Orbital period

The orbital period (also called the revolution period) is the time it takes for an object in space to go around another object once. This idea is very important in astronomy. It helps us understand how long it takes for planets to go around the Sun, for moons to go around planets, or even for stars to go around each other.

We can talk about the orbital period for many different things in space, like asteroids going around the Sun, exoplanets going around other stars, or binary stars going around each other. It also works for satellites that orbit around planets or moons.

In science, we measure orbital periods using units of time, such as hours, days, or years. The opposite of the orbital period is called the orbital frequency, which tells us how many times something goes around another object in a certain amount of time.

Small body orbiting a central body

According to Kepler's Third Law, the time it takes for a small object to go around a bigger object in space depends on how far apart they are and how heavy the bigger object is. This law works for objects moving in circles or stretched-out paths called ellipses.

For example, if a small object needs to go around a 100-kilogram mass every 24 hours, it would have to stay about 1.08 meters away from the center of that mass. If the path is a perfect circle, the speed stays the same.

Two bodies orbiting each other

In space science, when two objects move around each other, we can find out how long it takes for them to make one full circle. This helps us learn how long it takes for planets, stars, or moons to go around one another.

We use a special formula that looks at how far apart the objects are and how heavy they are. It also uses a value called the gravitational constant to figure out the time for one orbit. If the path is not a closed loop, the object will keep moving forever without repeating its path.

Related periods

See also: Lunar month § Types

For objects in space, the orbital period means the time it takes for one object to go around another. For Earth going around the Sun, this is called the sidereal year.

There are different ways to talk about orbital periods. The tropical period focuses on the parent star’s position. It helps us understand the solar year and our calendar year.

The synodic period looks at how objects move in relation to each other. It helps us know when planets return to the same place in the sky. For example, Jupiter has a synodic period of 398.8 days from Earth.

There are many other kinds of periods used in astronomy. Each one helps scientists study how objects move in space.

Examples of sidereal and synodic periods

The synodic period is the time it takes for a moon to go through all its phases, as seen from the planet it orbits. This is different from the sidereal period, which is the time it takes to complete one full orbit around the planet. For example, Deimos, one of Mars' moons, has a synodic period of 1.2648 days, which is slightly longer than its sidereal period of 1.2624 days.

The idea of a synodic period can also be used for planets relative to each other, not just for moons and Earth. The same basic formula is used to calculate these periods.