Mon Prof Augmenté
Kepler’s Laws of Orbits
Age 18 · Physics
Planets travel around the Sun along ellipses, with the Sun at one focus. A planet sweeps out equal areas in equal times, and the square of its orbital period grows in proportion to the cube of its average distance from the Sun.
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In the picture
- The Sun: one focus of the elliptical orbit
- The large bright oval: the planet’s closed elliptical path around the Sun
- The short wide wedge: the area swept close to the Sun in a fixed time
- The long narrow wedge: the same area swept farther from the Sun in the same time
- The smaller dotted oval: a smaller orbit with a shorter period, where T² is proportional to a³
Examples
- A comet speeds up as it swings near the Sun and slows down as it moves far away.
- Outer planets such as Neptune take much longer to orbit the Sun than inner planets such as Mercury.
Key takeaways
- Planetary orbits are ellipses, with the Sun at one focus.
- Equal areas are swept in equal times, and larger orbits have longer periods, with T² proportional to a³.
Quiz: Kepler’s Laws of Orbits (Age 18)
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According to Kepler, what shape do planetary orbits around the Sun have?
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What does the equal-area law say about a planet’s motion?
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How does Kepler’s third law relate orbital period T to average distance a from the Sun?
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