How did Kepler figure out that planets orbit is elliptical?

Answer 1

Kepler, the person who first wrote about elliptical orbits, had a Trinitarian view of the solar system in which it made perfect sense theologically to have the Sun at the centre of the universe. Copernicus also believed in a Sun-centred system.

Answer 2

He inferred the orbits were circular.

He presumed Copernicus was correct.

Incorrect prior knowledge can delay good reasoning.

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Copernicus's model of circles and epicycles represents the elliptical orbit quite accurately, provided the eccentricity is small. The main effect of an eccentricity of less than 0.1 is to displace the Sun from the centre, while the second-order effect of a noticeably elliptical shape, different from a circle, is very small.

Kepler was lucky to choose Mars to study because its eccentricity is larger than Venus's, Jupiter's and Saturn's, but even so the shape of the orbit is very nearly circular, which is why it took years of painstaking effort for Kepler to arrive at this solution.

With an eccentricity of 9%, as Mars's orbit has, the minor axis of the ellipse is only 0.4% smaller than the major axis. For the other planets (except Mercury) the minor axis is even closer to the major axis in size.

This is also the reason that it took over 1400 years for the old models of circles and epicycles to be questioned.

The other point is that there was no-one to tell Kepler his inital reasoning was 'wrong'. He was working with a blank sheet of paper, so to speak, and was working on an entirely new problem which no-one had solved before. He was the first person in 1400 years clever enough to tackle this unsolved problem and find a solution that we still accept today, 400 years later.

Answer 3

Kepler proposed that the planets orbit the sun in elliptical orbits with the sun situated
at one focus of each ellipse.

You have called Kepler's proposal "correct" because that's what you have always been
told, and you find it comfortable. In reality, Kepler's proposal simply explains all of the
planetary motions we see in the sky better than any other proposal that has come along,
and it exactly meshes with Newton's theory of gravity that came along a hundred years
after Kepler. But it's still "only a theory", and it can never be proven.

Answer 4

Kepler didn't prove that statement. In the 400 years or so since he worked on it,

it hasn't been proved yet, and it's unlikely that it can ever be proved. It's still "just

a theory".

Kepler only demonstrated that if you make that simple assumption, it comes very close

to explaining every motion of the real planets and moons that we see in the real sky,

so it's been adopted as the customary assumption by people who work with the motions

of astronomical bodies. It's worked out pretty good so far ... like, if you assume it's true

and use it to launch and steer a probe to Mars, then your probe actually hits Mars.

But it's still just a theory, and it probably always will be. So if you come up with a theory

that comes closer to explaining what we actually see in the sky, then your theory will be

adopted and Kepler's will be junked. That's how Science works.

Answer 5

By analyzing observations of the planets' positions.

Answer 6

Johannes Kepler didn't discover anything about the rotation of the other planets. His work was focused on the orbits of the planets.

Answer 7

Johannes Kepler