How does Kepler's second law explain changes in a planet's speed?

Kepler’s second law of planetary motion states that a radial line joining a planet to the Sun sweeps out equal areas in equal intervals of time. Because planets move in elliptical orbits, their distance from the Sun constantly changes, which necessitates a variation in their orbital speed to maintain this constant rate of area coverage.123

Specifically, the law explains speed changes as follows:

• Near Perihelion: When a planet is closest to the Sun, it must travel at a higher-than-average velocity to sweep out the same area as it would at greater distances.12
• Near Aphelion: When a planet is at its farthest point from the Sun, it moves at a lower-than-average velocity.12

This phenomenon is a direct consequence of the conservation of angular momentum, which remains constant throughout the planet's orbit. As the distance between the planet and the Sun decreases, the planet's orbital speed must increase to conserve this momentum.13