How Science Works

Scientific theories have to agree with observations and fit together into a consistent whole. Science is like an immense picture puzzle: Each time a new piece fits snugly among the other pieces, it both enhances its own plausibility and contributes an appropriate increment of knowledge to the entire puzzle.

Developments in theoretical astronomy during Seventeenth Century provide an example of the way ideas in science fit together to support further developments. The ideas of Kepler and Galileo were milestones on the road to Newton's laws of motion.

Up until this time, the visible planets, like all other celestial objects, were thought to be unlike the earth in their composition and to move in circular orbits. Then in 1609, after analysing observations of the planet Mars made by Tycho Brahe, Johannes Kepler declared that planetary orbits are oval, and probably elliptical. Kepler also stated his law of equal areas in equal times, which describes the way the planets move faster when nearer the sun.

Other pieces of the puzzle included Copernicus's book De Revolutionibus Orbium Celestium, published in 1543, which stated that the planets move around the sun, not the earth, and Galileo's telescopic observations, in 1610, of the orbital motion of Jupiter's moons, and of the earth-like appearance of the surface of the moon.

Once those ideas were accepted, it became apparent that the planets are material bodies held in their orbits by the sun's gravity. The big question then was how the force of gravity weakens with distance. It was reasonable to think that gravity, like light spreading out in all directions, weakens as the square of the distance, though some thought otherwise.

In 1684, Newton published a mathematical proof that a planet attracted by a gravitational force that weakens in just that fashion moves in an elliptical orbit and satisfies Kepler's law of equal areas.

Here is a page that shows how inverse-square gravitation generates elliptical orbits.

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