Sun Orbits Center of Mass w/ Jupiter & Earth: How To Calculate?

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Discussion Overview

The discussion revolves around calculating the orbital dynamics of the Sun, Earth, and Jupiter as a three-body system, specifically focusing on the center of mass and the implications of adding Jupiter to the Earth-Sun system. Participants explore how the presence of multiple bodies affects the orbital periods and velocities around the center of mass.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant notes that in a two-body system, the Sun and Earth orbit their common center of mass every 365 days, but questions how this changes with the addition of Jupiter, given their differing orbital periods.
  • Another participant suggests that the motion around the common center of mass can be viewed as a 'wobble' in the Sun-Jupiter system, with the period of these wobbles being equal to the period of the perturbing body.
  • A participant expresses a desire to calculate the velocities of the Sun, Earth, and Jupiter around the center of mass but is uncertain about how to determine the Sun's orbital period in this context.
  • It is proposed that to find the velocities, one must consider the positions of all planets in their orbits, with the Sun positioned opposite the center of mass for each, and that while this is numerically feasible, it may be non-trivial analytically.
  • There is a mention that assuming circular orbits could serve as a preliminary approach to the problem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to calculate the Sun's orbital period around the center of mass in the presence of Jupiter. Multiple perspectives on the approach and complexity of the calculations are presented, indicating ongoing uncertainty and exploration.

Contextual Notes

Limitations include the assumptions made regarding the orbits (e.g., circular orbits) and the complexity of calculating the velocities and positions of the bodies involved, which may not be straightforward.

whatisreality
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I know that if I have a two-body system, the sun and the earth, then the common centre of mass is orbited every 365 days. What about if I add another planet in, say jupiter? Does the sun still orbit the centre of mass every 365 days? With two bodies, their orbital period about the centre of mass is the same. But Jupiter and Earth have different orbital periods, so how do you know how often the sun orbits now, when there are two periods to choose from?
 
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The motion around common centre of mass of the Earth-Sun system can be seen as a 'wobble' in the motion of the Sun-Jupiter system. Or vice versa, depending on chosen reference frame, but considering how the latter is much more pronounced, it's easier to treat as perturbations the influence of the smaller planet.
The period of those wobbles are equal to the period of the perturbing body.
 
Bandersnatch said:
The motion around common centre of mass of the Earth-Sun system can be seen as a 'wobble' in the motion of the Sun-Jupiter system. Or vice versa, depending on chosen reference frame, but considering how the latter is much more pronounced, it's easier to treat as perturbations the influence of the smaller planet.
The period of those wobbles are equal to the period of the perturbing body.
I want to do everything in the centre of mass frame. So I want the orbital period of the sun, Earth and Jupiter about the centre of mass, so I can work out their velocities around the centre of mass and therefore also the velocity of the centre of mass itself.

But I really don't know how to work out the velocity (particularly of the sun) around the centre of mass. Because I don't know its orbital period about the centre of mass.
 
Last edited:
whatisreality said:
I want to do everything in the centre of mass frame. So I want the orbital period of the sun, Earth and Jupiter about the centre of mass, so I can work out their velocities around the centre of mass and therefore also the velocity of the centre of mass itself.

But I really don't know how to work out the velocity (particularly of the sun) around the centre of mass. Because I don't know its orbital period about the centre of mass.

You have to figure the positions of all the planets in their orbits. The Sun is on the opposite side of the center of mass for each. Then you add them all up. Not terribly difficult, but non-trivial. That's the numerical way to do it. Analytically, I don't know. You could assume circular orbits as a first cut.
The center of mass itself has no velocity relative to all of that motion.
 

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