AskDefine | Define declination

Dictionary Definition

declination

Noun

1 a condition inferior to an earlier condition; a gradual falling off from a better state [syn: decline] [ant: improvement]
2 (astronomy) the angular distance to a point on a celestial object measured north or south from the celestial equator; expressed in degrees; used with right ascension to specify positions on the celestial sphere [syn: celestial latitude, DEC]
3 a downward slope or bend [syn: descent, declivity, fall, decline, declension, downslope] [ant: ascent]
4 a polite refusal of an invitation [syn: regrets]

User Contributed Dictionary

English

Pronunciation

Etymology

From declinacioun, from declination, from declination.

Noun

  1. At a given point, the angle between magnetic north and true north.
  2. At a given point, the angle between the line connecting this point with the geographical center of the earth and the equatorial plane.
  3. A refusal.
  4. Declension.

Translations

at a given point, the angle between magnetic north and true north
at a given point, the angle between the line connecting this point with the geographical center of the earth and the equatorial plane
refusal
declension

Related terms

Derived terms

Extensive Definition

In astronomy, declination (abbrev. dec or δ) is one of the two coordinates of the equatorial coordinate system, the other being either right ascension or hour angle. Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator. Therefore, points north of the celestial equator have positive declinations, while those to the south have negative declinations.
The sign is customarily included even if it is positive. Any unit of angle can be used for declination, but it is often expressed in degrees, minutes, and seconds of arc.
A celestial object that passes over zenith, has a declination equal to the observer's latitude, with northern latitudes yielding positive declinations. A pole star therefore has the declination +90° or -90°. Conversely, at northern latitudes φ > 0, celestial objects with a declination greater than 90° - φ, are always visible. Such stars are called circumpolar stars, while the phenomenon of a sun not setting is called midnight sun.
If instead of measuring from and along the equator the angles are measured from and along the horizon, the angles are called azimuth and altitude (elevation).

Stars

Because a star lies in a nearly constant direction as viewed from earth, its declination is approximately constant from year to year. However, both the right ascension and declination do change gradually due to the effects of precession of the equinoxes and proper motion.

Varying declination

The declinations of all solar system objects change much more quickly than those of stars.

Sun

The declination of the Sun (δ) is the angle between the rays of the sun and the plane of the earth's equator. Since the angle between the earth axis and the plane of the earth orbit is nearly constant, δ varies with the seasons and its period is one year, that is the time needed by the earth to complete its revolution around the sun.
When the projection of the earth axis on the plane of the earth orbit is on the same line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is maximum and its value is 23°27'. This happens at the solstices. Therefore δ = +23°27' at the northern hemisphere summer solstice and δ = -23°27' at the northern hemisphere winter solstice. Due to the changes in the tilt of the Earth's axis, the angle between the rays of the sun and the plane of the earth equator is slightly decreasing.
When the projection of the earth axis on the plane of the earth orbit is perpendicular to the line linking the earth and the sun, the angle between the rays of the sun and the plane of the earth equator is null. This happens at the equinoxes. Therefore δ is 0° at the equinoxes.
Since the eccentricity of the earth orbit is quite low, it can be approximated to a circle, and δ is approximately given by the following expression:
\delta = -23.45^\circ \cdot \cos \left [ \frac \cdot \left ( N + 10 \right ) \right ]
where cos operates on degrees; if cos operates on radians, 360° in the equation needs to be replaced with 2π and will still output δ in degree; N is Day of the Year, that is the number of days spent since January 1.
An alternative form is given as:
\delta = 23.45^\circ \cdot \sin \left [ \frac \cdot \left ( N + 284 \right ) \right ]
A more precise formula is given by:
\ \delta = \frac \cdot (0.006918 - 0.399912 \cos \gamma + 0.070257 \sin \gamma - 0.006758 \cos 2\gamma + 0.000907 \sin 2\gamma - 0.002697 \cos 3\gamma + 0.00148 \sin 3\gamma)
where
\gamma = \frac ( N - 1 )
is the fractional year in radians.
More accurate daily values from averaging the four years of a leap-year cycle are given in the Table of the Declination of the Sun.

Moon

The Moon also has an annual cycle, with maximum declination at northern hemisphere midwinter and minimum at midsummer. There is also an approximately 19 year long cycle, varying the maximum declination from +28°35' to +18°18' and the minimum from -18°18' to -28°35'.

See also

Declination is used in some contexts that rule out astronomical declination, to mean the same as magnetic declination.
Declination is occasionally and erroneously used to refer to the linguistic term declension.

References

declination in Arabic: میل
declination in Asturian: Declinación (astronomía)
declination in Bulgarian: Деклинация
declination in Czech: Deklinace
declination in Danish: Deklination (astronomi)
declination in German: Deklination (Astronomie)
declination in Estonian: Kääne (astronoomia)
declination in Modern Greek (1453-): Απόκλιση αστέρος
declination in Spanish: Declinación (astronomía)
declination in Esperanto: Deklinacio (astronomio)
declination in Persian: میل
declination in French: Déclinaison (astronomie)
declination in Irish: Diallas
declination in Korean: 적위
declination in Croatian: Deklinacija (astronomija)
declination in Indonesian: Deklinasi
declination in Italian: Declinazione (astronomia)
declination in Latin: Declinatio (astronomia)
declination in Luxembourgish: Deklinatioun (Astronomie)
declination in Lithuanian: Deklinacija
declination in Latvian: Deklinācija (astronomijā)
declination in Malayalam: ഡെക്ലിനേഷന്‍
declination in Dutch: Declinatie (astronomie)
declination in Japanese: 赤緯
declination in Norwegian: Deklinasjon
declination in Norwegian Nynorsk: Deklinasjon
declination in Polish: Deklinacja (astronomia)
declination in Portuguese: Declinação
declination in Slovak: Deklinácia
declination in Serbo-Croatian: Deklinacija (astronomija)
declination in Finnish: Deklinaatio
declination in Swedish: Deklination (astronomi)
declination in Thai: เดคลิเนชัน
declination in Vietnamese: Xích vĩ
declination in Chinese: 赤纬

Synonyms, Antonyms and Related Words

Cartesian coordinates, aberrancy, aberration, abjuration, abjurement, abnegation, abscissa, altitude, azimuth, bend, bias, branching off, cascade, cataract, chucking, chucking out, chute, circuitousness, collapse, comedown, contempt, contradiction, coordinates, corner, crash, crook, crosswiseness, curve, cylindrical coordinates, debacle, debasement, decadence, decadency, declension, declinature, decline, declining, deflection, deflexure, defluxion, deformation, degeneracy, degenerateness, degeneration, degradation, demotion, denial, departure, depravation, depravedness, depreciation, deprivation, derogation, descending, descension, descent, despisal, despising, deterioration, detour, deviance, deviancy, deviation, deviousness, devolution, diagonality, digression, disagreement, disallowance, disapproval, discard, disclaimer, disclamation, discounting, discursion, dismissal, disobedience, disownment, disregard, dissent, divagation, divarication, divergence, diversion, dogleg, double, down, downbend, downcome, downcurve, downfall, downflow, downgrade, downpour, downrush, downtrend, downturn, downward mobility, downward trend, drift, drifting, drop, dropping, dying, ebb, ebbing, effeteness, equator coordinates, errantry, exception, exclusion, excursion, excursus, exorbitation, fading, failing, failure, failure of nerve, fall, falling, falling-off, gravitation, hairpin, holding back, ignoring, inclination, indirection, indirectness, involution, lapse, latitude, longitude, loss of tone, nay, negation, negative, negative answer, nix, no, nonacceptance, nonapproval, noncompliance, nonconformity, nonconsent, nonconsideration, nonobservance, obliqueness, obliquity, ordinate, passing by, pererration, plummeting, polar coordinates, pounce, putting away, putting out, rambling, rapids, rebuff, recantation, refusal, regression, rejection, renouncement, repudiation, repulse, retention, retrocession, retrogradation, retrogression, right ascension, scouting, sheer, shift, shifting, shifting course, shifting path, skew, skewness, slant, slippage, slump, spurning, squint, stoop, straying, sweep, swerve, swerving, swinging, swoop, tack, throwing out, thumbs-down, transverseness, turn, turndown, turning, turning out, twist, unwillingness, vagary, variation, veer, wandering, wane, waning, warp, waterfall, withholding, yaw, zigzag
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