Aristarchus of Samos (310-230 BC) was the first to believe in a heliocentric system, but his idea of a sun-centered system was never accepted by the ancient Greeks. The Ptolemaic (geocentric) view of the solar system was held, instead, by scholars and was the predominant view throughout the Middle Ages. It wasn’t until the Polish canon, physician and astronomer, Nicolaus Copernicus (1473-1543), published his work De Revolutionibus Orbium Coelestium ("On the Revolutions of the Heavenly Spheres"), that the idea of a heliocentric system was reintroduced to the world.

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Even though Copernicus had correctly placed the sun at the center of the solar system, he still maintained that the orbits of the planets were circular. It wasn’t until Johannes Kepler formulated his first law of planetary motion that the true shape of the orbits (i.e., an ellipse) was known.

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Johannes Kepler (1571-1630)

The relationship between Johannes Kepler and Tyhco Brahe (1546-1601) could, at best, be described as tempestuous. Being very different in their temperament and respective personalities (Kepler being a very introspective and reticent man, while Tycho was gregarious and boisterous) the two men clashed frequently during their short collaboration with each other; Kepler became Tyhco’s assistant in 1600 and remained his assistant until Tycho's death in 1601.

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But, while neither man really liked the other, they, at the same time, had a great admiration for each other’s abilities. Kepler was the brilliant mathematician and theoretician, while Tycho was the observational genius of his time. And, by using Tycho's planetary observations as a foundation for his own work--in particular, Tycho's observational data of the planet Mars--Johannes Kepler was able to derive his three laws of planetary motion.

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Even though Tycho's data made it possible for Kepler to formulate his three laws and thus helped establish the Copernican (heliocentric) model, Tycho, himself, did not believe in the Copernican system. He, instead, devised his own peculiar, geo-heliocentric system (above), in which the sun orbits the Earth and the stars and other planets orbit the sun. But, on Tycho’s deathbed, he handed over to Kepler the observational data, which he had so long withheld, and instructed him to make good use of it, which as we know, Kepler did. It is reputed that Tycho on his deathbed said to Kepler: " Let it not seem that I lived in vain". It was by this union of labors that the two men were able to revolutionize our view of the cosmos.

Although he is often attributed with its invention, Galileo did not invent the telescope--the telescope actually having been invented by Hans Lippershey (b. c.1570-d. c.1619) in 1608. But, even though he didn’t invent it, Galileo made extensive use of the telescope and built a strong body of evidence in support of the Copernican model with his observations. When he observed the moon with his occiale (i.e., telescope) he saw that, contrary to the Aristotelian view that all celestial objects were smooth and perfect, it was covered with craters and mountains. He observed that the sun had spots, and that it, too, was not perfect. But, perhaps, what was the most devastating blow to the Aristotelian/Ptolemaic (geocentric) system was Galileo’s observations of the phases of Venus and the discovery of four of the moons of Jupiter. Galileo published his observations in his book Sidereus Nuncius ("The Starry Messenger") on March 12, 1610. In 1611, Johannes Kepler, with whom Galileo had been corresponding for some time, gave his support to Galileo’s observations.

The title page from Sidereus Nuncius ("The Starry Messenger"), Venice, 1610

Galileo’s contribution to our knowledge of the universe is not confined to his astronomical observations. His experiments with falling objects increased our understanding of gravity. It was, in fact, Galileo who first realized that it was an external force which caused objects to fall to Earth and that all objects, in free fall, accelerate at the same rate. These ideas were later developed by Sir Isaac Newton in his law of universal gravitation. Galileo was also the first to gain insight into inertia, an insight which later led to Newton formulating his first law of motion.

Kepler and Galileo were two of the greatest defenders of the Copernican model and their work helped establish the Copernican vision of a heliocentric system as scientific fact, in place of the archaic, Ptolemaic (geocentric) system.

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Copyright © Adler Planetarium and Astronomy Museum.

This armillary sphere is a part of the historic instruments collection at the Adler Planetarium and Astronomy Museum in Chicago. The armillary sphere may have been invented by Eratosthenes around 255 BC. The instrument consists of a set of graduated rings representing circles on the celestial sphere, such as the meridian, equator, ecliptic horizon, tropics, and colures. The whole globe, in many instances, revolved around a central axis representing the polar axis. With an armillary sphere, one could track the path of the sun for any given day of the year or determine a star's coordinates. The armillary sphere was still in use until the 17th century. Image Credit: Adler Planetarium and Astronomy Museum, Chicago, IL.