You probably remember what an ellipse is. A circle has a center and if you tie a string to the center and to a pencil, you can draw a circle. In contrast, an ellipse has 2 foci, which are 2 spots separated slightly from one another. If you take a string and tie it to both of the foci, you can draw the ellipse by kind of doing the same thing you did with the circle, and get a squashed circle.
Learn more about the myths of orbital motion. Now, it turns out that the orbits of the planets are pretty circular. But they are actually ellipses, and this was first worked out in the early s by Johannes Kepler. That honor is held by Aristarchus of Samos, a Greek philosopher who lived in the second century B. Given the lack of proper instrumentation, the debate over whether the Earth or Sun was the center of the universe continued over many centuries.
In , Polish astronomer Nicolaus Copernicus published a mathematical treatise that promoted the idea of the Sun being the center of the solar system. But his treatment was complicated, and it was Kepler who used data to come up with the realization that the orbit of planets were ellipses. In fact, Kepler came up with three laws. They are: 1 the orbit of a planet is an ellipse, with the Sun at one of the two foci; 2 the line connecting the planet and Sun sweeps out equal areas during equal intervals of time and; 3 the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.
Why not circular? Donald Fleming, London, UK Orbits are eliptical because of Newtons Law of Gravity bodies attract each other in proportion to their mass and inversly proportional to the square of the distance between them.
All worked out by Kepler some years ago. A circular orbit is a special and very unlikely case of an eliptical orbit. Bob Kirk, Bangkok, Thailand Yes. Isaac Newton. Read his Principia Mathematica. Peter Brooke, By Kinmuck, Scotland, UK It's not easy to arrange a perfectly circular orbit for an inverse-square law - just a small tweak either in the initial conditions, or from interplanetary interactions or impacts will change the path from a circular orbit to an elliptical one.
But, if you look at an ellipse from the right angle, it will appear circular. So, consider it a matter of viewpoint! Michael Hall, Canberra, Australia The shape of planetary orbits follows from the observed fact that the force of gravity between two objects depends on the square of the distance between them. If you double the distance between two objects, the attractive force between them drops to a quarter of it's original value.
If you triple the distance it drops to a ninth. Isaac Newton demonstrated mathematically that this law implied that the path followed by an object in a gravity field would be a parabola, a hyperbola or an ellipse. Astronomy Stack Exchange is a question and answer site for astronomers and astrophysicists. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search.
I am a student of class 9 and recently came to know about kepler's law of planetary motion and got confused about the foci and elliptical orbits of planets and some stars. Not sure if you're looking for a more mathematical answer or just the "why", but to answer the why, I'll start with some history on this.
Everyone who worked out a model for the Solar System, from Aristotle to Copernicus, liked circles. Even though Copernicus correctly reasoned that the Earth moved around the Sun and not the Sun around the Earth, he continued to use circles in his models of the motion of the planets. After Copernicus, Tycho Brahe, funded by the King of Denmark, had the best equipment at the time for observing the motion of the stars and planets and he was able to make star charts that were ten times as accurate as anyone before him.
Brahe used equipment like this mural quadrant , and a large private observatory to take extremely accurate records. Kepler, who was a better mathematician than Brahe, desperately wanted to get his hands on Brahe's star charts and the use of his observatory and equipment so much so that when Brahe died, there were rumors that Kepler had poisoned him, though that probably didn't happen. When Kepler finally had everything at his disposal, he was able to work things out and study the Solar System more accurately.
However, he still didn't know why the planets moved in ellipses; he'd only worked out that the ellipses fit the movement so well that it almost certainly had to be true, but he had no idea why. Kepler, in fact, didn't care for ellipses. He liked circles better, but he couldn't deny that ellipses worked.
Nobody knew why planets moved in ellipses until Isaac Newton was asked that question and had to invent calculus to answer it.
Calculus explains why planets orbit in ellipses, and that's the real answer. Earth's orbit has an eccentricity of 0. This is why it's easy to mistake it for a perfect circle. Mercury , with an eccentricity of 0. Kepler's Laws. Latest Gallery Images. Contact Us Privacy Policy Proud to be part of. All Rights Reserved.
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