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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. The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth.
The Sun, in its apparent motion along the ecliptic, crosses the celestial equator at these points, one from south to north, the other from north to south. [6] The crossing from south to north is known as the March equinox, also known as the first point of Aries and the ascending node of the ecliptic on the celestial equator. [10]
The celestial equator (blue) and the equatorial coordinates (blue), being inclined to the ecliptic, appear to wobble as the Sun advances. See also: Axial precession and Astronomical nutation The celestial equator and the ecliptic are slowly moving due to perturbing forces on the Earth , therefore the orientation of the primary direction, their ...
The following table lists the common coordinate systems in use by the astronomical community. The fundamental plane divides the celestial sphere into two equal hemispheres and defines the baseline for the latitudinal coordinates, similar to the equator in the geographic coordinate system.
The right ascension symbol α, (lower case "alpha", abbreviated RA) measures the angular distance of an object eastward along the celestial equator from the March equinox to the hour circle passing through the object. The March equinox point is one of the two points where the ecliptic intersects the celestial equator.
The angle between the Earth's equatorial plane and the ecliptic, ε, is called the obliquity of the ecliptic and ε ≈ 23.4°. An equinox occurs when the earth is at a position in its orbit such that a vector from the earth toward the sun points to where the ecliptic intersects the celestial equator.
Solar ascending node on ecliptic: Solar rotational axis Heliocentric Aries ecliptic: HAE: First point of Aries: Ecliptic north pole Heliocentric Earth ecliptic: HEE: Sun–Earth line: Ecliptic north pole Heliocentric Earth equator: HEEQ: Intersection between solar equator and solar central meridian as seen from Earth: Solar rotational axis
The declination of the Sun, δ ☉, is the angle between the rays of the Sun and the plane of the Earth's equator. The Earth's axial tilt (called the obliquity of the ecliptic by astronomers) is the angle between the Earth's axis and a line perpendicular to the Earth's orbit. The Earth's axial tilt changes slowly over thousands of years but its ...