AP® European History · Unit 4 · Topic 4.2
Kepler, Galileo, and Newton
Kepler showed planets move in ellipses and sweep equal areas in equal times, Galileo's telescope found evidence no Earth-centered model could explain, and Newton tied it all together with one law of gravity that works on apples and moons alike.
Copernicus had put the Sun at the center but kept circular orbits, which never quite matched observation and needed leftover fudges to work. Kepler broke that last habit. Using Tycho Brahe's precise data, he showed planets travel in ellipses with the Sun at one focus, not the center, and that a planet sweeps out equal areas in equal times, moving faster when close to the Sun and slower when far. A third law linked period and distance: period squared equals distance cubed. Once orbits stopped fighting to be circles, the math finally fit the sky.
Galileo supplied the physical evidence. His telescope revealed mountains and craters on the Moon, meaning the heavens were not perfect unchanging spheres, moons orbiting Jupiter, proving not everything circled Earth, and the phases of Venus, which only made sense if Venus circled the Sun. None of this fit a strict Earth-centered universe. Newton then unified the whole picture mathematically. His law of universal gravitation showed that one inverse-square force, weakening with the square of distance, pulls the apple to the ground and holds the Moon in orbit around Earth, the same rule governing heaven and earth alike.
This chain, Kepler's laws, Galileo's observations, Newton's synthesis, turned astronomy into physics, and that same confidence in reason and evidence over inherited authority spread next into philosophy, politics, and religion, setting up the Enlightenment's challenges to monarchy and church.
Key terms
- Ellipse
- An oval-shaped orbit with two foci, where the Sun sits at one focus rather than at the center.
- Equal areas in equal times
- Kepler's second law: a planet sweeps the same amount of orbital area in any equal time interval, moving faster near the Sun.
- Inverse-square law
- A force that weakens in proportion to the square of the distance between two objects, as gravity does.
- Kepler's third law
- The rule that a planet's orbital period squared equals its distance from the Sun cubed, in years and astronomical units.
- Phases of Venus
- The changing crescent-to-full appearance of Venus seen through Galileo's telescope, possible only if Venus orbits the Sun.
Watch it explained
- How it worksDraw an ellipse with the Sun at one focus and sweep equal areas in equal times.
- How it worksShow me what Galileo saw through his telescope that no geocentric model could survive.
- How it worksHow does one inverse-square law explain a falling apple and the orbit of the Moon at once?
- Worked exampleFind how long Jupiter takes to orbit using Kepler's third law and its distance of 5.2 astronomical units.
- Common mistakeWhy isn't Galileo's trial best described as science versus religion?
Check yourself
End of topic.