Earth  Planets and Moons  ID: 4642

Kepler's Laws of Planetary Motion Described Using Earth Satellites

Johannes Kepler was born on December 27, 1571, in Weil der Stadt, Württemberg, in the Holy Roman Empire of German Nationality. Kepler moved to Prague to work with the renowned Danish astronomer, Tycho Brahe. He inherited Tycho's post as Imperial Mathematician when Tycho died in 1601. Using the precise data that Tycho had collected, Kepler discovered that the orbit of Mars was an ellipse. In 1609 he published Astronomia Nova, delineating his discoveries, which are now called Kepler's first two laws of planetary motion. Kepler noticed that an imaginary line drawn from a planet to the Sun swept out an equal area of space in equal times, regardless of where the planet was in its orbit. If you draw a triangle out from the Sun to a planet’s position at one point in time and its position at a fixed time later, the area of that triangle is always the same, anywhere in the orbit. For all these triangles to have the same area, the planet must move more quickly when it is near the Sun, but more slowly when it is farthest from the Sun. This discovery (which became Kepler’s second law of orbital motion) led to the realization of what became Kepler’s first law: that the planets move in an ellipse (a squashed circle) with the Sun at one focus point, offset from the center. In 1619 he published Harmonices Mundi, in which he describes his "third law." Kepler’s third law shows that there is a precise mathematical relationship between a planet’s distance from the Sun and the amount of time it takes revolve around the Sun.

Kepler’s laws of planetary motion can also be used to describe the motion of satellites in orbit around Earth.

Visualization Credits

Horace Mitchell (NASA/GSFC): Visualizer
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NASA's Scientific Visualization Studio