UCSB Science Line
 How the orbit of earth around the sun is elliptical and not circular? Thank you very much. Question Date: 2015-07-06 Answer 1:When the solar system formed, gravity pushed a cloud of dust and rock together to form the Earth. Let’s look at all the options for orbits: 1) If the new Earth moved too fast toward the sun, it would have too much speed to be pulled back around by gravity. Instead of orbiting, it would be flung out of the solar system. 2) If the new Earth moved too slowly toward the sun, it wouldn’t have enough speed to stop it from falling toward the Sun. The orbit would continue to get closer and closer to the sun until it fell in. 3) If the Earth had just the right amount of speed and came in at the right angle, it would move in a circular orbit, not moving closer or farther from the Sun. This is very unlikely, because there are so many speeds/angles that wouldn’t lead to a circular orbit. 4) If the Earth had a speed and approach that was somewhere in between, it’s orbit would be elliptical (a circle that isn’t perfectly round). When it’s closer to the sun, the Earth is going faster but the force pulling it closer is stronger. When it’s farther from the sun, it’s slower but the force is also slower. Over time, these forces will make the Earth’s elliptical orbit more and more round until it is a perfect circle, but there may be other forces (from the other planets) that will pull it out of this alignment. This is what happened to the Earth. Scientists define zero “eccentricity” as a perfect circle and one “eccentricity” as no longer in orbit – Earth’s eccentricity is 0.0167, the most circular orbit of any planet in our solar system. Answer 2:It's not a perfect circle. However, the elliptical shape of the Earth's orbit is so close to circular that you would have a hard time telling the difference. It wasn't always that way, though - one of the things that causes ice ages is changes in the Earth's orbit. Click Here to return to the search form.