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Definition of Orbits

An orbit is the path one object follows around another object. The object that is revolving in orbit is commonly known as a satellite. Smaller, less massive objects will orbit around larger, more massive objects. Planets in our solar system all orbit around the sun in the same orbital plane in elliptical orbits. Planets and other satellites in the solar system follow Kepler's Laws of Planetary Motion for their orbits.

The Ellipse
- Most satellite and planetary orbits are shaped as an ellipse. An ellipse is essentially an oval, with each ellipse having two foci. Imagine you had a circle with one center. Now imagine that you duplicated the center and started pulling the two center points away from each other. The resulting shape would be an ellipse with two foci. The further the foci move away from each other, the more elliptical or eccentric the ellipse becomes. Most of the orbits of the planets are elliptical; however, the foci are so close together that they almost look circular.
Kepler's First Law
- Kepler's first law states that the planets revolve around the sun in elliptical orbits.The sun is located at one focus of the ellipse, while the other focus of the ellipse is empty. The distance from the planet to the sun constantly changes in its orbit. Perihelion is defined as the point in the orbit where the planet is closest to the sun; aphelion is defined as the point in the orbit where the planet is farthest from the sun. Kepler's first law of motion also applies to moons orbiting planets.
Kepler's Second Law
- Kepler's second law states that a planet will sweep out equal areas of an ellipse in equal time as the planet travels in its orbit, meaning that the speed of the planet changes in its orbit around the sun. The planet will travel fastest in its orbit at perihelion and will move slowest in its orbit when it is at aphelion.
Kepler's Third Law
- Kepler's third law states that the ratio of the squares of amount of time two planets take to orbit is equal to the ratio of cubes of the semi-major axis. In other words, the amount of time it takes a planet to orbit the sun increases dramatically with its distance to the sun. Kepler's third law allows us to calculate the period in years it takes a planet to orbit the sun. According to Kepler's third law:
  Period in years² = Radius in A.U. ³
Barycenter
- The barycenter is the center of mass between two objects. In reality, planets do not revolve around the center of the sun, but both the sun and the planet revolve around the center of mass between the two objects. This fact is also true between moons and the planets that they orbit. The orbit of each planet will wobble just a little, as both the moon and the planet orbit around the center of gravity between them.

Astronomy