What Is an Orbit?

The Short Answer:

An orbit is a regular, repeating path that one object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like Earth or the Moon. Since the Earth orbits the Sun, you’re actually in orbit right now! Many planets, like Earth, have moons that orbit them. A satellite can also be man-made, like the International Space Station. Planets, comets, asteroids and other objects in the solar system orbit the Sun.

Watch this quick video to see how the Moon orbits Earth! Credit: NASA's Scientific Visualization Studio. Click here to download this video (28 MB, video/mp4).

What shape is an orbit?

Orbits come in different shapes. All orbits are elliptical, which means they are an ellipse, similar to an oval. For the planets, the orbits are almost circular. The orbits of comets have a different shape. They look like a "squashed" circle. 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, 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 orbital period. For example, Earth has an orbital period of one year.

An illustration of Earth’s orbit around the Sun, and the Moon’s orbit around Earth. The aphelion, perihelion, perigee, and apogee points are labeled.

The point at which a planet is closest to the Sun is called perihelion. The farthest point is called aphelion. 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.

A satellite orbiting Earth. Two dashed lines appear to represent the pull of gravity and the satellite’s momentum. The two dashed lines illustrate balance between gravity and momentum, which keep the satellite orbiting around Earth.

Credit: NASA/JPL-Caltech

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.

Where Do Satellites Orbit Earth?

The International Space Station is in low Earth orbit, or LEO. LEO is the first 100 to 200 miles of space. LEO is the easiest orbit to get to and stay in. One complete orbit in LEO takes about 90 minutes.

Astronaut James H. Newman waves during a spacewalk preparing for release of the first combined elements of the International Space Station. The astronaut wears a big white space suit and reflective helmet and waves at the camera in the foreground. The ISS’ solar panels and metallic body are behind him. Earth’s horizon is in the distance, and the rest of the background is deep, dark space.

Astronauts and scientists living on the International Space Station are in orbit around Earth, while you, on Earth, are orbiting around the Sun. Phew! So many orbits! Credit: NASA

Satellites that stay above a location on Earth are in geosynchronous Earth orbit, or GEO. These satellites orbit about 23,000 miles above the equator and complete one revolution around Earth precisely every 24 hours. Geosynchronous orbits are also called geostationary.

A satellite in geostationary orbit around the Earth. The satellite is simple; it has a single body and two solar panels on the top and bottom. A yellow line shows what geostationary orbit looks like. This yellow line runs parallel to the Earth’s equator, which means the satellite orbits above the equator and completes one revolution around Earth precisely every 24 hours.

A satellite in geostationary orbit around Earth. Credit: NASA

Any satellite with an orbital path going over or near the poles has a polar orbit. Polar orbits are usually low Earth orbits. Eventually, Earth's entire surface passes under a satellite in polar orbit.