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Elliptical Orbits

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Sreeda asks:

  1. Why do planets move in elliptical paths and not in circular ones?


Ellipse is a more general shape than a circle. Circle is a special case of ellipse when its semimajor axis and the semiminor axis are of equal lengths.

The shape of orbit depends on various factors.

Johannes Kepler in the 1600s was the first to start discussing the shape of planetary orbits, formulating a series of laws to explain their shape and characteristics. Isaac Newton and Albert Einstein contributed additional research and theories.

Shape of a planet’s orbit is affected by the gravitational force of sun as well as other planets.

Also Remember that the sun is also in motion, which makes elliptical orbit an easy one than a perfectly circular one.

[Comments solicited from esteemed visitors]


  1. well, i think this question is not a question to be asked. if it was some other orbit insted of ellipse for example straight line(an example ofcourse) you would have asked “why only straight line”.

    but it can be proved that any two bodies (free bodies ie.. no external force acting on them except their gravity) in free space can either be in state of constant velocity (which also includes the can be in rest, after all a special case of velocity=0), or can go around a ‘conic’ orbit (Ex binary star) , if we assume that newton’s law of gravity is true. this is because of the dependency of the trajactory and force. since according to newtons law force and distance are inversely proportional to their square we get a conic as tragectory. if it had not been this relation there would have been more possibilities!

    now comes the question having all the conic and also rest as a possibility why did the planet choose ellipse ? well, it is not that planet chooses its orbit but we hav to keep in track that we still dont know how the universe originated. the shape of the orbit (ellipse,circle,hyperbola,rest etc..) is determined by the boundary conditions or initial conditions. to be more mathematic the constant that we get in the solution while solving the tragectory eq. determines the shape of the orbit. and this constant depends on the initial conditions.

    the eq. is
    ( d^2(r)/dt^2) = G(m2)/r^2
    where r is the position vector
    G-univ. constat
    m2-mass of the object….

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