In astronomy, celestial coordinate systems locate objects in the sky, which is considered to be an infinitely large celestial sphere. There are four conventional celestial coordinate systems: horizontal, equatorial, ecliptic, and galactic.
Horizontal coordinates (also called alt/az coordinates) refer to an observer on the Earth’s surface at the center of an imagined celestial sphere. The observer’s local horizon is the fundamental plane. The sky is divided into an upper hemisphere (that the observer can see) and the lower hemisphere (that cannot be seen).
Altitude (h) is an object’s arc distance along a vertical circle from the horizon, positive for objects above the horizon [between it and the zenith (h =+90°)] and negative for objects below it [between it and the nadir (h =-90°)].
Azimuth (A) is the arc distance from the north point of the horizon (h =0°, A=0°) eastward along it to where it meets the object’s vertical circle, going from 0 to 360° from the north point to full circle back to the north point. Azimuth is undefined at the Earth’s North and South Poles, where the north and south celestial poles (NCP and SCP) coincide with the zenith and nadir, and the celestial meridian becomes undefined.
Equatorial coordinates are based on the Earth’s rotation, which produces an apparent westward rotation of the celestial sphere around the NCP and SCP. The projection of the equator of the Earth onto the celestial sphere is called the celestial equator. The Earth’s geographic poles are projected onto the celestial sphere, which defines the north and south celestial poles.
Right ascension (a ) is measured eastward along the celestial equator from the vernal equinox to where an object’s hour circle meets the celestial equator, usually measured full circle in time units from 0h to 24h.
Declination d is an object’s arc distance along its hour circle from the celestial equator: positive north of the equator and negative south of it. d =+90° for the NCP, and d =-90° for the SCP.
An object’s hour angle (t) is the arc distance westward along the celestial equator from its intersection with the celestial meridian above the horizon to where the celestial equator meets the object’s hour circle. Its value increases with time from 0h to 24h.
Right ascension and declination on the celestial sphere are analogous to geographic longitude and latitude, respectively, on Earth, but their measurements differs somewhat.
The ecliptic coordinates uses the ecliptic for its fundamental plane. The ecliptic is the Sun’s path as it
Celestial equator— The projection into space of the Earth’s equator.
Celestial merdian— The circle passing through the zenith, zadir, north celestial pole, and south celestial pole.
Ecliptic— The intersection of the Earth’s orbital plane with the celestial sphere.
Horizon— The circle on the celestial sphere 90° from the zenith and nadir.
Hour circles— Half circles from the north celestial pole to the south celestial pole.
Nadir— The point on the celestial sphere directly below (by downward extension of the plumb line through the Earth) the observer.
North point of the horizon— The intersection of the horizon and celestial meridian closer to the north celestial pole (NCP).
North (south) celestial poles (NCP, SCP)— The intersection(s) of the Earth’s rotation axis extended beyond the North (South) geographic poles, respectively, with the celestial sphere.
Secondaries to the ecliptic— Half circles from the north ecliptic pole to the south ecliptic pole. Secondaries to the galactic equator—Half circles from the north galactic pole to the south galactic pole.
Vernal equinox— The intersection of the celestial equator and ecliptic that the Sun appears to reach on or about March 21.
Vertical circles— Half circles from the zenith to the nadir.
Zenith— The point on the celestial sphere directly above (by upward extension of the local direction of gravity (plumb line) the observer.
crosses the sky over a one-year period. The ecliptic is the basic circle (or the Earth’s orbital plane onto the celestial sphere) and the vernal equinox is the zero point for these coordinates. The north (NEP) and south (SEP) ecliptic poles are 23°.5 from the NCP and SCP, respectively, and are everywhere 90° from the ecliptic. The ecliptic coordinate system is often used to chart the position of solar system objects.
Celestial longitude (l) is the arc distance eastward along the ecliptic from the vernal equinox to where an object’s secondary to the ecliptic meets the ecliptic. It is expressed in arc units from 0 to 360°.
Celestial latitude (b ) is an object’s arc distance from the ecliptic along its secondary to the ecliptic. It is positive north of the ecliptic and negative south of it. b =+90° at the NEP and b =-90° at the SEP. These coordinates may be either geocentric (Earth-centered) or heliocentric (Sun-centered).
The galactic coordinates uses the Milky Way galaxy as its fundamental plane. It involves the galactic equator, which is its basic circle. The origin of the galactic coordinate system is the point of the rotation of the Sun around the Milky Way’s axis of rotation.
The north (NGP) and south (SGP) galactic poles are 90° from the galactic equator and are 62.6° from the NCP and SCP, respectively.
The galactic longitude (l) is the arc distance eastward along the galactic equator from the direction to the center of the Milky Way galaxy to where an object’s secondary to the galactic equator crosses the galactic equator. It ranges from 0 to 360°.
Galactic latitude (b) is the arc distance of an object from the galactic equator along its secondary to the galactic equator. Values of b vary from +90° at the NGP to -90° at the SGP. Galactic coordinates of the direction to the center of the Milky Way galaxy are l =0°, b =0°. Galactic coordinates are usually heliocentric but can also be centered at the center of the Milky Way galaxy.
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Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2007.
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Millar, William. The Amateur Astronomer’s Introduction to the Celestial Sphere. Cambridge, UK: Cambridge University Press, 2006.
"Celestial coordinates." The Gale Encyclopedia of Science. . Encyclopedia.com. (July 19, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/celestial-coordinates
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