Orbit (Encyclopedia of Science)
An orbit is the path a celestial object follows when moving under the control of another's gravity. This gravitational effect is evident throughout the universe: satellites orbit planets, planets orbit stars, stars orbit the cores of galaxies, and galaxies revolve in clusters.
Without gravity, celestial objects would hurtle off in all directions. Gravity pulls those objects into circular and elliptical (oval-shaped) orbits. Indeed, gravity was responsible for the clumping together of dust and gas shortly after the beginning of the universe, which led to the formation of stars and galaxies.
Kepler's laws and planetary motion
Since ancient times, astronomers have been attempting to understand the patterns in which planets travel throughout the solar system and the forces that propel them. One such astronomer was the German Johannes Kepler (1571630). In 1595, he discovered that the planets formed ellipses in space. In 1609, he published his first two laws of planetary motion. The first law states that a planet travels around the Sun on an elliptical path. The second law states that a planet moves faster on its orbit when it is closer to the Sun and slower when it is farther away.
Ten years later, Kepler added a third law of planetary motion. This law makes it possible to calculate a planet's relative distance from the Sun...
(The entire section is 879 words.)
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