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Kepler's second law describes how fast a planet moves at different points in its orbit. What does the law state?
AThe planet moves at exactly the same speed everywhere in its orbit
BThe planet moves fastest when farthest from the Sun
CThe line from the Sun to the planet always points at a fixed star
DThe line from the Sun to the planet sweeps out equal areas in equal times
Answer & Solution
Correct answer: D. The line from the Sun to the planet sweeps out equal areas in equal times
1. Kepler's second law states that an imaginary line drawn from the Sun to the planet sweeps out equal areas in equal times.
2. Because the areas swept are equal in equal times, the planet must move fastest when it is closest to the Sun and slowest when it is farthest away, not the reverse.
3. The orbit does not keep the Sun-planet line fixed on a distant star, since that line rotates continuously as the planet moves around its ellipse.
4. The planet's speed is therefore not constant throughout the orbit, which rules out the claim that it moves at exactly the same speed everywhere.
_Source: OpenStax College Physics (CC BY 4.0), Ch 6 "Uniform Circular Motion and Gravitation", section 6.6 Satellites and Kepler's Laws: An Argument for Simplicity_
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