Wednesday, April 10, 2013

History of Science -- Part Two: Medieval and Renaissance Astronomy

From ancient times, the position of the stars in the sky foretold the change of the seasons. What, then, was the significance of the five bright objects that wandered through the starry background of the fixed stars? An obvious conclusion was that the motion of these planets (“planet” means “wanderer") foretold erratic human affairs and warranted serious attention. Astronomy’s roots are in astrology.

In the second century A.D., Ptolemy of Alexandria described the heavenly motions so well that calendars and navigation based on his model worked beautifully. The astrologer’s predictions — at least regarding the positions of the planets — were likewise accurate.

Although the stars moved in a steady progression across the sky on a yearly cycle, the planets not only moved differently (from the stars and even from each other). What was most odd is that some planets would actually change the direction of their motion against the fixed stars and go backward for a short time before reversing again. This was called “retrograde” motion.

Ptolemy’s explanation was based on a stationary Earth as the cosmic center, requiring planets to move on “epicycles,” complicated loopy curves made up of circles rolling on circles within yet further circles. Thus was preserved Aristotle’s “perfect circles” in the perfect heavens.

King Alfonso X of Castile, having the Ptolemaic system explained to him, supposedly remarked “If the Lord God Almighty had consulted me before embarking on Creation, I would have recommended something simpler.” Nevertheless, the combination of Aristotle’s physics and cosmology with Ptolemy’s astronomy was accepted as both practical truth and religious doctrine, and enforced by the Holy Inquisition.

In the sixteenth century, an insight upsetting the whole apple cart appeared within the Church itself. The Polish cleric and astronomer Nicolas Copernicus felt Nature had to be simpler than Ptolemy’s cosmology. He suggested that the Earth and five known planets orbited a central, stationary sun. The retrograde motion of planets such as Mars against the starry background was the result of observing the orbit from the third planet from the Sun, also in motion. The outer planets would seem to move backward when the Earth was overtaking them in its own orbit. (Inner orbits move faster than outer orbits. That is to come too.) The Earth was just the third planet from the Sun. It was a simpler picture.

However, simplicity was hardly a compelling argument. The Earth obviously stood still. One felt no motion. A dropped stone would be left behind on a moving Earth! If the Earth moved, since air occupied all space, a great wind would blow!! Moreover, a moving Earth conflicted with the wisdom of the Golden Age and the authority of Aristotle. Such arguments were hard to refute. And, most disturbingly, the Copernican system was seen to contradict the Bible, and doubting the Bible threatened salvation.

Copernicus’ work, published shortly after his death, included a foreword, probably added by a colleague, announcing his description as a mathematical convenience only — it did not describe actual motions. Any contradiction of the Church’s teachings was disavowed.

Enter Tycho Brahe … the man with a metal nose. (Don’t ask.) He was a Danish nobleman who refuted the Aristotelian belief in an unchanging celestial realm. His reputation started when he discovered a new star in the sky, a nova in the constellation Cassiopeia. Prior to his time, comets were considered atmospheric phenomenon. After all, the heavenly realm was perfect and unchanging. Through Brahe’s careful observations and calculations, he proved that comets existed beyond the orbit of the moon … amongst the celestial spheres. So much for the unchanging heavens.

Tycho Brahe was granted an estate on the island of Hven and the funding to build an early research institute by the King of Denmark. He built large astronomical instruments and took many careful measurements. On the island he founded small factories such as paper-making to provide material for printing his results. After disagreements with the new Danish king Christian IV in 1597, he was invited to Prague, where he became the official imperial astronomer. He built the new observatory at Benátky nad Jizerous. Here, from 1600 until his death in 1601, he was assisted by Johannes Kepler.

Brahe died under unusual circumstances in 1601. Not suspicious, just unusual. He suddenly contracted a bladder or kidney ailment after attending a banquet in Prague, and died eleven days later. According to Kepler's first hand account, Brahe had refused to leave the banquet to relieve himself because it would have been a breach of etiquette since the royalty in attendance had not left the party. After he had returned home he was no longer able to urinate, except eventually in very small quantities and with excruciating pain.

Upon his death, Kepler “inherited” the results of over twenty years of precise planetary measurements. With that windfall of data, Kepler began working on a system of planetary movements and orbits.

Although he believed in the Copernican system with the Sun at the center of the solar system, he adopted many of Aristotle’s views. He tried to fit the measurements and data to circular orbits and attempted to model the space between the orbits using the five Platonic Solids (for example, tetrahedron, cube, octahedron, etc.). He even invented new planets to try to get the scheme to work.

For many years he struggled trying to find a model that matched the data. Sadly, the solution actually was a common shape, one of the conical sections. Although the orbits were nearly circular, they were actually an oblong shape called an ellipse or elliptical.

Kepler eventually developed his first two laws: (1) The obit of every planet is an ellipse with the Sun at one of the two foci; and (2) A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

These laws stated that, not only weren’t the orbits perfect circles, but the planets actually sped up when approaching closer to the sun and slowed down on the distant part of their orbit.

Several years later he added his third law: (3)The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. These models not only worked for the known planets, but, years later, when additional planets were discovered, they also followed these laws.

Kepler's laws were not immediately accepted. Several major figures such as Galileo and René Descartes completely ignored Kepler's results. Many astronomers, including Kepler's teacher, Michael Maestlin, objected to Kepler's introduction of physics into his astronomy. Some adopted compromise positions. Ismael Boulliau accepted elliptical orbits but replaced Kepler's area law with uniform motion in respect to the empty focus of the ellipse while Seth Ward, English Astronomer and Mathematician, used an elliptical orbit with motions defined by an “equant,” an element of Ptolemy’s theory.

Kepler did great astronomy and was an excellent mathematician, but science didn’t guide his worldview. Initially, he considered the planets to be pushed along their orbits by angels, and as a sideline he drew horoscopes, in which he likely believed. He also had to take time from his astronomy to defend his mother from accusations of witchcraft.

However, his three laws were good scientific progress based on careful observation and cunning calculations and insight. However, Kepler did not explain any reason for why these relatively simple relationships existed. But they did provide a much simpler and very precise method to analyze planetary motions. An explanation for why Nature worked this way would have to wait for eight decades and Sir Isaac Newton to provide, but Kepler did live to see discoveries using the new instrument of the telescope and the discovery of moons on several of these planets and observation of the phases of Venus, definitely proving the solar-centric theory.

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