Tuesday, April 04, 2017

What is an Orbit?

or-bit (ˈȯr-bət) noun. 1. a. A path described by one body in its revolution about another (as by the earth about the sun or by an electron about an atomic nucleus); also, one complete revolution of a body describing such a path. b. A circular path[Middle English, from Medieval Latin orbita, from Latin, rut, track, probably from orbis]

An orbit is a regular, repeating path that one heavenly body takes around another body. A body which orbits another body is a satellite of the other body. So, the moon is a satellite of Earth, while at the same time, Earth is a satellite of the sun, along with the other planets, comets, asteroids, and many other bodies in the solar system. And remember that the sun orbits the center of our Milky Way galaxy. So, the sun is a satellite of the Milky Way.

A satellite can be classified as natural or artificial. The moon, Earth and other heavenly bodies are natural satellites. All orbiting objects made by humans are artificial satellites. The first artificial satellite was Sputnik 1, launched in 1957. A more recent example is the International Space Station, which was launched in pieces and assembled in orbit.

Most orbiting bodies move along or close to an imaginary flat surface. This imaginary surface is called the ecliptic plane.

What Shape Is an Orbit?

Orbits come in different shapes. All orbits are elliptical, which means they are the shape of an ellipse, similar to an oval. For the planets, the orbits are almost circular. The orbits of comets have a different shape. They are highly eccentric or "squashed." They look more like thin ellipses than circles.

Satellites that orbit Earth, including the moon, do not always stay the same distance from Earth. Sometimes they are closer, and at other times they are farther away. The closest point a satellite comes to Earth is called its perigee. The farthest point is the apogee. For planets and other bodies that orbit the sun, the point in their orbit closest to the sun is perihelion. The farthest point is called aphelion. Earth reaches its aphelion during summer in the Northern Hemisphere. The time it takes a satellite to make one full orbit is called its period. For example, Earth has an orbital period of one year. The inclination of an orbit is the angle the orbital plane when compared with Earth's equator.

This diagram of an Earth orbit demonstrates an elliptical orbit and shows apogee, perigee, aphelion, and perihelon. Image Credit: NOAA

How Do Objects Stay in Orbit?

An object in motion will stay in motion unless something pushes or pulls on it. This statement is called Newton's first law of motion. Without gravity, an Earth-orbiting satellite would go off into space along a straight line. With gravity, it is pulled back toward Earth. A constant tug-of-war takes place between the satellite's tendency to move in a straight line, or momentum, and the tug of gravity pulling the satellite back.

An object's momentum and the force of gravity have to be balanced for an orbit to happen. If the forward momentum of one object is too great, it will speed past and not enter into orbit. If momentum is too small, the object will be pulled down and crash. When these forces are balanced, the object is always falling toward the planet, but because it's moving sideways fast enough, it never hits the planet. Orbital velocity is the speed needed to stay in orbit. At an altitude of 150 miles (242 kilometers) above Earth, orbital velocity is about 17,000 miles per hour. Satellites that have higher orbits have slower orbital velocities.


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