isn't the earth pulling away from the moon enough to make the moon
exit orbit?
"
To make what? Are asking why moon orbits the Earth, rather then
being on some hyperbolical trajectory?
The 'why' in Newtonian physics is given by laws of motion
Are you looking for any other 'why'?
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Request for Answer Clarification by
crythias-ga
on
26 Jul 2004 10:14 PDT
I'm sorry. All I wanted to know was how the earth can maintain orbit
around the sun if the sun moves so fast.
The "in tact" meant that the comment below about the moon keeping
orbit around the earth while the earth orbits the sun is the same
concept -- the concept didn't disappear -- the same concept as the
earth orbiting the sun as the sun orbits the Milky Way galaxy.
I suppose the correct answer to my question is that the sun isn't
moving fast relative to the planar orbit of the earth around the sun,
if we can assume that the earth's orbit around the sun is roughly a
plane. And that the whole plane is also moving with the sun.
No, I'm not trying to advance any strange theories. I wanted to be
sure, ref: http://answers.google.com/answers/threadview?id=378584, the
speed of the sun, and how the sun travels the average distance between
the earth and the sun in about 7 days,... I wanted to be sure that the
earth/sun orbital plane moves in tandem with the sun's motion in the
sun's own orbit (that is the whole orbit of the earth around the sun
moves in the same direction and speed as the sun's own motion.) And,
how is that motion maintained.
The comment suggested that the motion of the moon around the earth is
the same concept. The earth moves very fast around the sun, yet the
moon stays around the earth.
Ref:
" why isn't the earth pulling away from the moon enough to make the moon
exit orbit?"
You asked: Are asking why moon orbits the Earth, rather then
being on some hyperbolical trajectory?
I hoped the rfc above is enough, but I think the answer is simply the
earth's gravitational pull on the moon is enough to keep the moon
orbiting the earth, even as the earth moves in orbit around the sun.
I imagine a yo-yo swinging in a circle in front of you, the string on
your finger allows the yoyo to continue to orbit your finger at a
fixed distance. Then you move your finger away from your body. What
happens to the yo-yo? It has to catch up to where your finger is.
I'm sorry this was such a bother. I really don't think I have any
better explanation for my question.
|
Clarification of Answer by
hedgie-ga
on
26 Jul 2004 22:55 PDT
OK,
crythias-g, let's clarify it further
You say:
The "in tact" meant that the comment below about the moon keeping
orbit around the earth while the earth orbits the sun is the same
concept -- the concept didn't disappear -- the same concept as the
earth orbiting the sun as the sun orbits the Milky Way galaxy.
And you right. This is the universal concept, valid through the
whole hierarchy of motions. It has a name: Inertial Frame of Reference.
The concept of Frame of reference is shown here:
http://www.wcsscience.com/frames/ofreference.html
There is another. less fundamental concept involved, which has to do
with the facts that most
planet move the same way, which is also repeated (at larger scale :
galaxy is (sort of) flat ,rotating disk)
(at smaller scale, e.g. in asteroids belts and Jupiter rings and
moons). This is well described,here:
Which way does the earth spin? Well, the stars, sun, moon, and planets
all seem to be moving east to west. So the earth is spinning in the
opposite direction, west to east. Hey, the moon is also moving west to
east, coincidence? Well, the earth is moving around the sun from west
to east. In fact all nine of the planets are moving around the sun
from west to east. And, all of the planets, except Uranus (which is
lying on its side) and Venus (which is spinning slowly backward), are
rotating from west to east. And most of the moons, of the various
planets, are orbiting their planets from west to east. And most of the
asteroids are orbiting the sun from west to east.
http://www.jimloy.com/astro/moon0.htm
see also
http://www.jimloy.com/astro/earth.htm
The fundamental concept is that of the coordinate system.I guess that
is what you mean when you are saying:
" I suppose the correct answer to my question is that the sun isn't
moving fast relative to the planar orbit of the earth around the sun,
if we can assume that the earth's orbit around the sun is roughly a
plane. And that the whole plane is also moving with the sun ..."
In the inertial frame attached to the solar system the Sun is indeed not moving
very fast. It is still moving. In the case of Sun this is a very fine
distinction, but
when we apply the same concept to Earth-Moon system we can see the difference:
In exact terms, Earth does not orbit the sun in a perfect ellipse.
Both Earth and Moon
orbit a center of the gravity of the Moon-Earth system. In this
inertial system the
pull of the sun is exactly compensated by inertial force as this system orbits the
Sun in perfect orbit. In the Moon-Earth inertial system then, we can
ignore the sun,
it's pull and travel around the galaxy, ... and apply the Newton's
laws (which "hold good"
in such a system) and derive the motion of Moon - to be a perfect
ellipse, not around
the Earth, of course, but around the origin of this frame (the center
of gravity of
Moon-Earth) .. etc
http://en.wikipedia.org/wiki/Coordinate
So, finally, to use your nice example of Yo-Yo: In that shuttle I
mentioned before,
there were two astronauts: One had an apple, and he just let it go. The other one
had a Yo-Yo, and he was spinning it above his head, in a circle which
had an center
on the axis defined by his body. Now we can say that the Yo-Yo is orbiting the
astronaut, as the whole shuttle (with all the astronauts and props)
is orbiting the Earth.
As long as, all the interactions are between the bodies contained the
inertial system
attached to the shuttle's center of gravity - they can be solved
without worrying about
the Earth. This applies also to your example, of Yo-Yo orbiting a
finger, attached to
a astronauts body, floating inside the shuttle. As the finger moves,
the string (gravity)
will reposition the Yo-Yo (to the left), while astronaut's body moves
a bit to the right.
At the same time, the whole shuttle, it's center of gravity, orbits
the earth, in perfect
ellipse, and is not affected by the internal motions within the
inertial frame of reference.
So, yes, we are in agreement and I hopr this additonal exchange did
help to clarify things,
for both of us. And thank you for interesting and thought provoking questions.
hedgie
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