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The Earth Doesn’t Actually Orbit The Sun?

You were taught that Earth and other planets in our solar system orbit the sun, and that our solar system orbits the center of the Milky Way. Is it possible that your science teacher had it wrong? Is everything you’ve learned about physics and gravity incorrect?

Does The Earth Technically Orbit The Sun?

Let us first start by saying that for all practical purposes, what you were taught isn’t entirely wrong. The Earth does in fact orbit the sun. In the strictest sense, however, it doesn’t.

“But how can this be?”, you might ask. It’s been tested, verified and is common knowledge. It’s one of the first science lessons taught to kids in in grade school. How can our fundamental understanding of the solar system be wrong? Well, it’s actually due to a technicality.

The Barycenter Of Our Solar System

Everything that has mass has gravity. The more massive something is, the more gravity it has. The sun and other planets all each have their own “gravity well” which interact and pull on one another. Because there are several different gravity wells interacting with each other, it means that everything in an orbital system (like our solar system) orbit the center of mass of the system. This center of mass is called the barycenter. The Earth, the sun and everything else in our solar system actually orbit this barycenter – not the sun.

Where Is The Barycenter?

In a single star system like ours, most of the time (but not always), the barycenter is located somewhere within the star itself. However, most barycenters continuously change as massive objects (like planets) orbit a star.

If an unusual alignment happens where a large percentage of mass is on one side of the star, the barycenter can exist outside of the star’s radius. In cases of two star systems, the barycenter will be located between the two stars. If the stars aren’t of the same mass, it will be closer to the heavier, or more massive one.

Bonus Fact: The Earth doesn’t even orbit where the barycenter currently is. It orbits where it was roughly eight minutes ago. If the sun suddenly vanished, we would continue to our orbit it for another 8 minutes before shooting off on some random tangent because gravity isn’t instantaneous – it propagates at roughly the speed of light.

References:
http://arxiv.org/pdf/gr-qc/9909087 (PDF)
– Murray, Carl; Stanley Dermott (1999). Solar System Dynamics. Cambridge UP. ISBN 0-521-57295-9.
– Hartle, JB (2003). Gravity: An Introduction to Einstein’s General Relativity. Addison-Wesley. p. 332.
– Taylor, Edwin F. and Wheeler, John Archibald, Spacetime Physics, 2nd edition, 1991, p. 12.

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1. G Jeevan

May 9, 2012 at 1:33 pm

Does our sun move around the barycenter when the barycenter is outside of the sun?

• Nick

July 22, 2012 at 7:25 am

No. The sun has its own path, whilst the baycentre is old point of gravity (sort ofspeak).

• bda

November 2, 2012 at 9:02 am

Yes, the sun orbits the barycenter. Everything in the system goes around the center of mass.

2. Nigel Fox

July 20, 2014 at 4:28 pm

Gravity or rather the speed of gravity is instantaneous (or this perhaps the wrong term)gravity has no speed – its either there or it isn’t. For example you couldn’t just disperse the mass of the sun in an instant – its center of mass would still be the same!

• Willy Roentgen

August 13, 2016 at 10:41 am

There is still some controversy about that. Recent results from star observations seem to indicate that the speed of gravity (waves) is the Speed of Light. Reference: http://en.wikipedia.org/wiki/Speed_of_gravity

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