Throughout history, astronomers have come up with, studied, and refined their theories of how the universe and the objects in it came to exist and how they behave. 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.

The way science works, you need actual data (from experiments and observations). To make the best possible conjectures about what you observe and measure, you need a LOT of data.

The more you have, the better your eventual explanation of phenomena will be. Data help shape the theory, not the other way around. This is at the heart of the scientific method.

### The Rise of Rational Thought in Astronomy

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. If planets moved in pure circles, their observed motions in the sky would not look the way they do. So, he applied mathematics and geometry to the problem of understanding orbits. As a mathematician, Kepler 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 that 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 them to understand effects on a body's motion.

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.

Using these experiments, astronomers could figure out WHY the planets were moving, but why did they stay in elliptical orbits? Why didn't they keep traveling in straight lines and fly off into deep space?

That question was answered by Sir Isaac Newton through a great many experiments that he wrote up in his publication *Philosophiae Naturalis Principia Mathematica*. He theorized that the external force that keeps the planets in orbit is 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. Newton's discoveries began to revolutionize our thoughts about motions of objects in space.

Meanwhile, humanity's 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. Instead, they took data and used it to help them shape their theories of the universe. This would lead to a real renaissance in astronomy and cosmology. Directly and indirectly, their work brought us to the brink of space travel and exploration.

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