From an Earth-centered to a Sun-centered universe: Ptolemy to Newton

For about 1,400 years, most astronomers used Ptolemy's geocentric model (around 150 CE), which put Earth at the center and used small circles called epicycles to explain why planets sometimes seem to move backward. In 1543 Nicolaus Copernicus proposed a heliocentric model with the Sun at the center, which explained that backward (retrograde) motion simply as Earth overtaking or being overtaken by other planets. Tycho Brahe then made the most precise naked-eye measurements of planetary positions ever recorded, and Johannes Kepler used Tycho's data to show that planets move in ellipses, not perfect circles. Starting in 1609 and 1610, Galileo Galilei turned a telescope to the sky and saw moons orbiting Jupiter and a full set of phases of Venus, evidence that did not fit Ptolemy's model. In 1687 Isaac Newton explained why planets move as Kepler described: the same force of gravity that makes an apple fall holds the planets in orbit around the Sun.

This explanation is AI-generated and can occasionally be wrong.

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