This lesson talks about what an orbit is and defines some common terms associates with orbit. We also talk specifically about orbits of planets and satellites.

## What is Orbit?

**Orbit** is the path of a body as it moves under the influence of a second body. An example is the path of a planet or comet as it moves around the Sun. Planets and satellites that orbit other bodies trace out a path called an **ellipse**. An ellipse is a closed curve of oval shape wherein the sum of the distances from any point on the curve to two internal focal points is constant. In everyday life you probably just call this an oval. As shown in the picture below, an ellipse has a major axis and a minor axis.

The major axis is always at least as long or longer than the minor axis. When both the major and minor axes are the same length, this is a special case of an ellipse we commonly call a circle. Therefore, orbiting bodies can also trace out a circular path. Although a circle is a special type of ellipse, people commonly refer to satellite and planetary orbits as either circular or elliptical. The **orbital period** is the time to complete one full orbit.

## Orbits of Planets

Kepler’s laws of planetary motion govern the orbits of planets around the Sun. At first, Kepler expected the planets to move around the Sun in perfect circles, but after years of observation he found that this was not true. **Kepler’s first law** of planetary motion states that the path of each planet around the Sun is an ellipse with the Sun at one focus. This is illustrated by the picture in the section above. Kepler also found that the planets do not move around the Sun at a uniform speed, but move faster when they are closer to the Sun and slower when they are farther away. **Kepler’s second law** states that the line from the Sun to any planet sweeps out equal areas of space in equal time intervals. This is shown in the picture below.

After ten years of work, Kepler discovered the relationship between the time it takes a planet to orbit the Sun and its distance from the Sun. **Kepler’s third law** says that the square of the orbital period of a planet is directly proportional to the cube of the average distance of the planet from the Sun. Mathematically, this is given by the ratio *T*^2/*r*^3 and applies to all planets. The practical application of Kepler’s third law is to calculate the radius of a planet’s orbit by observation of that planet’s orbital period.

## Orbits of Satellites

A **satellite** is a projectile or smaller celestial body that orbits a larger celestial body. An Earth satellite is a projectile that falls around the Earth rather than into the Earth. Stated another way, a satellite is a fast-moving projectile. The speed of the satellite must be great enough that its falling distance matches the curvature of the Earth. The surface of the Earth drops 5 meters vertically for every 8 kilometers traveled horizontally. Therefore, a projectile must travel at 8 kilometers per second in order to follow the curvature of the Earth and not fall into it.

Unlike planetary orbits, satellite orbits can be circular or elliptical. A satellite traveling at 8 kilometers per second follows the curvature of the Earth and orbits in a circular manner. If it moves any slower, it falls to Earth. If it moves any faster, it traces out an elliptical path.

Kepler’s laws apply to satellites as well as planetary motion. Based on the third law, the period of an Earth satellite depends on the distance from the Earth. Periods of typical Earth satellites range from about 90 minutes for satellites close to the Earth, to about 24 hours for communication satellites in geostationary orbits approximately 36,000 kilometers above the Earth’s surface. The pictures below compare some typical circular and elliptical orbits of Earth satellites.

## Lesson Summary

An orbit is the path of a body as it moves under the influence of a second body. Planets and satellites move in an elliptical path around the body that they orbit. Planetary motion, as well as satellite motion, is governed by Kepler’s laws. Earth satellites move in a circular path when they travel at 8 kilometers per hour and follow an oval path if they move faster.