Declination

In astronomy, declination (abbreviated dec; symbol δ) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.[1]

Ra and dec demo animation small
Right ascension and declination as seen on the inside of the celestial sphere. The primary direction of the system is the vernal equinox, the ascending node of the ecliptic (red) on the celestial equator (blue). Declination is measured northward or southward from the celestial equator, along the hour circle passing through the point in question.

The root of the word declination (Latin, declinatio) means "a bending away" or "a bending down". It comes from the same root as the words incline ("bend toward") and recline ("bend backward").[2]

In some 18th and 19th century astronomical texts, declination is given as North Pole Distance (N.P.D.), which is equivalent to 90 - (declination). For instance an object marked as declination -5 would have a NPD of 95, and a declination of -90 (the south celestial pole) would have a NPD of 180.

Explanation

Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, and hour angle is likewise comparable to longitude.[3] Points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of angular measure can be used for declination, but it is customarily measured in the degrees ( ° ), minutes ( ′ ), and seconds ( ″ ) of sexagesimal measure, with 90° equivalent to a quarter circle. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere.

An object at the

The sign is customarily included whether positive or negative.

Effects of precession

Ra and dec on celestial sphere
Right ascension (blue) and declination (green) as seen from outside the celestial sphere.

The Earth's axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates (including declination) are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch.[4]

The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, and B1950.0.[5]

Stars

A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change gradually due to precession of the equinoxes and proper motion, and cyclically due to annual parallax. The declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity.

As seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90° − φ (where φ = observer's latitude) appear to circle daily around the celestial pole without dipping below the horizon, and are therefore called circumpolar stars. This similarly occurs in the Southern Hemisphere for objects with declinations less (i.e. more negative) than −90° − φ (where φ is always a negative number for southern latitudes). An extreme example is the pole star which has a declination near to +90°, so is circumpolar as seen from anywhere in the Northern Hemisphere except very close to the equator.

Circumpolar stars never dip below the horizon. Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface (except extremely close to the equator. Upon flat terrain, the distance has to be within approximately 2 km, although this varies based upon the observer's altitude and surrounding terrain). Generally, if a star whose declination is δ is circumpolar for some observer (where δ is either positive or negative), then a star whose declination is −δ never rises above the horizon, as seen by the same observer. (This neglects the effect of atmospheric refraction.) Likewise, if a star is circumpolar for an observer at latitude φ, then it never rises above the horizon as seen by an observer at latitude −φ.

Neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − |φ|, and at the south point, −90° + |φ|. From the poles, declination is uniform around the entire horizon, approximately 0°.

Stars visible by latitude
Observer's latitude (°) Declination
of circumpolar stars (°) of non-circumpolar stars (°) of stars not visible (°)
+ for north latitude, − for south   − for north latitude, + for south
90 (Pole) 90 to 0 N/A 0 to 90
66.5 (Arctic/Antarctic Circle) 90 to 23.5 +23.5 to −23.5 23.5 to 90
45 (midpoint) 90 to 45 +45 to −45 45 to 90
23.5 (Tropic of Cancer/Capricorn) 90 to 66.5 +66.5 to −66.5 66.5 to 90
0 (Equator) N/A +90 to −90 N/A

Non-circumpolar stars are visible only during certain days or seasons of the year.

Stars and dec
The night sky, divided into two halves. Declination (green) begins at the equator (green) and is positive northward (towards the top), negative southward (towards the bottom). The lines of declination (green) divide the sky into small circles, here 15° apart.

Sun

The Sun's declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the local summer solstice, leading to the phenomenon of it being above the horizon at midnight, which is called midnight sun. Likewise, near the local winter solstice, the Sun remains below the horizon all day, which is called polar night.

Relation to latitude

When an object is directly overhead its declination is almost always within 0.01 degrees of the observer's latitude; it would be exactly equal except for two complications.[6] [7]

The first complication applies to all celestial objects: the object's declination equals the observer's astronomic latitude, but the term "latitude" ordinarily means geodetic latitude, which is the latitude on maps and GPS devices. In the continental United States and surrounding area, the difference (the vertical deflection) is typically a few arcseconds (1 arcsecond = 1/3600 of a degree) but can be as great as 41 arcseconds.[8]

The second complication is that, assuming no deflection of the vertical, "overhead" means perpendicular to the ellipsoid at observer's location, but the perpendicular line does not pass through the center of the earth; almanacs provide declinations measured at the center of the Earth. (An ellipsoid is an approximation to sea level that is mathematically manageable).[9] For the moon this discrepancy can reach 0.003 degrees; the Sun and planets are hundreds of times more distant and for them the discrepancy is proportionately smaller (and for the stars is immeasurable).

See also

Notes and references

  1. ^ U.S. Naval Observatory, Nautical Almanac Office (1992). P. Kenneth Seidelmann, ed. Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. p. 724. ISBN 0-935702-68-7.
  2. ^ Barclay, James (1799). A Complete and Universal English Dictionary.
  3. ^ Moulton, Forest Ray (1918). An Introduction to Astronomy. New York: Macmillan Co. p. 125, art. 66.
  4. ^ Moulton (1918), pp. 92–95.
  5. ^ see, for instance, U.S. Naval Observatory Nautical Almanac Office, Nautical Almanac Office; U.K. Hydrographic Office, H.M. Nautical Almanac Office (2008). "Time Scales and Coordinate Systems, 2010". The Astronomical Almanac for the Year 2010. U.S. Govt. Printing Office. p. B2,.
  6. ^ "Celestial Coordinates". www.austincc.edu. Retrieved 2017-03-24.
  7. ^ baylor.edu
  8. ^ "USDOV2009". Silver Spring, Maryland: U.S. National Geodetic Survey. 2011.
  9. ^ P. Kenneth Seidelmann, ed. (1992). Explanatory Supplement to the Astronomical Almanac. Sausalito, CA: University Science Books. pp. 200–5.

External links

Boötes void

The Boötes void (or The Great Nothing) is a notably large, approximately spherical region of space, containing very few galaxies. It is located in the vicinity of the constellation Boötes, hence its name. Its center is located at approximately right ascension 14h 50m and declination 46°.

Compass

A compass is an instrument used for navigation and orientation that shows direction relative to the geographic cardinal directions (or points). Usually, a diagram called a compass rose shows the directions north, south, east, and west on the compass face as abbreviated initials. When the compass is used, the rose can be aligned with the corresponding geographic directions; for example, the "N" mark on the rose points northward. Compasses often display markings for angles in degrees in addition to (or sometimes instead of) the rose. North corresponds to 0°, and the angles increase clockwise, so east is 90° degrees, south is 180°, and west is 270°. These numbers allow the compass to show magnetic North azimuths or true North azimuths or bearings, which are commonly stated in this notation. If magnetic declination between the magnetic North and true North at latitude angle and longitude angle is known, then direction of magnetic North also gives direction of true North.

Among the Four Great Inventions, the magnetic compass was first invented as a device for divination as early as the Chinese Han Dynasty (since c. 206 BC), and later adopted for navigation by the Song Dynasty Chinese during the 11th century. The first usage of a compass recorded in Western Europe and the Islamic world occurred around 1190.

Equatorial coordinate system

The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention.The origin at the center of Earth means the coordinates are geocentric, that is, as seen from the centre of Earth as if it were transparent. The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.

Equatorial mount

An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescopes and cameras. The advantage of an equatorial mount lies in its ability to allow the instrument attached to it to stay fixed on any celestial object with diurnal motion by driving one axis at a constant speed. Such an arrangement is called a sidereal or clock drive.

Fourth Cambridge Survey

The Fourth Cambridge Survey (4C) is an astronomical catalogue of celestial radio sources as measured at 178 MHz using the 4C Array. It was published in two parts, in 1965 (for declinations +20 to +40) and 1967 (declinations -7 to + 20 and +40 to +80), by the Radio Astronomy Group of the University of Cambridge. References to entries in this catalogue use the prefix 4C followed by the declination in degrees, followed by a period, and then followed by the source number on that declination strip, e.g. 4C-06.23.

The 4C Array, which used the technique of aperture synthesis, could reliably position sources with flux densities of around 2 Jy, to within about 0.35 arcmin in Right ascension and 2.5 arcmin in declination.

List of NGC objects (5001–6000)

This is a list of NGC objects 5001–6000 from the New General Catalogue (NGC). The astronomical catalogue is composed mainly of star clusters, nebulae, and galaxies. Other objects in the catalogue can be found in the other subpages of the list of NGC objects.

The constellation information in these tables is taken from The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, which was accessed using the "VizieR Service". Galaxy types are identified using the NASA/IPAC Extragalactic Database. The other data of these tables are from the SIMBAD Astronomical Database unless otherwise stated.

Lunar standstill

A lunar standstill is the gradually varying range between the northern and the southern limits of the Moon's declination, or the lunistices, over the course of one-half a sidereal month (about two weeks), or 13.66 days. (Declination is a celestial coordinate measured as the angle from the celestial equator, analogous to latitude.) One major, or one minor, lunar standstill occurs every 18.6 years due to the precessional cycle of the lunar nodes at that rate.

At a major lunar standstill, the Moon's range of declination, and consequently its range of azimuth at moonrise and moonset, reaches a maximum. As a result, viewed from the middle latitudes, the Moon's altitude at upper culmination (the daily moment when the object appears to contact the observer's meridian) changes in just two weeks – from highest to lowest above the horizon due north or south, depending on the observer's hemisphere. Similarly, its azimuth at moonrise changes from northeast to southeast and at moonset from northwest to southwest. Solar eclipses at ascending node occurs in March, solar eclipses at descending node occurs in September. Lunar eclipses at descending node occurs in March, lunar eclipses at ascending node occurs in September.

This time appears to have had special significance for the Bronze Age societies, who built the megalithic monuments in Britain and Ireland. It also has significance for some neopagan religions. Evidence also exists that alignments to the moonrise or moonset on the days of lunar standstills can be found in ancient sites of other ancient cultures, such as at Chimney Rock in Colorado and Hopewell Sites in Ohio.

Magnetic declination

Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) and true north (the direction along a meridian towards the geographic North Pole). This angle varies depending on position on the Earth's surface and changes over time.

Somewhat more formally, Bowditch defines variation as “the angle between the magnetic and geographic meridians at any place, expressed in degrees and minutes east or west to indicate the direction of magnetic north from true north. The angle between magnetic and grid meridians is called grid magnetic angle, grid variation, or grivation.”By convention, declination is positive when magnetic north is east of true north, and negative when it is to the west. Isogonic lines are lines on the Earth's surface along which the declination has the same constant value, and lines along which the declination is zero are called agonic lines. The lowercase Greek letter δ (delta) is frequently used as the symbol for magnetic declination.

The term magnetic deviation is sometimes used loosely to mean the same as magnetic declination, but more correctly it refers to the error in a compass reading induced by nearby metallic objects, such as iron on board a ship or aircraft.

Magnetic declination should not be confused with magnetic inclination, also known as magnetic dip, which is the angle that the Earth's magnetic field lines make with the downward side of the horizontal plane.

NGC 11

NGC 11 is a spiral galaxy located in the Andromeda constellation. It is located at right ascension 00h 08m 42.5s; declination +37° 26′ 53″; under J2000.0 coordinates and was discovered by Édouard Stephan on October 24 1881.

NGC 4790

NGC 4790 is a Barred Spiral Galaxy (SBc) located in the vicinity of the constellation Virgo. It has a declination of -10° 14' 52" and a right ascension of 12 hours, 54 minutes and 51.9 seconds. In 2012, a possible supernova, SN 2012au, was detected in NGC 4790.The galaxy was discovered on 25 March 1786 by William Herschel.

Nolle prosequi

Nolle prosequi (; Classical Latin: [ˈnolːe ˈproːsekwiː]) is a legal term of art and a Latin legal phrase meaning "be unwilling to pursue", a phrase amounting to "do not prosecute". It is a phrase used in many common law criminal prosecution contexts to describe a prosecutor's decision to voluntarily discontinue criminal charges either before trial or before a verdict is rendered. It contrasts with an involuntary dismissal.

North

North is one of the four cardinal directions or compass points. It is the opposite of south and is perpendicular to east and west. North is a noun, adjective, or adverb indicating direction or geography.

North Magnetic Pole

The North Magnetic Pole is the wandering point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downwards (in other words, if a magnetic compass needle is allowed to rotate about a horizontal axis, it will point straight down). There is only one location where this occurs, near (but distinct from) the Geographic North Pole and the Geomagnetic North Pole.

The North Magnetic Pole moves over time due to magnetic changes in the Earth's core. In 2001, it was determined by the Geological Survey of Canada to lie west of Ellesmere Island in northern Canada at 81.3°N 110.8°W / 81.3; -110.8 (Magnetic North Pole 2001). It was situated at 83.1°N 117.8°W / 83.1; -117.8 (Magnetic North Pole 2005 est) in 2005. In 2009, while still situated within the Canadian Arctic territorial claim at 84.9°N 131.0°W / 84.9; -131.0 (Magnetic North Pole 2009), it was moving toward Russia at between 55 and 60 kilometres (34 and 37 mi) per year. As of 2017, the pole is projected to have moved beyond the Canadian Arctic territorial claim to 86.5°N 172.6°W / 86.5; -172.6 (Magnetic North Pole 2017 est).Its southern hemisphere counterpart is the South Magnetic Pole. Since the Earth's magnetic field is not exactly symmetrical, the North and South Magnetic Poles are not antipodal, meaning that a straight line drawn from one to the other does not pass through the geometric centre of the Earth.

The Earth's North and South Magnetic Poles are also known as Magnetic Dip Poles, with reference to the vertical "dip" of the magnetic field lines at those points.

Ohio Sky Survey

The Ohio Sky Survey was an astronomical survey of extragalactic radio sources. Data were taken between 1965 and 1971 using the Big Ear radio telescope at the Ohio State University Radio Observatory (OSURO), also known as the "Big Ear Radio Observatory (BERO)".

The survey covered 94% of the sky area between the limiting declinations of 63°N and 36°S with a resolution at 1415 MHz of 40 arc minutes in declination. The survey was carried out primarily at a frequency of 1415 MHz but observations were also made at 2650 MHz and 612 MHz. Roughly 19,620 sources were identified over the course of the survey of which 60% were previously uncatalogued.

The survey was unique in that it covered a larger portion of the sky, to a greater depth, and at a higher frequency, than any previous survey. In addition, all previously catalogued sources were tabulated and maps of the areas surveyed were included with the positions of all catalogued sources.

Sources discovered in the course of the survey were assigned names according to a coordinate numbering system consisting of a two-letter prefix followed by three digits. The first letter, O, stood for Ohio, and the second letter, B–Z inclusive (omitting O) indicated the source right ascension in hours (0–23 inclusive). The first digit indicated the declination zone in increments of 10°, while the last two digits give the right ascension to the nearest one-hundredth of an hour.

Data reduction for the survey was done using a computer program developed by John D. Kraus and Robert S. Dixon.The Ohio Sky Survey was published in seven installments and two supplements.

Position of the Sun

The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the celestial sphere, along a circular path called the ecliptic.

Earth's rotation about its axis causes the fixed stars to apparently move across the sky in a way that depends on the observer's geographic latitude. The time when a given fixed star transits the observer's meridian depends on the geographic longitude.

To find the Sun's position for a given location at a given time, one may therefore proceed in three steps as follows:

calculate the Sun's position in the ecliptic coordinate system,

convert to the equatorial coordinate system, and

convert to the horizontal coordinate system, for the observer's local time and location.This calculation is useful in astronomy, navigation, surveying, meteorology, climatology, solar energy, and sundial design.

Prosodic unit

In linguistics, a prosodic unit, often called an intonation unit or intonational phrase, is a segment of speech that occurs with a single prosodic contour (pitch and rhythm contour). The abbreviation IU is used and therefore the full form is often found as intonation unit, despite the fact that technically it is a unit of prosody rather than intonation, which is only one element of prosody.

Prosodic units occur at a hierarchy of levels, from the metrical foot and phonological word to a complete utterance. However, the term is generally restricted to intermediate levels which do not have a dedicated terminology. Prosodic units do not generally correspond to syntactic units, such as phrases and clauses; it is thought that they reflect different aspects of how the brain processes speech, with prosodic units being generated through on-line interaction and processing, and with morphosyntactic units being more automated.

Right ascension

Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.

When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle.

Smithsonian Astrophysical Observatory Star Catalog

The Smithsonian Astrophysical Observatory Star Catalog is an astrometric star catalogue. It was published by the Smithsonian Astrophysical Observatory in 1966 and contains 258,997 stars. The catalogue was

compiled from various previous astrometric catalogues, and contains only stars to about ninth magnitude for

which accurate proper motions were known. Names in the SAO catalogue start with the letters SAO, followed by a number. The numbers are assigned following 18 ten-degree bands of declination, with stars sorted by right ascension within each band.

Uppsala General Catalogue

The Uppsala General Catalogue of Galaxies (UGC) is a catalogue of 12,921 galaxies visible from the northern hemisphere. It was first published in 1973.

The catalogue includes essentially all galaxies north of declination -02°30' and to a limiting diameter of 1.0 arcminute or to a limiting apparent magnitude of 14.5. The primary source of data is the blue prints of the Palomar Observatory Sky Survey (POSS). It also includes galaxies smaller than 1.0 arcminute in diameter but brighter than 14.5 magnitude from the Catalogue of Galaxies and of Clusters of Galaxies (CGCG).

The catalogue contains descriptions of the galaxies and their surrounding areas, plus conventional system classifications and position angles for flattened galaxies. Galaxy diameters are included and the classifications and descriptions are given in such a way as to provide as accurate an account as possible of the appearance of the galaxies on the prints. The accuracy of coordinates is only what is necessary for identifications purposes.

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