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The celestial equator is part of the celestial sphere, an imaginary object used to locate celestial objects. It corresponds to the terrestrial equator and is fixed in position, while the observer moves with the rotation of the Earth. The celestial equator’s position in the sky varies depending on the observer’s location, and it is used in a coordinate system based on declination and right ascension to locate celestial objects.
The celestial equator is part of a larger system called the celestial sphere which is used as a coordinate system for locating and describing the position of celestial objects. The celestial sphere is not a physical object, but rather an imagined object of infinite size with the Earth at its center. The celestial equator corresponds exactly to the terrestrial equator, and is essentially a projection of the terrestrial equator onto the imaginary celestial sphere.
The idea of an imaginary line on an imaginary sphere of infinite size in the sky can be difficult for many people to grasp. For this reason, representations of the celestial sphere and celestial equator are often depicted in educational materials as having a set size, slightly larger than the Earth, simply for illustration purposes. This creates a viewer’s perspective of looking down on the sphere, with the Earth at the center. On the sphere, the celestial equator occupies the corresponding path and position of the terrestrial equator.
The rotation of the Earth makes the celestial sphere appear to move, but it doesn’t. It is fixed in position, but the observer moves as the Earth rotates. This causes the sphere to appear to rotate once every 24 hours, the same amount of time it takes for the Earth to complete one rotation or one day. The celestial equator, however, will always appear at exactly the same point from every single place on earth. This is because the earth’s equator does not move relative to any point on its surface, and since the celestial equator exactly matches the earth’s equator, it does not move either.
The position of the celestial equator in the sky will differ, however, depending on the position of the observer on the earth’s surface. It will always follow a path that describes a straight line joining two points, due east and due west. When at the equator, it will pass directly overhead. As the observer moves north, the center point of the line moves south. Conversely, if the observer moves south of the equator, the path of the celestial equator will appear to move north. From the point of view of someone standing at either pole, the celestial equator follows the horizon exactly.
The celestial sphere-based coordinate system is used to both locate and quantify the position of objects in the sky. Coordinates for this system are based on declination and right ascension. Declination refers to an object’s position above or below the celestial equator and is measured in degrees. Right ascension roughly corresponds to longitude, but uses a specific point on the celestial equator called the vernal equinox as a reference point. Right ascension is measured in hours, reflecting the apparent rotation of the sphere relative to the Earth.
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