page_title: Celestial equator
hierarchical_section_title: Celestial equator
caption_attribution_description: Description of relations between Axial tilt (or Obliquity), rotation axis, plane of orbit, celestial equator and ecliptic. Earth is shown as viewed from the Sun; the orbit direction is counter-clockwise (to the left).
context_page_description: The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. Due to Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic. The inclination has varied from about 22.0° to 24.5° over the past 5 million years.
  An observer standing on Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. As the observer moves north, the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant; thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on 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 celestial equator itself.
context_section_description: The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. 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). The inclination has varied from about 22.0° to 24.5° over the past 5 million years.
  An observer standing on 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 infinitely distant (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 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 celestial equator itself.
  Astronomical objects near the celestial equator appear above the horizon from most places on earth, but they culminate (reach the meridian) highest near the equator. The celestial equator currently passes through these constellations:
  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
  These are the most globally visible constellations.
  Celestial bodies other than Earth also have similarly defined celestial equators.
caption: The celestial equator is currently inclined by about 23.44° to the ecliptic plane. The image shows the relations between Earth's axial tilt (or obliquity), rotation axis, and orbital plane.
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