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Further, the current usage of "Kepler's Second Law" is something of a misnomer. Kepler had two versions, related in a qualitative sense: the "distance law" and the "area law". The "area law" is what became the Second Law in the set of three; but Kepler did himself not privilege it in that way. [11]
Kepler's laws apply only in the limited case of the two-body problem. Voltaire and Émilie du Châtelet were the first to call them "Kepler's laws". Nearly a century later, Isaac Newton had formulated his three laws of motion. In particular, Newton's second law states that a force F applied to a mass m produces an acceleration a given by the ...
Mathematically, the second epicycle and the equant produce nearly the same results, and many Copernican astronomers before Kepler continued using the equant, as the mathematical calculations were easier. Copernicus' epicycles were also much smaller than Ptolemy's, and were required because the planets in his model moved in perfect circles.
The second section establishes relationships between centripetal forces and the law of areas now known as Kepler's second law (Propositions 1–3), [21] and relates circular velocity and radius of path-curvature to radial force [22] (Proposition 4), and relationships between centripetal forces varying as the inverse-square of the distance to ...
Kepler's 2nd law: equal areas are swept out in equal times (area bounded by two radial distances and the orbital circumference): = | | where L is the orbital angular momentum of the particle (i.e. planet) of mass m about the focus of orbit,
This is Kepler's second law of planetary motion. The square of this quotient is proportional to the parameter (that is, the latus rectum) of the orbit and the sum of the mass of the Sun and the body. This is a modified form of Kepler's third law. He next defines: 2p as the parameter (i.e., the latus rectum) of a body's orbit,
Next Newton proves his "Theorema II" which shows that if Kepler's second law results, then the force involved must be along the line between the two bodies. In other words, Newton proves what today might be called the "inverse Kepler problem": the orbit characteristics require the force to depend on the inverse square of the distance. [3]: 107
The two-body problem is interesting in astronomy because pairs of astronomical objects are often moving rapidly in arbitrary directions (so their motions become interesting), widely separated from one another (so they will not collide) and even more widely separated from other objects (so outside influences will be small enough to be ignored safely).