Celestial equator

The great circle on the celestial sphere directly above Earth’s equator, dividing the sky into northern and southern celestial hemispheres. It is the reference line for zero declination.

Celestial sphere: An imaginary sphere centered on an observer, used to describe the apparent positions of celestial objects. The celestial equator is a great circle drawn on this geometric model of the sky.
Right ascension: An angular coordinate measuring eastward position along the celestial equator from the vernal equinox. It measures position along the circle that serves as its coordinate baseline.
Equatorial mount: A telescope mount with one axis aligned parallel to Earth’s rotational axis. Its polar axis parallels the celestial poles, allowing tracking aligned with the equatorial frame.
Ecliptic: The apparent annual path of the Sun on the celestial sphere, also the plane of Earth’s orbit projected onto the sky. Unlike the celestial equator, it follows Earth’s orbital plane and is tilted relative to it.
Earth's equator: The imaginary circle around Earth equidistant from the geographic poles. The celestial equator is the projection of this terrestrial circle onto the sky.
Equatorial coordinate system: A celestial coordinate system that locates objects by right ascension and declination. Its two coordinates are defined with respect to the celestial equator and equinox.
Transit circle: A telescope designed to measure when celestial objects cross a local meridian. Meridian crossings support precise measurements in equatorial coordinates.
Ecliptic coordinate system: A celestial coordinate system that uses the ecliptic as its fundamental plane. It locates objects relative to the Sun’s apparent path rather than Earth’s equator.
Earth's rotation: Earth’s spinning motion around an axis through its geographic poles. Earth’s rotation makes the celestial sphere appear to turn around the celestial poles.
Axial tilt: The angle between a body's rotational axis and the perpendicular to its orbital plane. Earth’s tilt makes the celestial equator inclined to the ecliptic.

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