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I've had this question for a long time, and I thought this might be a place to ask it. In my school, we learnt about frames of reference. So, all I want to ask is, why do we say that all planets orbit the Sun? Isn't that just by the Sun's frame of reference? If we take the frame of reference as the Earth, don't we get a geocentric model, even if the celestial bodies follow more seemingly complicated paths?

Qmechanic
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Supernerd411
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2 Answers2

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This is actually a deep and subtle question, which can be answered in several different ways at different levels. Sean Carroll has a good discussion of it here.

You should read Carroll's post for the details, but the short version is: yes, there is an Earth-centered frame of reference in which the Sun orbits the Earth, but it is not an inertial frame of reference, which is a particularly useful and convenient class of reference frame. The sun-centered frame of reference isn't quite inertial either, but it's a lot closer. (That said, the Earth-centered reference frame is often more useful! It all depends on what you want to use it for.)

As Carroll discusses, the story gets more complicated once we consider general relativity.

tparker
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Velocity is only a relative quantity, meaning that it is just a choice of observer coordinates what point is taking to be (instantaneously) at rest. Acceleration, on the other hand, is not relative; there is an absolute difference between an observer in uniform motion and one that is accelerating. All the bodies in the solar system are experiencing gravitational forces, but their accelerations differ tremendously because of their different masses. The largest internal forces in the system are the mutual gravitational forces between Jupiter and the sun. The forces are equal in magnitude (by Newton's third law), but the sun is more than a thousand times as massive. That means the sun's acceleration is a thousand times less than Jupiter's. So to a reasonably good approximation, we can take the sun to be stationary, because it's acceleration is so much smaller than that of every other major body in the solar system.

Buzz
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