Kepler’s first law of planetary motion

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Kepler’s first law of planetary motion

astronomy

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June 17, 2026

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Kepler's first law of planetary motion, formulated by Johannes Kepler, states that planets move around the Sun in elliptical orbits, with the Sun at one of the two foci of the ellipse. This law revolutionized the understanding of celestial mechanics, moving away from the ancient idea of perfect circular orbits. An ellipse is a flattened circle, and its shape is defined by its eccentricity. The first law also implies that a planet's distance from the Sun varies throughout its orbit. This variation in distance is directly related to Kepler's second law, which states that a line connecting a planet to the Sun sweeps out equal areas in equal times, meaning planets move faster when closer to the Sun and slower when farther away. These laws were foundational for Isaac Newton's development of the law of gravitation.123456

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conic section
Post-Greek applications
…Johannes Kepler derived his first law of planetary motion: A planet travels in an ellipse with the Sun at one focus. Galileo Galilei published the first correct description of the path of projectiles—a parabola—in his Dialogues of the Two New Sciences (1638). In 1639 the French engineer Girard Desargues initiated…
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…Johannes Kepler derived his first law of planetary motion: A planet travels in an ellipse with the Sun at one focus. Galileo Galilei published the first correct description of the path of projectiles—a parabola—in his Dialogues of the Two New Sciences (1638). In 1639 the French engineer Girard Desargues initiated…
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mathematics
Newton and Leibniz
…the planets were known by Kepler’s laws to move in ellipses with the Sun at one focus, this result supported his inverse square law of gravitation. To establish the proposition, Newton derived an approximate measure for the force by using small lines defined in terms of the radius (the line…
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…the planets were known by Kepler’s laws to move in ellipses with the Sun at one focus, this result supported his inverse square law of gravitation. To establish the proposition, Newton derived an approximate measure for the force by using small lines defined in terms of the radius (the line…
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Kepler’s laws of planetary motion
…be stated as follows: (1) All planets move about the Sun in elliptical orbits, having the Sun as one of the foci. (2) A radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time. (3) The squares of the sidereal periods (of…
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…be stated as follows: (1) All planets move about the Sun in elliptical orbits, having the Sun as one of the foci. (2) A radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time. (3) The squares of the sidereal periods (of…
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principles of physical science
The development of quantitative science
…by the German astronomer Johannes Kepler, based on the inspired interpretation of centuries of patient observation that had culminated in the work of Tycho Brahe of Denmark, may be regarded fairly as the first great achievements of modern quantitative science.
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…by the German astronomer Johannes Kepler, based on the inspired interpretation of centuries of patient observation that had culminated in the work of Tycho Brahe of Denmark, may be regarded fairly as the first great achievements of modern quantitative science.
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