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What is an orbit?
An orbit is the gravitationally curved path of one object around a point or another body, for example the gravitational orbit of a planet around a star.
An orbit is a regular, repeating path that an object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the moon, or human made.
In our solar system, the Earth orbits the Sun, as do the other eight planets. They all travel on or near the orbital plane, an imaginary disk-shaped surface in space. All of the orbits are circular or elliptical in their shape. In addition to the planets' orbits, many planets have moons which are in orbit around them.

Path and Shape of Orbits
Contrary to popular belief, orbits are not perfect circles. Copernicus discovered that our Solar System was heliocentric (planets orbiting around the sun) instead of geocentric (all the planets rotating around the Earth) in 1514, but he mistakenly thought that the planets' paths were perfect circles. Johannes Kepler, another astronomer who lived in the 1600's, introduced his three laws of planetary motion stating his beliefs on the movement of the planets. If you ever need a good simulation for visual learning, then go to this site: http://vimeo.com/2015277 it has a very nice demonstration of Kepler's laws.

Kepler's 3 Laws of Planetary Motion:
1. Planets orbit around the sun in elliptical patterns, not circular, around the sun.

2. Planets orbiting around the sun "sweep out" with equal areas in equal times.
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The location where the Earth is closest to the sun is called the Perihelion, and the point where it is furthest is called the Aphelion. There must be two foci in an orbit, the sun being at one of the two points.

3. T2/R3=1 (or any other constant depending on units). This means the Period of a planet's orbit squared divided by the distance of the radius to the sun cubed equals a constant.

Orbit of the moon
The orbit of the Moon around the Earth is completed in approximately 27.3 days. Because of this motion, the Moon appears to move about 13° against the stars each day, or about one-half degree per hour. If you watch the Moon over the course of several hours one night, you will notice that its position among the stars will change by a few degrees. The changing position of the Moon with respect to the Sun leads to lunar phases. The Moon stays in orbit around the Earth because of the gravitational attraction force between the Earth and Moon. The Moon has a nearly circular orbit (e=0.05) which is tilted about 5° to the plane of the Earth's orbit. Its average distance from the Earth is 384,400 km. The combination of the Moon's size and its distance from the Earth causes the Moon to appear the same size in the sky as the Sun, which is one reason we can have total solar eclipses.

What causes an orbit to happen?
Orbits are the result of a perfect balance between the forward motion of a body in space, such as a planet or moon, and the pull of gravity on it from another body in space, such as a large planet or star. An object with a lot of mass goes forward and wants to keep going forward; however, the gravity of another body in space pulls it in. There is a continuous tug-of-war between the one object wanting to go forward and away and the other wanting to pull it in.

These forces of inertia and gravity have to be perfectly balanced for an orbit to happen. If the forward movement (inertia) of one object is too strong, the object will speed past the other one and not enter orbit. If inertia or momentum is much weaker than the pull of gravity, the object will be pulled into the other one completely and crash.


Different kinds of orbits:

Satellites put in space by people serve different purposes. Some use orbits to move from planet to planet. Others stay moving around one planet to do a specific job. The kinds of orbits they travel in help them to achieve this purpose. Some kinds of planetary orbits include:
Geosynchronous Orbits. A geosychronous orbit (GEO) is a circular, low orbit about Earth having a period of 23 hours 56 minutes 4 seconds--that is, the same amount of time it takes for the Earth to turn, so as the Earth spins, the satellite moves in time with it. Geosynchronous means "in time with the Earth." A spacecraft in geosynchronous stays over the same line of longitude. (A line of longitude marks one slice of the world from north to south pole.) Often a satellite in geosynchronous orbit stays above the same spot on Earth. When it does, it is called geostationary. This orbit is ideal for certain kinds of communication satellites, or meteorological (weather) satellites that have a job to do over one part of the world.
Polar Orbits. Polar orbits are useful for spacecraft that carry out mapping or surveillance operations. A satellite in polar orbit goes around the Earth from pole to pole. The planet spins underneath it as the satellite goes from north to south. This gives the spacecraft access to virtually every point on the surface. The Magellan spacecraft used a nearly-polar orbit at Venus. When the planet rotated once, all 360 degrees longitude had been exposed to Magellan's surveillance.
Walking Orbits. There are some things that interfere with making spacecraft follow perfect orbits easily. Planets are not perfectly spherical (ball shaped) and they do not have evenly distributed mass. Some parts of the planet might weigh a little more than others. For example a huge iron concentration could be in one part of the planet, making that side weigh a little more. Also, gravity can be uneven in space. Other bodies such as the Sun, the moon or other satellites, pull on spacecraft in orbit about a planet. Sometimes, scientists choose the path of a spacecraft's orbit to use this other gravity to slowly change the orbit over time. The result is called a walking orbit.
Suns-Synchronous Orbits. Sometimes a walking orbit can be designed so that the orbit changes slowly in time with the planet moving around the Sun, and in time with the planet's rotation so that the spacecraft is always at the same angle to the Sun. This is called a Sun-synchronous orbit. On Earth, this would work out so that the orbit always passes a low point at the same local time every day. This can be useful if instruments on board depend on a certain angle of solar illumination on the surface. Mars Global Surveyor's intended orbit at Mars is a 2-PM Mars local time Sun-synchronous orbit.