<Picture 1 ; The portrait of Kepler>
Recently, I read 『Cosmos』 which is a popular science book by astronomer and Pulitzer Prize-winning author Carl Sagan. In Chapter 3 'Harmony of the Worlds', there is a story of the development of astronomical observation. Reviewing the development of astronomy, we can find out the differences between the science and other studies.
1. History of Astronomy before Kepler ; The Geocentric Theory and The Copernican Theory
There was a time before television, before radio, before books. On a moonless night, our ancestors watched the stars. Unlike the future hard to predict, there are patterns in the night sky. Like the Sun and the Moon, stars always rise in the east and set in the west. So, measuring the stars and its motion, they could understand the seasons, the tides and the life circles of many animals. Therefore, there were a number of astrologers in the past and they were strictly forbidden by government rulers because they can predict some bad accident in their country. At that time, there were many people both astronomers and astrologers simultaneously.
Claudius Ptolemaeus, who claimed the Geocentric theory, is also an astronomer and an astrologer. His theory can be represtented by the imagination that the planets go around the Earth and the Earth is a center of the universe. At that time, the Geocentric theory was generally adopted as true. Supported by the Church through the Dark Ages, Ptolemy's model helped prevent the advance of astronomy for a millenium.
By 1543, a Polish Catholic cleric Nicholas Copernicus claimed a new theory. It was the theory proposing the Sun, not the Earth, is at the center of the universe. The Earth was demoted to just one of the planets, third from the Sun, moving in a perfect circular orbit. the retrogradation of mars that can't be explained by Ptolemy's theory was explained clearly in the Copernican theory. But in 1616 the Catholic Church placed Copernicus' work on its list of forbidden books "until corrected" by local ecclesiastical censors, where it remained until 1835. His lonely struggle between the Earth-centered and Sun-centered was to ignite the modern scientific revolution.
Johannes Kepler, born in Germany in 1571, found his three laws explaining the planetary motions. In his work to find these laws, there is some differences between the Science and other studies.
2. Kepler's Three Law
Kepler worked as a mathematic teacher in a secondary school in Austria. He wasn't a master of the Astronomy for the first time. He proceed his work with an absurd hypothesis that there are only six planet in the universe because of the five perfect solids of Pythagoras and Plato. (At that time, there are only 6 planet known to people.) To access to more accurate observations of planetary positions, he visited Tycho Brahe the Imperial Mathematician in the Court of the Holy Roman Emperor, Rudolf Ⅱ.
<Picture 2 ; Kepler's first law, second law, and third law>
Kepler realized that his hypothesis was not supported by the Tycho's observations. He decided to correct his theory instead of correcting Tycho's data. Analyzing his data, Kepler came to feel that his fascination with the circle had been a delusion. He tried various oval-like curves, calculated away and tried the formula for an ellipse. He found that it matched Tycho's observations beautifully. His first law of planetary motions is as follows.
Kepler's first law: A planet moves in an ellipse with the Sun at one focus.
In addition, Kepler realized that the more closely a planet reaches the Sun, the faster the planet goes. So, he found the second law of planetary motion: Planets sweeps out equal areas in equal times. In the B of the picture 2, it takes as long to travel along the left line as along the right line.
Kepler's second law: A planet sweeps out equal areas in equal times.
The last law was called the harmonic law because he found the law from the assumption that planetary motions are harmonic like a music.
Kepler's third law: the more distant the planet, the more slowly it moves, but according to a precise mathematical law: 
,
P represents the period of revolution of the planet about the Sun, measured in years, and a the distance of the planet from the Sun measured in "astronomical units." An astronomical unit is the distance of the Earth from the Sun. The graph of it is as follows.
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<Picture 3 ; The graph of the Kepler's third law>
Uranus, Neptune and Pluto were not found at that time. But his third law also makes sense for these planets.
3. The Aspect of Science from Kepler's Theory
According to the process to find the three laws, Kepler is not just an astrologer but a great astronomer. Ancient astrologers hide or distort the data not fit in their claims, and even their claims are not falsifiable. But Kepler daringly threw his theory away instead of correcting the Tycho's observations when his theory was not in accordance with the data. After that, he founded another hypothesis and made a great advance in the Astronomy with his mathematical proof. In addition, his theory is not only falsifiable but also predictable because the third law makes sense for Uranus, Neptune and Pluto.
Bibiology
Sagan, C. E., 1980, Cosmos(1st ed.), New York: Random House.
2016.01.01 Han Kwang Hee
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