## ::Axial tilt

### ::concepts

First::journal    Title::earth    Earth's::author    Style::years    Volume::orbit    Bibcode::issue

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• Top: Earth's axial tilt (obliquity) is currently about 23.4°. To understand axial tilt, the right-hand rule can be employed. When the fingers of the right hand are curled around in the direction of the planet's rotation, the thumb points in the direction of the north pole.
• Bottom: The axial tilt of Earth, Uranus and Venus. It is the angle between the perpendicular to the plane of the ecliptic (vertical black line) and the planet's rotational north pole (red line).

In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital plane.<ref> {{#invoke:citation/CS1|citation |CitationClass=book }}</ref> It differs from orbital inclination.

At an obliquity of zero, these lines point in the same direction i.e. the rotational axis is perpendicular to the orbital plane. Axial tilt differs from inclination. Because the planet Venus has an axial tilt of 177° its rotation can be considered retrograde, opposite that of most of the other planets.<ref name="CorreiaVenusI">{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref><ref>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref> The north pole of Venus is "upside down" relative to its orbit. The planet Uranus has a tilt of 97°, hence it rotates "on its side", its north pole being almost in the plane of its orbit.<ref name="NASA" />

Over the course of an orbit, the obliquity does not change, and the orientation of the axis remains the same relative to the background stars. This causes one pole to be directed toward the Sun on one side of the orbit, and the other pole on the other side — the cause of the seasons on the Earth. Earth's obliquity oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. It is currently 23.44 degrees and decreasing.

Axial tilt sections