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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 equatorial describes the sky as seen from the Solar System, and modern star maps almost exclusively use equatorial coordinates. The equatorial system is the normal coordinate system for most professional and many amateur astronomers having an equatorial mount that follows the movement of the sky during the night. Celestial objects are found ...
A position in the equatorial coordinate system is thus typically specified true equinox and equator of date, mean equinox and equator of J2000.0, or similar. Note that there is no "mean ecliptic", as the ecliptic is not subject to small periodic oscillations. [5]
Like right ascension in the equatorial coordinate system, the primary direction (0° ecliptic longitude) points from the Earth towards the Sun at the March equinox. Because it is a right-handed system, ecliptic longitude is measured positive eastwards in the fundamental plane (the ecliptic) from 0° to 360°.
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 ...
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 United States Naval Observatory states "the Equation of Time is the difference apparent solar time minus mean solar time", i.e. if the sun is ahead of the clock the sign is positive, and if the clock is ahead of the sun the sign is negative. [6] [7] The equation of time is shown in the upper graph above for a period of slightly more than a ...
TT differs from Geocentric Coordinate Time (TCG) by a constant rate. Formally it is defined by the equation = +, where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, is the constant difference in the rates of the two time scales, and is a constant to resolve the epochs (see below).