Nicolaus Copernicus (1473-1543) a Polish astronomer, was best known for his theory that the Sun is at the center of the universe, and that the Earth, spinning on its axis once daily, revolves yearly around the Sun. This is called the heliocentric, or Sun-centered, system.

Before Copernicus’ theory, astronomers believed in Ptolemy’s geocentric universe in which the Earth was motionless at the center of several rotating spheres. These spheres contained (in order from the earth outward) the following heavenly bodies: the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The outermost sphere contained the other stars.

Copernicus's theory looked at the universe in a very different way. He believed the Earth rotates daily on its axis, and it revolves yearly around the Sun. More importantly, he argued that the planets also circled the Sun.

There were only ten scientists that believed Copernicus’ theory between 1543 and 1600.The most famous were Galileo and the German astronomer Johannes Kepler. In 1588, Danish astronomer Tycho Brahe came up with a theory that tried to combine Ptolemy’s Earth-centered theory with Copernicus’ Sun-centered theory. Brahe’s theory said the Earth remained at rest and all the planets revolved around the Sun as it revolved around the Earth.

Not until the late 17th century, and the rise of English scientist Sir Isaac Newton, did most astronomers begin to accept Copernicus’ theory.

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