Astronomy › Section C · Orbits and gravity › Topic C.1
The laws of planetary motion
Around 1600, a meticulous observer and a stubborn mathematician turned the Sun-centred idea into a precise description of how planets move. Kepler's three laws still predict orbits today, from asteroids to planets around other stars.
In this topic
- Tycho Brahe's record of the sky
- Johannes Kepler and the problem of Mars
- What an ellipse is
- The first law: orbits are ellipses
- Worked example: how far Earth's distance swings
- The second law: equal areas in equal times
- The third law: periods and distances
- Worked example: using the third law both ways
- Worked example: Halley's Comet
- What the laws do and do not explain
- Common mistakes to avoid
- Key takeaways
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