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Why does the velocity of a planet change as it orbits the sun?
The Earth's orbit around the sun isn't a perfect circle: it's an ellipse, or an oval.
This means sometimes it will be closer to the sun than others. When it's close, the pull of the sun's gravity makes the Earth speed up. That strong pull "slingshots" the Earth away, and when it's farther out, it slows down. Kepler's second law describes how fast bodies orbit others.