What is Kepler first law?

Answer 1

Kepler's Laws of Planetary Motion:

1] Each planet moves in an elliptical orbit with the sun at one focus

2] The line form the sun to any planet sweeps out equal areas of space in equal time intervals

3] The squares of the times of revolution (days, months or years) of the planets are proportional to the cubes of their average distances from the sun.

Answer 2

Kepler's three laws of planetary motion describe the orbits of the planets, and Kepler's model is the one that is generally used today.

The three laws describe the shape of a planet's orbit, how the planet's speed varies as it goes round an orbit of a given shape, and how quickly the planet completes an orbit of a given size.

The first major model was that of Claudio Ptolemy who placed the Earth at the centre, but as a result of this he was forced to use somewhat complicated geometric paths to explain the observed positions of the planets, especially the inner planets.

The model used combinations of circles and epicycles, and in this respect it was similar to the next model, published by Copernicus in 1543. The model put the Sun at the centre and as a result the orbits were geometrically less complicated. Galileo supported this model.

At the end of the 1500s Tycho Brahe made a series of more accurate measurements of the planet's positions over a period of time, and Johannes Kepler acquired the results after Tycho's death in 1601.

Kepler's model (1609) kept the Sun at the centre but introduced the novel idea of elliptical orbits. He showed that his model represented the planets' positions more accurately (when compared with observations) than the earlier models.

The above three models were alternative ways of describing the planets' positions as observed, and none of them attempted to give reasons for the planets' behaviour. However Newton's book 'Principia' gave details of the theory of gravity and the laws of motion, and he was able to show that a planets moving under the Sun's gravity must follow an elliptical orbit.

So Kepler's model was eventually given a theoretical backup and the two earlier models were discarded.