## Brahe, Kepler, Galileo

Previous: The Copernican Revolution

Here I mention the three main astronomers between Copernicus and Newton, and see what are their contributions.

Tycho Brahe. Wikipedia.

• Tycho Brahe (1546-1601, Danish): The foremost and most accurate naked-eye observer of the sky. He observed the motion of the planets, and had tons of carefully collected accurate empirical data (4′ accuracy or better, approx. 1/8 the angular diameter of the full moon).
– Comets are not an atmospheric phenomenon, they are much more distant than the Moon.
– By observing a supernova (stella nova) in 1572, he demonstrated that the heavens above the Moon are not immutable (a belief held by the Church).

Johannes Kepler. Wikipedia

• Johannes Kepler (1571-1630, German): He inherited the data by Brahe and analyzed it to come up with a mathematical body to explain the planetary motion that was consistent with these observations.
– He abandoned the assumption of circular motion, since it wouldn’t allow him to match the data with his model, and proposed elliptical orbits.
– Kepler’s Laws:
1. A planet orbits the Sun in an ellipse, with the Sun at one focus of the ellipse.
2. A line connecting a planet to the Sun sweeps out equal areas in equal time intervals.
3. P2 = a3, P == orbital period in years, a == average distance of the planet from the Sun in astronomical units (AU). There is a linear relationship between the semimajor axis and the period of a planet’s orbit around the Sun.

See also Kepler’s Laws Revisited.

Galileo Galilei. Wikipedia

• Galileo Galilei (1564-1642, Italian): Observed the heavens with a self-made telescope.
– Objects near the surface of the Earth fall with the same acceleration, independent of their weight.
– The Milky Way is not a cloud. It contains an enormous number of individual stars.
– The Moon has craters, therefore is not a perfect sphere (as believed previously).
– The phases of Venus indicate that the planet does not shine by itself, but rather reflects sunlight.
– The Sun has dark spots varying in number and location.
– Discovery of four moons around Jupiter, indicating the existence of at least another center of motion in the universe. This greatly damaged the geocentric model, still supported by the Church.

In the year of Galileo’s death, on Christmas day, Isaac Newton was born, arguably the greatest of all scientific minds…

Next: Newton’s Laws

Isaac Newton. Wikipedia

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