📘 Kepler’s Second Law
Kepler's Second Law, the Law of Equal Areas, states that a line connecting a planet to the Sun sweeps out equal areas in equal intervals of time.
The Law of Equal Areas
While Kepler’s First Law describes the shape of planetary orbits, the Second Law explains how orbital speed changes along that path.
Equal Area in Equal Time
Imagine drawing a line from the Sun to a planet. As the planet moves over a fixed period (like one month), that line traces out a wedge-shaped area. Kepler found that:
- When the planet is closest to the Sun (perihelion), it moves faster, sweeping a wide, short area.
- When the planet is farthest from the Sun (aphelion), it moves slower, sweeping a narrow, long area.
No matter where the planet is in its orbit, the area swept in a given time is always the same.
Connection to Angular Momentum
Kepler’s Second Law reflects the conservation of angular momentum. With no outside torque acting on the planet:
- A smaller distance to the Sun requires higher speed to maintain constant angular momentum.
- A larger distance allows the planet to move more slowly.
Interactive: Orbital Motion Vocabulary
Match the key terms of Kepler’s Second Law to their correct descriptions.
Click a term and then its matching description. Complete all pairs to finish!
💡 Quick Concept Check:
At which point in its orbit—perihelion or aphelion—does a planet have the greatest kinetic energy?