The path of a body in revolution. Orbits are constrained by centripetal forces. Astronomical orbits are held by gravitational force between the orbiting body and a the body or bodies being orbited.
Orbits of one body about a single other, are easily modeled, but any system with three or more orbiting bodies is difficult, as described by the classical three body problem.
When a group G has a permutation action on a set S, the orbit of an element s is defined to be the subset of S consisting of those elements to which some element of G can send s.

--back to combinatorics--
Why stuff goes into orbit:

Imagine I'm on the moon, and I have a bunch of tennis balls. If I throw a ball parallel to the ground, but not very quickly, it'll go for a while, but eventually gravity will pull it to the ground and it'll stop. Now, as I throw the balls with greater and greater velocity, they'll get farther and farther, but they'll all fall eventually. But then I throw one really, really fast, and it never hits the ground! How can this be?

Let's examine the curved path of the ball as it travels any horizontal distance-- for simplicity's sake, 1 meter. Within that distance, it will fall away from a perfectly horizontal path (the path it would take if there were no gravity) by a certain amount; let's call it dvert, and it increases as horizontal velocity increases, since it can travel farther before it hits the ground. Now, let's look at the planet we're on (in this case, the moon); more specifically, 1 meter of it. Since the planet isn't flat (last time I checked), it'll curve away from a perfectly flat distance by a certain amount within this 1-meter length.

Now here's the crazy part: If dvert (the amount the ball falls away from horizontal as it travels 1 meter) ever equals the amount that the planet curves away from the horizontal, the ball will never hit the ground! Because the earth is curving away at the same rate that the ball is, it will go into orbit.
A brand of Wrigley's gum. Comes in a mostly blue box, cleverly modern font depicting the name of said gum, and the trade marked motto "Just brushed clean feeling". Sugar free. Not a low calorie food. The made for resale packets contain 14 individually wrapped pieces of chewable goodness.

If you are a phenylketonuric: beware. This product contains phenylalanine.

Or"bit (?), n. [L. orbita a track or rut made by a wheel, course, circuit, fr. orbis a circle: cf. F. orbite. See 2d Orb.]

1. Astron.

The path described by a heavenly body in its periodical revolution around another body; as, the orbit of Jupiter, of the earth, of the moon.

2.

An orb or ball.

[Rare & Improper]

Roll the lucid orbit of an eye. Young.

3. Anat.

The cavity or socket of the skull in which the eye and its appendages are situated.

4. Zool.

The skin which surrounds the eye of a bird.

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