# Why do all the planets orbit the sun?

This might seem like a silly question to most of you but I’m going to ask it anyway. Why do all the planets orbit the sun?

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chroot
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They actually don't orbit the Sun. Both the planets and the Sun orbit their common center of mass. However, the Sun is much, much more massive than the Earth, for example, so the center of mass of the Sun-Earth system is very close to the center of the Sun. That gives the (false) impression that the planets orbit the Sun while the Sun does not move.

- Warren

Originally posted by chroot
They actually don't orbit the Sun. Both the planets and the Sun orbit their common center of mass. However, the Sun is much, much more massive than the Earth, for example, so the center of mass of the Sun-Earth system is very close to the center of the Sun. That gives the (false) impression that the planets orbit the Sun while the Sun does not move.

- Warren
So if we don’t orbit the sun why have we got different seasons?

chroot
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We do orbit the Sun in a very loose sense. In a more precise sense, we actually orbit the Earth-Sun center of mass, which is quite close to the center of the Sun. The reason the center of mass is so close to the center of the Sun is because the Sun is much more massive than the Earth.

- Warren

Can it be explained exactly why celestial bodies orbit around other bodies, in a particular direction following the same path?

chroot
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Newton's law of gravitation explains orbits quite nicely. In some cases, however, you need to use the full power of general relativity.

- Warren

russ_watters
Mentor
Originally posted by repugno
Can it be explained exactly why celestial bodies orbit around other bodies, in a particular direction following the same path?
To elaborate, all the planets go the same direction because they all formed out of the same spinning disk of gas and dust.

Originally posted by russ_watters
To elaborate, all the planets go the same direction because they all formed out of the same spinning disk of gas and dust.
Why was the gas disk spinning in the first place? And can anyone tell me what is at the centre of our galaxy?
Thanks

enigma
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Originally posted by repugno
Why was the gas disk spinning in the first place?
Small, random motion at a great distance which is then brought into a small distance.

Think about how an ice skater speeds up rotation by bringing her arms in. If you now expand that idea out to millions of miles, it is fairly easy to imagine how the gas "spun up".

wolram
Gold Member
http://burtleburtle.net/bob/physics/orbit101.html

The further a world is from its sun, the slower it needs to move to stay in orbit. For a circular orbit, Velocityworld = sqrt(masssun/distance). Kinetic energy = 0.5*massworld*velocity2. Since velocity increases with 1/distance, the kinetic energy required for a circular orbit goes to infinity as the distance goes to zero.

The period of a world in a circular orbit is 2*pi*radius/velocity, and velocity is proportional to sqrt(1/radius), so the period is proportional to 2*pi*radius3/2. That's Kepler's third law of orbits. For example (as in the inner and outer worlds in the example), a world at 4 times the distance should have an orbit 8 times as long.
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i hope you dont mind but i thought this link might be helpful
to this discussion.

Phobos
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Asymmetrical collapse of an irrgular-shaped nebula resulting in a net spin. Knowing that a nebula (which was the source of the material for the solar system) is a literal cloud of atoms & dust...try and imagine how precisely it would have to come together so as not to spin. In space, unless 2 gravitationally bound are on a direct collision course, they follow spiraling paths around their common center of gravity. Anyway, as the material was pulled closer together (gravity), it spun faster (ice skater analogy)....conservation of angular momentum, y'know. Then the centrifugal force resulting from the spinning cloud cause it to flatten out into a disk shape...which is why the planets orbit the sun in more or less the same plane.

russ_watters
Mentor
Originally posted by repugno
Why was the gas disk spinning in the first place? And can anyone tell me what is at the centre of our galaxy?
Thanks
To elaborate on what others said, after it starts spinning due to asymetrical collapse, eventually, some objects/portions of the cloud will come into stable orbits and some will not. Those that do will coalesce into planets and those that don't play cosmic pinball for a few billion years, end up as comets, get ejected from the solar systm, or fall into the sun.

I don't know if you're familiar with the "anthropic principle," but applied to this example, the planets are in stable orbits because they are in stable orbits. If they weren't in stable orbits, they (we) wouldn't be here. Different configurations of a cosmic dust cloud will produce different solar systems, some with nice, orderly systems, and some without. In fact, the only solar systems we have been able to find other than our own are ones vastly different from our own. Because of the way we are looking, all we can see are massive planets orbiting close to (and quickly around) or highly elliptically around small stars.

Contrary to popular belief, the seasons on Earth do not exist because of its orbit around the Sun. Actually, the seasons happen because of Earth's obliquity. When the Earth rotates about its own axis, the central exis is off by 23 degrees to the perpendicular axis. This is the reason why when the Earth orbits the Sun, seasons occur and change.

enigma
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Originally posted by harsh
Contrary to popular belief, the seasons on Earth do not exist because of its orbit around the Sun.

...

This is the reason why when the Earth orbits the Sun, seasons occur and change.
So it is because the Earth orbits the Sun...

Staff Emeritus
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Dearly Missed
In fact then, both things, plus a thrird.

1. The earth's axis is tilted relative to its plane of motion.
2. It orbits the sun.
3. As it does so, it keeps its axis parallel to itself (the axis always points in the same direction).

So in January, at perihelion, the southern hemisphere is tilted toward the sun, and in June, at aphelion, the norrthern one is.

Jenab
chroot said:
They actually don't orbit the Sun. Both the planets and the Sun orbit their common center of mass. However, the Sun is much, much more massive than the Earth, for example, so the center of mass of the Sun-Earth system is very close to the center of the Sun. That gives the (false) impression that the planets orbit the Sun while the Sun does not move.

- Warren
The sun's center shifts about inside a highly oblate McLaurin spheroid about 2 solar diameters wide whose center is the solar system barycenter. The plane defined by the major axes is approximately the plane of Jupiter's orbit. The minor axis is (if I remember correctly) less than 10% the length of the two major axes. The sun's wandering inside this spheroid is dominated by Jupiter's gravity, but the movement is perturbed by the other planets so that it isn't a regular orbit around the barycenter. The Earth also responds to Jupiter's gravity, so the apparent movement of the sun isn't noticed so much.

Jerry Abbott

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Would the movement of the Sun's barycenter inside that ellipsoid be chaotic by any chance? Other net gravitaitonal effects in complex systems seem to be.

Jenab