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I like to answer reader questions from time to time on Universe Today, and here’s one I got recently, “how does the Earth rotate around the Sun?” I think I know what the questioner is getting at, but there’s a bit of a mistake in the question. The Earth rotates (or spins) on its axis, and the Earth revolves (or orbits) around the Sun. So I think a more proper question might be, “how does the Earth revolve around the Sun?” So did we end up with a situation where the Earth is orbiting around the Sun? How did this get going in the beginning?
The short answer is, through the conservation of angular momentum.
The entire Solar System formed from a cloud of gas and dust about 4.6 billion years ago. Some event, like a nearby supernova explosion caused the cloud to collapse inward through mutual gravity. Each individual particle of gas and dust had its own movement, and as they collected together through gravity, the whole cloud began to rotate in an average direction.
As the central star (the Sun) pulled together, it spun faster, and flattened out into a disk of material; the Sun at the middle, surrounded by the protoplanetary disk. Rocks collected together into larger and larger objects, becoming planetesimals, and eventually full-sized planets. Once the Sun compressed down far enough to ignite with solar fusion, it generated powerful solar winds that blew away all of the leftover gas and dust in the Solar System. All that left was the planets in their orbits, as they were too massive be moved by the Sun’s solar wind.
So, to understand why the Earth is orbiting around the Sun, you have to go right back to the very beginning, and follow the individual atoms and molecules that make up the Earth. It was through their original motion and their gravitational interactions that the Earth was formed and put into its current trajectory around the Sun.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.
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