why do planets orbit the sun in an elliptical shape
Why do the planets rotate around the Sun? First, please note that "rotate" actually is used to describe an celestial body's spin, and "revolve" is used to describe its orbital motion. For example, the Earth completes one rotation about its axis about every 24 hours, but it completes one revolution around the Sun about every 365 days. Anyway, the basic reason why the planets revolve around, or, the Sun, is that the
of the Sun keeps them in their orbits. Just as the Moon orbits the Earth because of the pull of Earth's gravity, the Earth orbits the Sun because of the pull of the Sun's gravity. Why, then, does it travel in an around the Sun, rather than just getting pulled in all the way? This happens because the Earth has a velocity in the direction perpendicular to the force of the Sun's pull. If the Sun weren't there, the Earth would travel in a straight line. But the gravity of the Sun alters its course, causing it to travel around the Sun, in a shape very near to a circle.
This is a little hard to visualize, so let me give you an example of how to visualize an object in orbit around the Earth, and it's analogous to what happens with the Earth and the Sun. Imagine Superman is standing on Mt. Everest holding a football. He throws it as hard as he can, which is incredibly hard because he's Superman. Just like if you threw a football, eventually it will fall back down and hit the ground. But because he threw it so hard, it goes past the horizon before it can fall. And because the Earth is curved, it just keeps on going, constantly "falling," but not hitting the ground because the ground curves away before it can. Eventually the football will come around and smack Superman in the back of the head, which of course won't hurt him at all because he's Superman. That is how orbits work, but objects like spaceships and moons are much farther from the Earth than the football that Superman threw. (We're ignoring air resistance with the football example; actual spacecraft must be well above most of a planet's atmosphere, or air resistance will cause them to spiral downward and eventually crash into the planet's surface. ) This same situation can be applied to the Earth orbiting the Sun - except now Superman is standing on the Sun (which he can do because he's Superman) and he throws the Earth.
The next question, then, is how did Earth get that velocity, since in real life there's no Superman throwing it. For that, you need to go way back to. This page was last updated on January 31, 2016. I am sort of repeating what others have already said. For an inverse-square force field like gravitation, the general closed orbit is an ellipse. But the orientation, size, eccentricity, and the sense (going clockwise or counter-clockwise) of the ellipse depend on the initial conditions.
Consider the thought experiment: Take a new planet, hold it at certain distance from the sun. As soon as you release it, it will start on some elliptical path. But what elliptical path? If you release it from rest, it will head straight for the sun - the straight line too is one extreme special case of an ellipse. If you give the planet a push sideways it will take an elliptical path, in the more familiar sense. If the initial push direction is perpendicular to the line joining the planet and the sun, and is of right magnitude, you will get a circular orbit. If you give too strong a push of this kind, it may go on a non-closed orbit - that is, it just doesn t return to to its point of start at all, but goes away to infinity, getting only deflected by the sun to some extent on its way.
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