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 the distance between them and the mass of the bigger object. This law works for objects moving in circular paths 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. In simple cases where the path is a perfect circle, the speed stays the same, and this speed is linked to how fast something would need to leave the object to escape its pull entirely.

Two bodies orbiting each other

In space science, when two objects are moving around each other, the time it takes to complete one full circle can be found using a special formula. This formula helps us understand how long it takes for planets, stars, or moons to orbit one another.

The formula looks at the distance between the objects and their masses, using a value called the gravitational constant to calculate the exact time needed for one orbit. If the path is not a closed loop, like in some special cases, the object will keep moving forever without repeating its path.

Related periods

See also: Lunar month § Types

For objects in space, the orbital period usually means the sidereal period. This is the time it takes for one object to go around another, measured by how it looks against distant stars. For Earth going around the Sun, this is called the sidereal year. This measures the orbit from a fixed point in space.

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 is different. It looks at how objects move in relation to each other, not just to their parent star. It helps us know when planets return to the same place in the sky, like when they line up with the Sun. For example, Jupiter has a synodic period of 398.8 days from Earth, so we see Jupiter opposite the Sun about once every 13 months.

There are many other kinds of periods used in astronomy. Each one helps scientists study how objects move in space. These include the draconitic period, which is the time between when an object passes through a certain point in its orbit, and the anomalistic period, which is the time between when an object gets closest to the body it orbits.

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 surface of 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.