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The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those ...
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 poles are located at ±90° from the fundamental plane.
In astronomy, an equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. [1] [2] [3] Although there are two such intersections, the equinox associated with the Sun's ascending node is used as the conventional origin of celestial coordinate systems and referred to simply as "the equinox".
In astronomy, the ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations [1] of Solar System objects. Because most planets (except Mercury ) and many small Solar System bodies have orbits with only slight inclinations to the ecliptic , using it as the ...
The epoch for which M 0 is defined is often determined by convention in a given field or discipline. For example, planetary ephemerides often define M 0 for the epoch J2000, while for earth orbiting objects described by a two-line element set the epoch is specified as a date in the first line. [5]
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).
Ancient astronomy was very concerned with the rise and set of celestial objects. The ascension was the point on the celestial equator (red) which rose or set at the same time as an object (green) on the celestial sphere. As seen from the equator, both were on a great circle from pole to pole (left, sphaera recta or right sphere).
The observation techniques are topics of positional astronomy and of astrogeodesy. Ideally, the Cartesian coordinate system (α, δ) refers to an inertial frame of reference. The third coordinate is the star's distance, which is normally used as an attribute of the individual star. The following factors change star positions over time: