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Topic: Celestial coordinate

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Horizontal coordinate system

	Celestial coordinate system - Wikipedia, the free encyclopedia
		There are different celestial coordinate systems each using a coordinate grid projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth.
		The coordinate systems differ only in their choice of the fundamental plane, which divides the sky into two equal hemispheres along a great circle.
		Popular choices of pole and equator are the older B1950 and the modern J2000 systems, but a pole and equator "of date" can also be used, meaning one appropriate to the date under consideration, such as that at which a measurement of the position of a planet or spacecraft is made.
	en.wikipedia.org /wiki/Celestial_coordinate_system (310 words)

	Celestial Coordinate System
		This coordinate system is illustrated in the following figure (for which you should imagine the earth to be a point at the center of the sphere).
		In the celestial coordinate system the North and South Celestial Poles are determined by projecting the rotation axis of the Earth to intersect the celestial sphere, which in turn defines a Celestial Equator.
		The zero point for celestial longitude (that is, for right ascension) is the Vernal Equinox, which is that intersection of the ecliptic and the celestial equator near where the Sun is located in the Northern Hemisphere Spring.
	csep10.phys.utk.edu /astr161/lect/time/coordinates.html (700 words)

	Astronomical Leage - Astro Note 11: Celestial Coordinate Systems
		Since all objects on the celestial sphere appear to be at the same, arbitrarily large distance from the observer, it is usually not necessary to know the object's true distance.
		Coordinates are then specified by angular measure around the great circle from the reference point and by angular distance from the reference plane along another great circle perpendicular to that plane.
		That intersection where the Sun appears to cross the celestial equator from south to north is chosen as the reference point and is known as the vernal equinox.
	www.astroleague.org /al/astrnote/astnot11.html (607 words)

	Basic Astronomical Terms
		The declination of -90 deg is the direction of the South Celestial Pole (SCP), +90 deg the (opposite) direction of the North Celestial Pole (NCP), and the circle of declination 0 deg is the Celestial Equator.
		The Null coordinate is defined by the vernal equinox, the direction where the Sun apparently crosses the Celestial Equator to the North around March 21 in each year, currently situated in Pisces.
		The celestial coordinate corresponding to latitude is the Declination.
	seds.lpl.arizona.edu /messier/xtra/Terms/terms.html (939 words)

	Meade Instruments Corporation - Understanding Celestial Movements and Coordinates
		Understanding celestial coordinates: Celestial objects are mapped according to the R.A. and Dec. coordinate system on the "celestial sphere," the imaginary sphere on which all stars appear to be placed.
		Celestial object positions on the Earth's surface are specified by their latitude and longitude.
		With all celestial objects therefore capable of being specified in position by their celestial coordinates of Right Ascension and Declination, the task of finding objects (in particular, faint objects) in the telescope can be simplified.
	www.meade.com /manuals/m4500/celest.html (892 words)

	Navigator Light Software
		The declination is measured in the plane North-South, from the Celestial Equator.
		The celestial sphere poles, being in the axis of rotation, remain fixed in the sky.
		Because both Earth and Celestial equators are in the same plane, the latitude of the GP is equal to the declination of the star.
	www.tecepe.com.br /nav/inav_c11.htm (1545 words)

	Coordinate conversion: celestial to horizon
		Since the location of the vernal equinox changes due to the precession of the Earth's axis of rotation, coordinates must be given with reference to a date or epoch.
		This is a local coordinate system to use for locating objects in the night sky as seen from a point on the Earth's surface.
		Azimuth is the angle of a celestial object around the sky from north.
	home.att.net /~srschmitt/celestial2horizon.html (643 words)

	The Celestial Sphere
		Superposed on the overall diurnal motion of the sky is "intrinsic" motion that causes certain objects on the celestial sphere to change their positions with respect to the other objects on the celestial sphere.
		Actually, all objects are slowly changing their relative positions on the celestial sphere, but for most the motion is so slow that it cannot be detected over timespans comparable to a human lifetime; only the "wanderers" have sufficiently fast motion for this change to be easily visible.
		Another important imaginary object on the celestial sphere is the "ecliptic" or "Road of the Sun", which is the imaginary path that the Sun follows on the celestial sphere over the course of a year.
	csep10.phys.utk.edu /astr161/lect/celestial/celestial.html (824 words)

	Astronomical Coordinate Systems
		The coordinate systems considered here are all based at one reference point in space with respect to which the positions are measured, the origin of the reference frame (typically, the location of the observer, or the center of Earth, the Sun, or the Milky Way Galaxy).
		The second coordinate of a position in the horizon system is defined by the point where the verticle circle of the position cuts the horizon.
		Celestial objects are at constant RA, but change their hour angle as time proceeds.
	www.seds.org /~spider/spider/ScholarX/coords.html (3777 words)

	Celestial Coordinates
		It is a great circle on the celestial sphere and crosses the celestial equator at two points.
		Examine the origin of the celestial coordinate presently in the constellation of Pisces (the fish).
		Find the celestial coordinates of the star in the south-east (lower left in the northern hemisphere) corner of the square (gamma in Pegasus).
	plato.acadiau.ca /courses/phys/astro/lessons/celcoord.html (774 words)

	PY 124: Right Ascnesion (Site not responding. Last check: 2007-10-26)
		Right Ascension is a coordinate on the celestial sphere that is similar to, but not identical to, longitude on the Earth's surface.
		In this situation we mean the point on the celestial sphere where the paths of the ecliptic and the celestial equator cross near the constellation Aries.
		The entire celestial sphere slowly moves from left to right over the course of the night, so the positions in relation to the horizon and the meridian of all the stars shown slowly change.
	www.physics.ncsu.edu /courses/astron/Right_Ascension.html (734 words)

	Zoom Astronomy Glossary: C
		Celestial coordinates are pairs of numbers (right ascension and declination) which are used to locate objects on the celestial sphere.
		Cetus is a constellation that straddles the celestial equator.
		Culmination is the highest altitude that a celestial object attains above the horizon (as seen from Earth); an object culminates as it crosses the observer's meridian.
	www.enchantedlearning.com /subjects/astronomy/glossary/indexc.shtml (5823 words)

	Attributes which Qualify Celestial Coordinate Systems
		Many celestial coordinate systems have some additional free parameters which serve to identify a particular coordinate system from amongst a broader class of related coordinate systems.
		This value is used to qualify celestial coordinate systems that are notionally based on the Earth's equator and/or the ecliptic (the plane of the Earth's orbit around the Sun).
		Coordinates expressed in any of these systems vary with time due to movement (by definition) of the coordinate system itself, and must therefore be qualified by a moment in time (the epoch of the mean equinox, or ``equinox'' for short) which specifies the position of the model coordinate system on the sky.
	star-www.rl.ac.uk /~dsb/ast/sun210.htx/node87.html (604 words)

	Sky Publishing - Understanding Celestial Coordinates (Site not responding. Last check: 2007-10-26)
		The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the faulty world-view of the ancients.
		The celestial dome with its starry decorations had to be a complete celestial sphere, early skywatchers figured out, because we never see a bottom rim as the dome tilts up and rotates around the Earth once a day.
		Parts of the celestial sphere are always setting behind the western horizon, while other parts are rising in the east.
	www.wwnorton.com /astro21/sandt/celcoords.html (1413 words)

	Representations of Celestial Coordinates in FITS
		Building on conventions in wide use within astronomy, this paper proposes changes to the simple methods for describing coordinates and proposes detailed conventions for describing most of the methods by which spherical coordinates may be projected onto a two-dimensional plane.
		The original authors chose to defer discussion of the technical details of coordinate specification until the basic FITS format was accepted generally and until a deeper understanding of image coordinate specification and computation could be obtained.
		For celestial coordinates, the proposed fourth step involves converting the linear offsets into longitudes and latitudes in the ``native coordinate system'' for the specified type of projection.
	www.adass.org /adass/proceedings/adass94/greisene.html (1114 words)

	Representations of World Coordinates in FITS
		The computation of the coordinates associated with each pixel is broken into three steps, a correction for arbitrary small image distortions, a linear transformation (including rotation, scale and skew), and transformation from intermediate world coordinates to the required world coordinate system via a predefined algorithm.
		The coordinates for integer voxel numbers refer to the center of the voxel, the only location within an individual voxel that is invariant to rotation and transposition.
		The native coordinate system is the one in which the projection is most conveniently defined and the reference values in the desired system must correspond to the ``reference point'' of the projection.
	www.adass.org /adass/proceedings/adass99/P1-29 (1279 words)

	ICRS Narrative
		The equinox, therefore, is a direction in space along the nodal line defined by the intersection of the ecliptic and equatorial planes; equivalently, on the celestial sphere, the equinox is at one of the two intersections of the great circles representing these planes.
		Because both of these planes are moving, the coordinate systems that they define must have a date associated with them; such a reference system must be therefore specified as "the equator and equinox of [some date]".
		At its General Assembly in 2000 [8] the IAU defined a system of space-time coordinates for (1) the solar system, and (2) the Earth, within the framework of General Relativity, by specifying the form of the metric tensors for each and the 4-dimensional space-time transformation between them.
	aa.usno.navy.mil /faq/docs/ICRS_doc.html (4516 words)

	Celestial Coordinate System Subroutines
		The following subroutines are provided to help calculate the transformation between pixel location in an image and the corresponding celestial coordinates on the sky.
		Get the values of all the standard FITS celestial coordinate system keywords from the header of a FITS table where the X and Y (or RA and DEC coordinates are stored in 2 separate columns of the table.
		Calculate the celestial coordinate corresponding to the input X and Y pixel location in the image.
	heasarc.gsfc.nasa.gov /docs/software/fitsio/user_f/node60.html (208 words)

	The Sky, Stars, and Celestial Sphere
		Actually, all objects are slowly changing their relative positions on the celestial sphere, but for most the motion is so slow that it cannot be detected over time spans comparable to a human lifetime; only the "wanderers" have sufficiently fast motion for this change to be easily visible.
		Since the celestial sphere does not incorporate the distance of a particular object in its location on the sky, to locate an object we must refer to its angular coordinates.
		Another important imaginary plane on the celestial sphere is the plane of the "ecliptic" or "Path of the Sun", which is the imaginary path that the Sun follows on the celestial sphere over the course of a year.
	www.pas.rochester.edu /~blackman/ast104/celestial5.html (2289 words)

	Penn AstroLab Lecture Notes: Celestial Navigation
		The coordinate system is also a direct analog, with declination (Dec) measured as the latitude is on the globe, and the right ascension (RA) corresponding to longitude - though for right ascension, the angle is measured in units of 0 to 24 hours from the zero-hour point.
		Using these coordinates, the North Celestial Pole (NCP), as discussed in the previous lecture, is at an azimuth of 0 degrees (due north), and an elevation equal to the observer's latitude.
		This is a reflection of the fact that the ecliptic is inclined to the celestial equator by 23.5 degrees, which is equivalent to saying that the Earth's rotation axis is misaligned by 23.5 degrees with the "axis" of Earth's orbit around the Sun.
	www.aoc.nrao.edu /~smyers/courses/astrolabs/navigation.html (4108 words)

	Creating a Mosaic World Coordinate System
		Similarly the celestial coordinates can be added after the pixel coordinates are determined or as the pixel coordinates are measured.
		This task is basically a fitting task that fits a transformation between pixel coordinates and celestial coordinates and records the result in both a database file and in the image header as a WCS.
		Starting with an approximately correct WCS what is done is to register a list of celestial coordinates to the objects in the image, fit a WCS of an appropriate order to describe the optical distortions, and update the header and write a WCS database file to be used to apply the WCS to other images.
	iraf.noao.edu /projects/ccdmosaic/astrometry/astrom.html (3590 words)

	AST 101 - FALL 2003 - Course Pack
		The observer's zenith moves from the celestial equator toward the celestial pole as the observer moves from zero latitude toward higher latitude.
		This is a nuisance requiring a rather straightforward calculation when the coordinates of a star are needed for a certain night, but its position has been given on a map, or in a catalogue of positions, for a different time.
		Latitude 0 degrees and declination 0 degrees occur on the terrestrial and celestial equators.
	www.pa.msu.edu /courses/2003fall/AST101/coursepk.html (7857 words)

	Spherical Coordinate System Lab
		This coordinate system is based upon the physical fact that we live on the earth and gravity pulls us to the earth establishing and up and a down locally.
		The declination of the celestial equator is 0.
		This coordinate system is based upon the physical fact that we live in a flattened disk of stars that the solar system revolves about in approximately 250 million years.
	www.mc.cc.md.us /Departments/planet/planet/Coordinate_Systems_Lab.htm (1626 words)

	Peter Alway Astronotes 1
		(In the norther hemisphere, the altitude of the north celestial pole is equal to the observer's lattitude.
		The south celestial pole is at -90 degrees declination.
		the celestial equator, of course is at 0 degrees declination.
	members.aol.com /satrnpres1/astronomy/unit1c.html (2711 words)

	Celestial Coordinate Systems
		Each uses a coordinate grid projected on the Celestial Sphere, in analogy to the Geographic coordinate system used on the surface of the Earth.
		It is also the most closely related to the Geographic coordinate system, because they use the same fundamental plane, and the same poles.
		Similarly, projecting the geographic Poles onto the celestial sphere defines the North and South Celestial Poles.
	kstars.sourceforge.net /handbook/ai-skycoords.html (781 words)

	Celestial Coordinate Systems
		And in some instances the coordinate system is slightly distorted, so that the ordinary rules of spherical trigonometry no longer strictly apply.
		Figure 1 shows how some of these systems are related, to one another and to the direction in which a celestial source actually appears in the sky.
		To help understand this diagram, and the SLALIB routines that can be used to carry out the various calculations, we will look at the coordinate systems involved, and the astronomical phenomena that affect them.
	www.hartrao.ac.za /nccsdoc/slalib/sun67.htx/node202.html (378 words)

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