Heliocentric, Tidal and Nebular Theory Help

By — McGraw-Hill Professional
Updated on Sep 16, 2011

Heliocentric Theory

The “thought police” of the church held less power in northern Europe than they did in Italy. Proponents of the heliocentric (Sun-centered) theory were taken seriously in places such as Germany, France, Poland, and England.

The Pioneers

Nicolaus Copernicus, a Polish astronomer, published a work in the early sixteenth century suggesting that the Sun, not the Earth, must be at the center of the Universe. (Remember that back in the sixteenth century the Earth, the Moon, the Sun, and the planets basically defined the entire Cosmos. No one knew what the stars were, much less how they were distributed throughout space.) The Earth, thought Copernicus, is a planet just like Mercury, Venus, or Mars insofar as its importance in the overall scheme of things. But Copernicus could not prove his theory to the complete satisfaction of the authorities in his part of the world. If the Earth is moving, asked the skeptics, why don’t we feel a constant wind from space? What force could push the Earth? Why should such a force exist?

Another astronomer, Tycho Brahe, was involved with an ongoing meticulous mapping and recording job. He kept careful records of the positions of all the planets over a period of time. Brahe had a German assistant named Johannes Kepler who eventually formulated the three fundamental rules for planetary motion, known as Kepler’s laws . Isaac Newton put it all together and finally changed mainstream thinking. The Earth had lost its exalted position, replaced by the Sun. The heliocentric theory had survived the test of time and had become the conventional wisdom.

Kepler’s Laws

Johannes Kepler published his famous rules of planetary motion early in the seventeenth century. They can be stated briefly as follows:

  • Each planet follows an elliptical orbit around the Sun, with the Sun at one focus of the ellipse.
  • An imaginary line connecting any planet with the Sun sweeps out equal areas in equal periods of time.
  • For each planet, the square of its “year” (sidereal period) is directly proportional to the cube (third power) of its average distance from the Sun.

Theoretically, it is possible for a planet’s orbit to be perfectly circular. A circle is an ellipse in which both foci are at the same point. In reality, however, there is always some imperfection, so all planets follow orbits that are slightly oblong.

Kepler did not originally call his rules laws. This label was attached later by others. Kepler came up with his three principles and refined them over a period of several years. The first two rules were finalized in 1609, and the last one came out in 1618. The first two laws are illustrated in Fig. 9-4, and third law is rendered graphically in Fig. 9-5.

Evolution of the Solar System
Heliocentric Theory Kepler’s Laws

Figure 9-4. Kepler’s first and second laws. The planet’s orbit is an ellipse with the Sun at one focus, and equal areas ( X ) are swept out in equal periods of time ( t ).

Evolution of the Solar System
Heliocentric Theory Kepler’s Laws

Figure 9-5. Kepler’s third law. The squares of planets’ orbital periods ( R and S ) are proportional to the cubes of their mean orbital radii ( r and s ).

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