Mon Prof Augmenté
How Planets Orbit the Sun
Age 18 · Physics
A planet traces an ellipse with the Sun sitting at one focus, not at the centre. As it travels, it sweeps out equal areas in equal times, which means it speeds up near the Sun and slows down far from it.
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In the picture
- The Sun, sitting at one focus of the elliptical path
- A wide, short area swept out close to the Sun
- A long, narrow area swept out far from the Sun, equal in size to the near one
- The planet, moving fastest where the orbit passes closest to the Sun
Examples
- Halley’s Comet swings in close to the Sun then travels far out into space for decades before returning.
- Earth moves slightly faster in January, when it is closest to the Sun, than in July, when it is farthest away.
Key takeaways
- Orbits are ellipses, not circles, with the Sun at one focus.
- A planet moves fastest at its closest point to the Sun and slowest at its farthest point.
Quiz: How Planets Orbit the Sun (Age 18)
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Which shape best describes the path a planet follows around the Sun?
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At which point in its orbit does a planet move fastest?
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A planet sweeps out a short, wide region close to the Sun and a long, narrow region far from the Sun in the same amount of time. What must be true about the areas of these two regions?
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