From Nick Greene,
Your Guide to Space / Astronomy.
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The early theories of the Universe and astronomy, while very clever, all had the same problem. They were all based on incorrect information and beliefs. To create an accurate model, you must first begin with accurate data.

Thanks to the observations of Tycho Brahe, his assistant, Johannes Kepler was able to determine that the circle was not the correct geometric form to explain planetary motions. As a mathematician, he knew that a circle is just a specialized ellipse. Utilizing non-circular ellipses, he was able to calculate orbits, which correctly predicted planetary positions. He couldn’t directly measure a planet's exact orbital sizes, but he was able to measure the ratio by using his equation and Brahe's observations.

Kepler had explained how planets moved, but he still couldn’t explain why. Up till this time, scientists believed that objects tended to stay at rest. Observation had shown that all motion eventually ceases and unmoving objects did not begin to move on their own. So, why would planets?

In the early 17th century, Galileo Galilei used surfaces of varying smoothness to slide blocks across. He found that rough tables made objects slow down at a faster rate than smooth ones. Extrapolating from these observations, he theorized that if a surface were completely smooth, objects would continue moving forever.

If you’ve studied physics, you’ll recognize this as the basis for the theory of inertia. Objects in motion tend to stay in motion in a straight line, and objects at rest tend to stay at rest, unless acted upon by an external force.

Now they knew why the planets were moving, but why in a circle? Why not keep traveling in straight lines and fly off into deep space?

This was answered by Sir Isaac Newton when he published Philosophiae Naturalis Principia Mathematica. He theorized that the external force that kept the planets in orbit was the pull of gravity. According to Newton, the same force that causes an apple falls to the ground also explains why the moon continually "falls" around the Earth.

Meanwhile, our view of the Universe kept evolving. While Tycho Brahe’s attempt to compromise with the Copernican theory and the Ptolemaic model led to an awkward mess, his observations helped Johannes Kepler calculate his three laws of planetary motion, which gave a more accurate picture. Galileo’s discovery of the moons of Jupiter with the newly invented telescope lent credence to the sun-centered model of the solar system.

Starting with Brahe’s years of observation, the work of Kepler, Galileo, and Newton were part of a new era of science, where observation, not philosophy was king. Scientists no longer tried to match data to theory. This would lead to a real renaissance in astronomy and cosmology as well as science in general.

Assignment

Read more about Galileo Galilei, Tycho Brahe, Johannes Kepler and Sir Isaac Newton. Don't forget the discussion Forum.

Third Lesson > Modern Astronomy > Lesson 3, 4, 5, 6, 7, 8, 9, 10