30/04/2026
Kepler’s Second Law of Planetary Motion
➡️ Johannes Kepler formulated the laws of planetary motion from the detailed astronomical observations of Tycho Brahe. Among these, the second law explains how a planet’s speed varies as it revolves around the Sun. Unlike uniform circular motion, planetary motion is not at a constant speed, and this law provides the reason for that variation.
➡️ Statement of the Law
Kepler’s Second Law states that a line joining a planet to the Sun sweeps out equal areas in equal intervals of time.
In practical terms, this means that when a planet is nearer to the Sun, it travels faster, and when it is farther away, it travels more slowly. The law therefore describes a continuous change in orbital speed depending on distance from the Sun.
➡️ Mathematical Formulation
The law can be expressed quantitatively in several related forms:
The rate at which area is swept out by the radius vector remains constant:
dA/dt = constant
In terms of distance and angular velocity:
(1/2) × r² × ω = constant
In terms of angular momentum:
m × r² × ω = constant
In these expressions, A represents the area swept out, t is time, r is the distance between the planet and the Sun, ω is the angular velocity, and m is the mass of the planet.
These formulations show that the areal velocity does not change throughout the motion.
This law is closely connected to the principle of Conservation of Angular Momentum, which states that angular momentum remains constant in the absence of external torque.
➡️ Significance of the Law
Kepler’s Second Law has several important implications in physics and astronomy. First, it explains why planets do not move with uniform speed in their orbits but instead accelerate and decelerate depending on their distance from the Sun.
Second, it played a crucial role in the development of classical mechanics. Isaac Newton used this law as a foundation in formulating his law of universal gravitation.
Third, the law reveals a deeper conserva