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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 other two cover the equatorial region of the celestial sphere, from the declination of 30° south to 30° north. The two equatorial charts are mercator projections, one for the eastern hemisphere of the celestial sphere and one for the western hemisphere. Note that unlike familiar maps, east is shown to the left and west is shown to the right.
This page was last edited on 3 November 2021, at 12:38 (UTC).; Text is available under the Creative Commons Attribution-ShareAlike 4.0 License; additional terms may apply.
A diagram of a typical nautical sextant, a tool used in celestial navigation to measure the angle between two objects viewed by means of its optical sight. Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the ...
However, all objects on the celestial sphere are subject to diurnal motion, which always appears to be westward. A northern observer can determine whether altitude is increasing or decreasing by instead considering the azimuth of the celestial object: If the azimuth is between 0° and 180° (north–east–south), the object is rising.
This description of the orientation of the reference frame is somewhat simplified; the orientation is not quite fixed. A slow motion of Earth's axis, precession, causes a slow, continuous turning of the coordinate system westward about the poles of the ecliptic, completing one circuit in about 26,000 years.
The International Astronomical Union defines it as the angle measured relative to the north celestial pole (NCP), turning positive into the direction of the right ascension. In the standard (non-flipped) images, this is a counter clockwise measure relative to the axis into the direction of positive declination .
Ecliptic latitude or celestial latitude (symbols: heliocentric b, geocentric β), measures the angular distance of an object from the ecliptic towards the north (positive) or south (negative) ecliptic pole. For example, the north ecliptic pole has a celestial latitude of +90°. Ecliptic latitude for "fixed stars" is not affected by precession.