According to Kepler's 2nd Law, a planet moves fastest in its orbit when it is

• A. At perihelion
• B. At aphelion
• C. At the ascending node
• D. At the farthest point from the Sun

Answer: (A)

Kepler's second law states that a planet sweeps out equal areas in equal times as it orbits the Sun. Because the area swept per unit time is constant, the planet must move faster when it is closer to the Sun and slower when it is farther away. At perihelion, the closest point to the Sun, the radius is smallest, so to keep the same area per unit time, the angular rate must be higher, making the linear speed the greatest. At aphelion, the farthest point, the opposite happens—the speed is the slowest. The ascending node is a geometrical crossing point of the orbit plane and doesn’t determine speed.