Knowra Kepler's second law Kepler's second law A line joining a planet and the Sun sweeps out equal areas in equal intervals of time. This rule expresses the conservation of angular momentum in orbital motion.
Angular momentum : A conserved physical quantity associated with rotational motion, determined by position, momentum, and their orientation. The Sun's gravitational force exerts no torque about the Sun, so orbital angular momentum stays constant.
Johannes Kepler : A German astronomer and mathematician who formulated three laws of planetary motion in the early seventeenth century. He published the equal-area rule in 1609 as part of his account of planetary motion.
Kepler's laws of planetary motion : Three empirical rules describing planetary orbits, orbital speeds, and the relation between orbital period and size. The equal-area rule is the second of the three laws.
Newton's laws of motion : Three principles relating an object's motion to inertia, net force, and action-reaction interactions. Combined with gravity, they derive Kepler's area rule from zero torque about the Sun.
Areolar velocity : The rate at which a moving body's radius vector sweeps out area. Kepler's law states that this rate remains constant throughout an orbit.
Astronomia Nova : Johannes Kepler's 1609 book presenting his discoveries about the orbit of Mars and planetary motion. It introduced the equal-area rule alongside Kepler's elliptical-orbit law.
Ellipse : A closed curve whose points have a constant sum of distances from two fixed foci. Kepler's first law places the Sun at one focus of each planetary ellipse.
Newton's Principia : Isaac Newton's 1687 mathematical work presenting laws of motion and universal gravitation. Newton showed how Kepler's empirical laws follow from gravitational dynamics.
Central force : A force directed along the line joining an object to a fixed center, with magnitude depending only on distance. A central force has zero torque about its center, producing the conserved angular momentum behind the law.
Tycho Brahe : A Danish astronomer whose precise pre-telescopic observations of planetary positions informed Kepler's work. His Mars observations gave Kepler the evidence needed to test how planets move.
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