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Celestial equator

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From Wikipedia, the free encyclopedia
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  1. Celestial equator
Celestial equator
Projection of Earth's equator out into space

The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of a planet, by convention generally Earth. By extension, it is also a plane of reference in the equatorial coordinate system.1 Because of the 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), but has varied from about 22.0° to 24.5° over the past 5 million years2 as a result of Milankovitch cycles and perturbation from other planets.

Celestial equator in relation to the galactic and ecliptic planes

An observer standing on the Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be of infinite reach (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on the Earth. At the poles, the celestial equator coincides with the astronomical horizon. At all latitudes, the celestial equator is a uniform arc or circle because the observer is only finitely far from the plane of the celestial equator, but infinitely far from the edge of the celestial equator itself.3

Astronomical objects near the celestial equator appear above the horizon from most places on the Earth, but they culminate highest near the equator. The celestial equator currently passes through these constellations:4

  • Pisces (contains the first point of Aries above its southern border)
  • Cetus
  • Taurus
  • Eridanus
  • Orion (near Orion's Belt)
  • Monoceros
  • Canis Minor
  • Hydra
  • Sextans
  • Leo
  • Virgo (contains the first point of Libra)
  • Serpens (caput)
  • Ophiuchus
  • Serpens (cauda)
  • Aquila
  • Aquarius

Over thousands of years, the orientation of the Earth's equator and thus the constellations the celestial equator passes through will change due to axial precession.

Celestial bodies other than the Earth also have similarly defined celestial equators.56

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Where Celestial equator leads. Every connection below has a source: a link in this article, a shared Wikipedia category, or CirrusSearch text similarity. Articles already on your trail are left out.

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Contents
Contents4 sections
  • (Top)
  • See also
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See also

  • Celestial pole
  • Declination
  • Rotation around a fixed axis (pole)

References

  1. 1"Celestial Equator". Retrieved 5 August 2011.
  2. 2Berger, A.L. (1976). "Obliquity and Precession for the Last 5000000 Years". Astronomy and Astrophysics. 51 (1): 127–135. Bibcode:1976A&A....51..127B.
  3. 3Millar, William (2006). The Amateur Astronomer's Introduction to the Celestial Sphere. Cambridge University Press. ISBN 978-0-521-67123-1.
  4. 4Ford, Dominic. "Map of the Constellations". in-the-sky.org. Retrieved 1 Feb 2021.
  5. 5Tarasashvili MV, Sabashvili ShA, Tsereteli SL, Aleksidze NG (26 Mar 2013). "New model of Mars surface irradiation for the climate simulation chamber 'Artificial Mars'". International Journal of Astrobiology. 12 (2): 161–170. Bibcode:2013IJAsB..12..161T. doi:10.1017/S1473550413000062. S2CID 120041831.
  6. 6"Equal length of day and night on Saturn: the start of spring in the northern hemisphere". German Aerospace Center. Archived from the original on 28 June 2021. Retrieved 1 Feb 2021.
Categories:
  • Astronomical coordinate systems
  • Dynamics of the Solar System
  • Technical factors of astrology
  • Circles
  • Planes (geometry)

321 words · 6 references · revision 1380318692 · CC BY-SA 4.0

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Key concepts

  • Ecliptic3
  • Axial tilt2
  • Serpens2
  • Rotation1
  • Orbital plane1
  • Galactic plane1
  • Great circle1
  • Celestial sphere1
  • Coplanarity1
  • Equator1

The articles this one links to most, weighted towards the introduction.

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Created
1 Dec 2002
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230 edits
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1 edit in the last 30 days
Last edited
15 Oct 2026

230 revisions in total · 160 contributors · 0 / −25 bytes across 1 edits by 1 contributor in the last 30 days

  1. 15 Oct 2026, 01:24 UTCJeffrey34555mPost-move cleanup: Changed link from Eridanus (constellation) to Eridanus using Move+−25diff
  2. 1 Jun 2026, 10:44 UTCArjayaymDuplicate word reworded−4diff
  3. 31 May 2026, 20:13 UTCMucketymuckTalk of distance to the celestial equator clarified; obviously, a polar observer is indeed “infinitely far from the celestial equator” in the sense of having it in their line of sight, but at other latitudes it’s the celestial equator’s edge, which stretches to infinity, from which they are infinitely far. Similarly, the celestial equator is not infinitely distant but spans an infinite range. Reperetive internal link removed, and adjective twice misused as preposition substituted for.+57diff
  4. 30 Dec 2025, 02:56 UTCNsae CompNo edit summary+213diff
  5. 21 Sept 2025, 00:13 UTCCommunityNotesContributormDisambiguating links to Cetus (link changed to Cetus (constellation)) using DisamAssist.+22diff
  6. 1 Aug 2025, 00:46 UTCIonixamI added articles.+24diff
  7. 5 Jul 2025, 14:52 UTCRandy Krynuppercase per direct link (Orion's Belt, Earth)0diff
  8. 5 Jun 2025, 12:52 UTCAnonymous contributor§ See also already linked−23diff

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Created
1 Dec 2002
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Go deeper: linked from this article

  • Listed under See also

    Rotation around a fixed axis

    Type of motion

    Rotation around a fixed axis or axial rotation is a special case of rotational motion around an axis of rotation fixed, stationary, or static in three-dimensional space.

    7 min readGo deeper

  • Linked from the introduction

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    Classical approach to the many-body problem of astronomy

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    Earth's orbit

    Trajectory of Earth around the Sun

    Earth orbits the Sun at an average distance of 149.60 million km (92.96 million mi), or 8.317 light-minutes, in a counterclockwise direction as viewed from above the Northern Hemisphere.

    6 min readGo deeper

  • Linked from the introduction

    Equator

    Imaginary line halfway between Earth's North and South poles

    The equator is the circle of latitude that divides Earth into the Northern and Southern Hemispheres.

    13 min readGo deeper

  • Linked from the introduction

    Astronomical object

    Natural physical entity in space

    An astronomical object, celestial object, stellar object or heavenly object is a naturally occurring physical entity, association, or structure that exists within the universe.

    11 min readGo deeper

  • Linked from the introduction

    Ecliptic

    Apparent path of the Sun on the celestial sphere

    The ecliptic or ecliptic plane is the apparent path of the Sun on the celestial sphere, resulting from Earth's orbit around the Sun.

    11 min readGo deeper

  • Linked from the introduction

    Rotation

    Movement of an object which leaves at least one point unchanged

    Rotation, also known as rotational motion or rotary motion, is the movement of an object that leaves at least one point unchanged.

    11 min readGo deeper

  • Linked from the introduction

    Axial tilt

    Angle between the rotational axis and orbital axis of a body

    In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbital plane.

    12 min readGo deeper

  • Linked from the introduction

    Orbital plane

    Concept in geometry and physics

    The orbital plane of a revolving body is the geometric plane in which its orbit lies. Three non-collinear points in space suffice to determine an orbital plane.

    1 min readGo deeper

  • Linked from the introduction

    Coplanarity

    Geometric property of objects being in the same plane

    In geometry, a set of points in space are coplanar if there exists a geometric plane that contains them all.

    2 min readGo deeper

  • Listed under See also

    Celestial pole

    Imaginary sky rotation points

    The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere.

    4 min readGo deeper

  • Linked from the introduction

    Culmination

    Passage of an astronomical body across the meridian

    In observational astronomy, culmination is the passage of a celestial object (such as the Sun, the Moon, a planet, a star, constellation or a deep-sky object) across the observer's local meridian.

    4 min readGo deeper

  • Linked from the introduction

    Serpens

    Constellation split into two non-contiguous parts

    Serpens (Ancient Greek: Ὄφις, romanized: Óphis, lit. 'the Serpent') is a constellation in the northern celestial hemisphere.

    45 min readGo deeper

  • Linked from the introduction

    Milankovitch cycles

    Global climate cycles

    Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years.

    19 min readGo deeper