Orbital inclination

Orbital inclination measures how tilted an object's path is as it moves around a big object in space, like a planet or the Sun. We describe this tilt using an angle between two flat surfaces, or planes.

One plane is a chosen reference, often the equator of the planet. The other plane is the path the object follows as it orbits.

For example, when a satellite flies around Earth right above the equator, its path matches Earth's equator perfectly. In this case, we say its orbital inclination is 0°. Most orbits are not flat like this. They are tilted, meaning the satellite spends part of its trip above the northern half of Earth and part above the southern half. If a satellite's path goes as far as 20° north and 20° south of the equator, its orbital inclination would be 20°. This helps scientists and engineers plan how satellites will move and how they can best help us from space.

Orbits

The inclination of an orbit is one of the six important features that describe the shape and direction of a path in space. It is the angle between the orbit's plane and a reference plane, usually measured in degrees.

For satellites orbiting a planet, the reference plane is often the planet's equatorial plane—the plane that runs along the planet's middle. For planets in our Solar System, the reference plane is usually the ecliptic, the plane in which Earth orbits the Sun.

For natural or human-made satellites, the inclination is measured compared to the equatorial plane of the body they orbit. An inclination of 0° means the satellite orbits in the same direction as the planet spins, along the equatorial plane. An inclination of exactly 90° is called a polar orbit, where the satellite passes over the planet's poles. Inclinations greater than 90° mean the satellite orbits backward compared to the planet's rotation.

Calculation

In space science, we can find the angle of an object's orbit using special math. This angle is called the inclination. It tells us how tilted the orbit is compared to a flat surface, like the Earth’s middle line. Scientists use vectors, which are directions in space, to do these calculations. They also use a rule called the cosine rule to find angles between two orbits.

Observations and theories

Most planets in our Solar System have orbits that stay close to each other and to the Sun's equator. Some objects, like the dwarf planets Pluto and Eris, and the large asteroid Pallas, have orbits that tilt more.

In 1966, Peter Goldreich studied how the orbits of moons around planets change over time. He found that moons close to their planet usually stay close to the planet's equator. Moons farther away stay close to the plane of the Solar System. Most of these close moons, except for Neptune's moon Triton, orbit near the equator because they formed there. Our Moon is special because its orbit never stayed close to Earth's equator. Scientists are still trying to understand why.

Other meaning

For planets and other spinning space objects, the angle between their middle imaginary line and their path around another object can also be called inclination. But clearer words to use are axial tilt or obliquity.