Copernicus' Universe

The Sun was Copernicusí center of the universe, encircled by Mercury, Venus, Earth, Mars, Jupiter, and Saturn.

Related Topics

Nicolas Copernicus


Ptolemyís Universe

Planetary Motion

Isaac Newton

Copernicus made a great leap forward by realizing that the motions of the planets could be explained by placing the Sun at the center of the universe instead of Earth. In his view, Earth was simply one of many planets orbiting the Sun, and the daily motion of the stars and planets were just a reflection of Earth spinning on its axis. Although the Greek astronomer Aristarchus developed the same hypothesis more than 1500 years earlier, Copernicus was the first person to argue its merits in modern times.

       In Copernicusí Sun-centered (heliocentric) view of the cosmos, the planetsí occasional backward, or retrograde, motion comes about naturally through the combined motions of Earth and the planets. As Earth speeds around the Sun in its faster orbit, it periodically overtakes the outer planets. Like a slower runner in an outside lane at a track meet, the more distant planet appears to move backward relative to the background scenery.

       Copernicusí model also explains why the two planets closest to the sun, Mercury and Venus, never stray far from the Sun in our sky. And it allowed Copernicus to calculate the approximate scale of the solar system for the first time. Thatís not to say Copernicusí model was without problems: He still clung to the classical idea that the planets should move in circular orbits at constant speeds, so like Ptolemy, he had to jury-rig a system of circles within circles to predict the planetsí positions with reasonable accuracy.

       Despite the basic truth of his model, Copernicus did not prove that Earth moved around the Sun. That was left for later astronomers. The first direct evidence came from Newtonís laws of motion, which say that when objects orbit one another, the lighter object moves more than the heavier one. Because the Sun has about 330,000 times more mass than Earth, our planet must be doing almost all the moving. A direct observation of Earthís motion came in 1838 when the German astronomer Friedrich Bessel measured the tiny displacement, or parallax, of a nearby star relative to the more distant stars. This minuscule displacement reflects our planetís changing vantage point as we orbit the Sun during the year.

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