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Progress in Understanding the Cosmos: A Selected Chronology | Pages 206-207

ca. 440 B.C.


Greek philosopher Leucippus and his student Democritus propose that dividing a piece of iron in half again and again eventually leads to "atoms," from the Greek word for "indivisible." These basic units compose all matter, the philosophers reason.

ca. 260 B.C.

Aristarchus of Samos, Greece, becomes the first philosopher to suggest that Earth orbits around the Sun within a giant universe. However, his writings on this topic do not survive; we have only allusions to his work.

ca. A.D. 140

Claudius Ptolemy, a Greek astronomer in Alexandria, Egypt, constructs a complex Earth-centered model of the universe. His model places the planets on small circles, called epicycles, that revolve around Earth on larger spheres. The resulting curlicues in planetary motion appear to explain the occasionally erratic wanderings of the planets in the sky.


Polish astronomer Nicolaus Copernicus publishes his life's work in On the Revolutions of the Celestial Spheres. His treatise describes his conviction that the Sun, not Earth, rests at the center of the universe. Copernicus receives a copy of the printed book on his deathbed and his contributions are not recognized until many years later.


Danish astronomer Tycho Brahe spies a bright new star: a supernova in the constellation Cassiopeia, showing that the realm of the stars is not permanent and unchanging. Brahe then establishes an observatory and assembles an exhaustive record of the motions of planets and comets in the sky. The work prepares the way for the discoveries of his assistant, Johannes Kepler.


German astronomer Johannes Kepler publishes his first two laws of planetary motion in The New Astronomy. The first and most famous law states that the orbits of planets trace ellipses in space, not circles, as astronomers had assumed. The second law describes how planetary orbits circumscribe equal areas in equal amounts of time. His third law, which describes how the length of a planet's year depends on the size of its orbit, appears 10 years later in The Harmonies of the World.


Italian physicist and astronomer Galileo Galilei publishes the first observations of the night sky through a telescope in The Starry Messenger. His discoveries of moons orbiting Jupiter and the phases of Venus support the Copernican view of a Sun-centered solar system, but the Catholic Church forces Galileo to recant his claims. Galileo also lays the foundation for modern physics with his studies of moving objects on Earth.


Dutch astronomer Christian Huygens realizes that the "ears" of Saturn, as Galileo had called them, actually form a ring that encircles the planet without touching its surface.


English physicist Sir Isaac Newton publishes The Mathematical Principles of Natural Philosophy. It contains his three laws of motion, still taught in physics classes today, and his law of universal gravitation, which explains the motions of falling objects and planets in orbit as the results of a single force. Newton is also known for his investigations of the nature of light, using prisms


Danish physicist Hans Christian Oersted shows that an electric current moving through a wire generates a magnetic field around it. This first convincing demonstration of a link between electricity and magnetism sets the stage for much of nineteenth-century physics.