Orbital inclination

Inclinations greater than 90° describe retrograde orbits (backward).

Captured bodies on distant orbits vary widely in their inclinations, while captured bodies in relatively close orbits tend to have low inclinations owing to tidal effects and perturbations by large regular satellites.

The inclination of exoplanets or members of multi-star star systems is the angle of the plane of the orbit relative to the plane perpendicular to the line of sight from Earth to the object.

[5] Since the word "inclination" is used in exoplanet studies for this line-of-sight inclination, the angle between the planet's orbit and its star's rotational axis is expressed using the term the "spin-orbit angle" or "spin-orbit alignment".

[5] In most cases the orientation of the star's rotational axis is unknown.

Because the radial-velocity method more easily finds planets with orbits closer to edge-on, most exoplanets found by this method have inclinations between 45° and 135°, although in most cases the inclination is not known.

[8] He showed that, for each planet, there is a distance such that moons closer to the planet than that distance maintain an almost constant orbital inclination with respect to the planet's equator (with an orbital precession mostly due to the tidal influence of the planet), whereas moons farther away maintain an almost constant orbital inclination with respect to the ecliptic (with precession due mostly to the tidal influence of the sun).

He concluded that these moons formed from equatorial accretion disks.

But he found that the Moon, although it was once inside the critical distance from the Earth, never had an equatorial orbit as would be expected from various scenarios for its origin.

This is called the lunar inclination problem, to which various solutions have since been proposed.

[9] For planets and other rotating celestial bodies, the angle of the equatorial plane relative to the orbital plane – such as the tilt of the Earth's poles toward or away from the Sun – is sometimes also called inclination, but less ambiguous terms are axial tilt or obliquity.