Axial tilt



Axial tilt, known to astronomers as obliquity, is the angle between an object's rotational axis and its orbital axis. It differs from orbital inclination.

To understand axial tilt, we employ the right-hand rule. 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.

At an obliquity of 0°, these lines point in the same direction i.e. the rotational axis is perpendicular to the orbital plane. Because the planet Venus has an axial tilt of 177° its rotation can be considered retrograde, or opposite that of most of the other planets of the Sol System. 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.

The Earth's axis remains tilted in the same direction with reference to the background stars throughout a year (throughout its entire orbit). This means that one pole (and the associated hemisphere of the Earth) will be directed away from the Sun at one side of the orbit, and half an orbit later (half a year later) this pole will be directed towards the Sun. This is the cause of the Earth's seasons.

Variations in Earth's axial tilt can influence the seasons and is likely a factor in long-term climate change.