SPEAKER: Four millennia ago, the Babylonians were skilled astronomers, able to predict the apparent motions of the moon, planets, and the sun and even predict eclipses. The Greeks, however, were the first to put these motions in the context of a cosmological model. They developed a geocentric model with a system of circular orbits around the Earth with the celestial sphere rotating about the Earth every 24 hours.
Aristarchus of Samos, using a geometric argument, estimated the sun must be much larger than the Earth. And it was inconceivable that such a giant would circle our diminutive planet.
This heliocentric model was firmly rejected by Aristotle and other contemporaries, however. The culmination of Greek cosmology was a geocentric epicycle model of Ptolemy using orbits within orbits to predict the planetary motions to impressive precision. The model fit the observational data so well that it survived for over 1,000 years. It was only with the work of Copernicus that Ptolemy's model was threatened.
In 1514, Copernicus distributed a book usually called the Little Commentary to his friends in which he sets out his heliocentric cosmology, that the sun is at the universe's center with planets orbiting in circular orbits. Although rejected by many of his contemporaries, the model was defended by Galileo and Kepler.
Kepler, using the meticulous measurements of Tyco Brahe, showed that a heliocentric model in which the orbits were elliptical rather than circular fitted observations better than Ptolemy's and Copernicus's circular orbits.
Galileo's telescopes revolutionized astronomical observation. And his observations of Venus showed it had phases, just like the moon, explained by it orbiting the sun rather than the Earth. 150 years later, Newton's laws of gravity gave the first underlying theoretical explanation for the celestial motions.
Galileo had observed that the Milky Way was comprised of innumerable stars invisible to the naked eye. In the 18th century, Kant proposed that the Milky Way was a lens shaped island universe or galaxy. It wasn't until the 19th century, however, that Bessel measured the distance to stars using parallax. He found that the stars were millions of times further away than the sun. And as the 20th century dawned, the great scale of the universe started to be realized.
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Cosmology uses observations of cosmic structures, like stars and galaxies, to understand the origin, evolution, and ultimate fate of the universe. Join Rachel Bean as she examines our current perception and evolving ideas of the universe.
This video is part 1 of 6 in The Puzzling Life of the Universe series.