Axial tilt

obliquityobliquity of the eclipticaxisEarth's axistilttiltedtilt of the Earth's axisaxial inclinationaxis is tiltedorientation
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.wikipedia
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Season

seasonsseasonalfour seasons
This causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other side—the cause of the seasons on Earth.
On Earth, seasons are the result of Earth's orbit around the Sun and Earth's axial tilt relative to the ecliptic plane.

Equator

equatorial planeThe Equator
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.
The precise location of the equator is not truly fixed; the true equatorial plane is perpendicular to the Earth's spin axis, which drifts about 9 m during a year.

Axial precession

precession of the equinoxesprecessionprecession of equinoxes
This value remains about the same relative to a stationary orbital plane throughout the cycles of axial precession.
For example, suppose that the Earth's orbital position is marked at the summer solstice, when the Earth's axial tilt is pointing directly toward the Sun.

Venus

Morning Starevening starplanet Venus
The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on Earth's north side of the invariable plane of the Solar System; under this system, Venus is tilted 3° and spins retrograde, opposite that of most of the other planets. Mercury and Venus have most likely been stabilized by the tidal dissipation of the Sun.
Venus's minute axial tilt—less than 3°, compared to 23° on Earth—also minimises seasonal temperature variation.

Celestial equator

equatorialequatorial planeequatorial sky
Earth's orbital plane is known as the ecliptic plane, and Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere.
Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit).

Orbital period

periodsynodic periodsynodic
Over the course of an orbital period, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background of stars.

Ecliptic

ecliptical orbitsecliptic planeplane of the ecliptic
Earth's orbital plane is known as the ecliptic plane, and Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere.
Because Earth's rotational axis is not perpendicular to its orbital plane, Earth's equatorial plane is not coplanar with the ecliptic plane, but is inclined to it by an angle of about 23.4°, which is known as the obliquity of the ecliptic.

Rotation around a fixed axis

axis of rotationaxisaxial
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.

Earth

Earth's surfaceterrestrialworld
This causes one pole to be directed more toward the Sun on one side of the orbit, and the other pole on the other side—the cause of the seasons on Earth. Earth's orbital plane is known as the ecliptic plane, and Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere.
The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.44 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth–Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane.

Solar System

outer Solar Systeminner Solar Systemouter planets
The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on Earth's north side of the invariable plane of the Solar System; under this system, Venus is tilted 3° and spins retrograde, opposite that of most of the other planets.
Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic.

Milankovitch cycles

Milankovitch cycleMilankovitch theoryMilankovich cycle
Variations in Earth's axial tilt can influence the seasons and is likely a factor in long-term climatic change (also see Milankovitch cycles).
In the 1920s, he hypothesized that variations in eccentricity, axial tilt, and precession of the Earth's orbit resulted in cyclical variation in the solar radiation reaching the Earth, and that this orbital forcing strongly influenced climatic patterns on Earth.

Planet

planetsFormer classification of planetsplanemo
The exact angular value of the obliquity is found by observation of the motions of Earth and planets over many years.
As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons.

Northern Hemisphere

NorthernNorth HemisphereNorthern Hemispheric
Summer occurs in the Northern hemisphere when the north pole is directed toward the Sun.
Owing to the Earth's axial tilt, winter in the Northern Hemisphere lasts from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer lasts from the June solstice through to the September equinox (typically on 23 September UTC).

Orbital inclination

inclinationinclinedtilted
It differs from orbital inclination.
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.

Pytheas

Pytheas of MassaliaPytheas of MassiliaPythéas
The ancient Greeks had good measurements of the obliquity since about 350 BC, when Pytheas of Marseilles measured the shadow of a gnomon at the summer solstice.
The angle added to the elevation by the tilt is known as the obliquity of the ecliptic and at that time was 23° 44′ 40″.

Ulugh Beg

Ulug BegMirzo UlugbekUlugbek
In 1437, Ulugh Beg determined the Earth's axial tilt as 23°30′17″ (23.5047°).
Ulugh Beg also determined the Earth's axial tilt as 23°30'17" in the sexagesimal system of degrees, minutes and seconds of arc, which in decimal notation converts to 23.5047°.

Climate change (general concept)

climate changeclimatic changeclimate variation
Variations in Earth's axial tilt can influence the seasons and is likely a factor in long-term climatic change (also see Milankovitch cycles).
The three types of kinematic change are variations in Earth's eccentricity, changes in the tilt angle of Earth's axis of rotation, and precession of Earth's axis.

Epsilon

ΕEGreek letter Epsilon
It is denoted by the Greek letter ε.

Mercury (planet)

MercuryMercurioplanet Mercury
Mercury and Venus have most likely been stabilized by the tidal dissipation of the Sun.
Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 degree).

Orbit of the Moon

Moon's orbitits orbitorbit
Frequency map analysis conducted in 1993 suggested that, in the absence of the Moon, the obliquity can change rapidly due to orbital resonances and chaotic behavior of the Solar System, reaching as high as 90° in as little as a few million years (also see Orbit of the Moon).
The Moon's orbital plane is inclined by about 5.1° with respect to the ecliptic plane, whereas the Moon's equatorial plane is tilted by only 1.5°.

Orbital resonance

1:1 resonanceresonancemean-motion resonance
Frequency map analysis conducted in 1993 suggested that, in the absence of the Moon, the obliquity can change rapidly due to orbital resonances and chaotic behavior of the Solar System, reaching as high as 90° in as little as a few million years (also see Orbit of the Moon).
A resonance between the precession of Saturn's rotational axis and that of Neptune's orbital axis (both of which have periods of about 1.87 million years) has been identified as the likely source of Saturn's large axial tilt (26.7°).

Trepidation (astronomy)

trepidationtrepidation of the equinoxes
It was widely believed, during the Middle Ages, that both precession and Earth's obliquity oscillated around a mean value, with a period of 672 years, an idea known as trepidation of the equinoxes.
Copernicus' version of trepidation combined the oscillation of the equinoxes (now known to be a spurious motion) with a change in the obliquity of the ecliptic (axial tilt), acknowledged today as an authentic motion of the Earth's axis.

Moon

lunarthe MoonLuna
The Moon has a stabilizing effect on Earth's obliquity.
The Moon's axial tilt with respect to the ecliptic is only 1.5424°, much less than the 23.44° of Earth.

Jupiter

JovianGioveplanet Jupiter
Jupiter's low axial tilt means that the poles constantly receive less solar radiation than at the planet's equatorial region.

Uranus

Uranian34 TauriMagnetosphere of Uranus
The Uranian axis of rotation is approximately parallel with the plane of the Solar System, with an axial tilt of 97.77° (as defined by prograde rotation).