Question

Explain the significance of Kepler’s laws for planetary motion. Name and describe the work of at least 2 other scientists who contributed to the theory of gravity which helped explain Kepler’s observations.

Answer 1

First lets state Kepler's laws

1. Planets move around the Sun in ellipses, with the Sun at one focus
2. The line connecting the Sun to a planet sweeps equal areas in equal times.
3. The square of the orbital period of a planet is proportional to the cube (3rd power) of the mean distance from the Sun

Kepler's laws describe the motion of planets around the Sun. Kepler knew 6 planets: Earth, Venus, Mercury, Mars, Jupiter and Saturn. All these (also the Moon) move in nearly the same flat plane. The solar system is flat and on a plane. The Earth is on the plane too, so we see the entire system edge-on--the entire plane occupies one line cutting across the sky, known as the ecliptic.

Every planet, the Moon and Sun too, move along or near the ecliptic. If you see a bunch of bright stars strung out in a line across the sky--with the line perhaps also including the Moon, or the place on the horizon where the Sun had just set--you are probably seeing planets. Ancient astronomers believed the Earth was the center of the Universe--the stars were on a sphere rotating around it (we now know it's actually the earth that is turning) and the planets were moving on their own "crystal spheres" with variable speed. They usually moved in the same direction, but sometimes their motion reversed for a month or two, and no one knew why.]

A Polish clergyman named Nicholas Copernicus figured out by 1543 that those motions made sense if planets moved around the Sun, if the Earth was one of them, and if the more distant ones moved more slowly. The Earth then sometimes overtakes the slower planets more distant from the Sun, making their positions among stars move backwards (for a while). The orbits of Venus and Mercury are inside that of Earth, so they are never seen far from the Sun.

The pope and church fought the idea of Copernicus because in one of the psalms (which are really prayer-poems) the bible says that God "set up the Earth that it will not move" [that was one translation: a correct one may be "will not collapse"]. Galileo, an Italian contemporary of Kepler who supported the ideas of Copernicus, was tried by the church for disobedience and was sentenced to house arrest for the rest of his life.

It was an age when people often followed ancient authors (like the Greek Aristotle) rather than check out with their own eyes what Nature was really doing. When people started checking, observing, experimenting and calculating, that brought the era of the scientific revolution and of technology. Our modern technology is the ultimate result, and Kepler's laws (together with Galileo's work, and that of William Gilbert on magnetism) are important because they started that revolution .