History of Astronomy
Here are some notes about the thousands of years of discoveries which have led to our present day understanding of the universe. We will study from Thales of ancient Greece to the stunning changes in Astronomy in the seventeenth century.
From the very beginning of human history, the universe was thought to surround the Earth, with our planet at the center of all things. Go out on a dark, clear night and you see what looks like the upper half of a celestial sphere sprinkled with stars arching over your head. The impression is very strong. That is why the dome of a planetarium can look so very much like the real night sky. This geocentric system was taught and refined by the ancient Greeks, among them, Aristotle. The great influence of Aristotle assured that the Earth-centered view would persist for centuries after his death. What any system of the universe needed to explain was the occasional backwards motion of the planets called retrograde motion. As planets seem to wander through the constellations, they seem to back up every year or so. Mars retrogrades about every 26 months. The geocentric system used a small circle called an epicycle to explain retrograde motion. Ptolemy explained the theory of epicycles in his great work on Astronomy, Almagest. Carl Sagan explains it all here.
But five hundred years ago, some European astronomers began to doubt the old geocentric system. The first and most famous was Nicholas Copernicus whose image we see at the top of this page. In 1543, Copernicus published a book called De Revolutionibus. In it, he described how the Earth was just one of many planets which circle the Sun in an heliocentric system. Retrograde motion can be explained by the fact that a moving Earth causes the planets only to appear to change direction. In an heliocentric system, the planets display an ever changing array of alignments; at times being easily visible, and at other times being hidden by the Sun.
The work of Copernicus influenced other astronomers of the late sixteenth century. Galileo, for example, came to believe in and promote the heliocentric theory. With his little telescope, he proved that it was the Earth which moved around the sun; not the other way around. Among his most revealing observations was the changing phases of the planet Venus. This was to get Galileo into serious trouble with the Pope and the Holy Office.
Not every astronomer was quick to accept the new thinking. Tycho Brahe never was convinced of the possible orbital motion of Earth. He devised an interesting hybrid system where the Earth was stationary. To support his modified geocentric system, Tycho made incredibly accurate observations of the positions of planets and stars from his observatory on the island of Hven. In his studies of heavenly bodies, Tycho failed to detect any . parallax These observations convinced him that the Earth could not be moving through space.
However, his assistant, Johannes Kepler, through his mathematical manipulations of Tycho's data, discovered otherwise. Kepler gave us three laws of planetary motion:
- Planets orbit the Sun in elliptical orbits with the Sun at one focus of this ellipse
- Planets sweep out equal areas of an orbit in equal intervals of time
- The cube of the semi-major axis of a planet's orbit is proportional to the square of its period
You must watch this somewhat amusing YouTube presentation explaning Kepler's laws!
Here is Sagan again to explain the significance of Kepler's accomplishment. These laws did a wonderful job of explaining exactly how the planets orbit the Sun if not why.
The physical reason for the planets motions was yet to be explained by Sir Isaac Newton.
More on the life of Tycho Brahe including his pet moose.
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Updated September 23, 2009