What is "declination"

Refraction \Re*frac"tion\ (r?*fr?k"sh?n), n. [F. r['e]fraction.]

1. The act of refracting, or the state of being refracted.

2. The change in the direction of ray of light, heat, or the like, when it enters obliquely a medium of a different density from that through which it has previously moved.

   Refraction out of the rarer medium into the denser, is made towards the perpendicular.
   --Sir I. Newton.

3. (Astron.)

   1. The change in the direction of a ray of light, and, consequently, in the apparent position of a heavenly body from which it emanates, arising from its passage through the earth's atmosphere; -- hence distinguished as atmospheric refraction, or astronomical refraction.

   2. The correction which is to be deducted from the apparent altitude of a heavenly body on account of atmospheric refraction, in order to obtain the true altitude.

      Angle of refraction (Opt.), the angle which a refracted ray makes with the perpendicular to the surface separating the two media traversed by the ray.

      Conical refraction (Opt.), the refraction of a ray of light into an infinite number of rays, forming a hollow cone. This occurs when a ray of light is passed through crystals of some substances, under certain circumstances. Conical refraction is of two kinds; external conical refraction, in which the ray issues from the crystal in the form of a cone, the vertex of which is at the point of emergence; and internal conical refraction, in which the ray is changed into the form of a cone on entering the crystal, from which it issues in the form of a hollow cylinder. This singular phenomenon was first discovered by Sir W. R. Hamilton by mathematical reasoning alone, unaided by experiment.

      Differential refraction (Astron.), the change of the apparent place of one object relative to a second object near it, due to refraction; also, the correction required to be made to the observed relative places of the two bodies.

      Double refraction (Opt.), the refraction of light in two directions, which produces two distinct images. The power of double refraction is possessed by all crystals except those of the isometric system. A uniaxial crystal is said to be optically positive (like quartz), or optically negative (like calcite), or to have positive, or negative, double refraction, according as the optic axis is the axis of least or greatest elasticity for light; a biaxial crystal is similarly designated when the same relation holds for the acute bisectrix.

      Index of refraction. See under Index.

      Refraction circle (Opt.), an instrument provided with a graduated circle for the measurement of refraction.

      Refraction of latitude, longitude, declination, right ascension, etc., the change in the apparent latitude, longitude, etc., of a heavenly body, due to the effect of atmospheric refraction.

      Terrestrial refraction, the change in the apparent altitude of a distant point on or near the earth's surface, as the top of a mountain, arising from the passage of light from it to the eye through atmospheric strata of varying density.

late 14c. as a term in astronomy, from Old French declinacion (Modern French déclinaison), from Latin declinationem (nominative declinatio), noun of action from past participle stem of declinare (see decline (v.)). It took on various other senses 15c.-17c., most now obsolete.

n. 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 (context grammar English) declension. 5 (context archaic English) The act or state of bending downward; inclination. 6 (context archaic English) The act or state of falling off or declining from excellence or perfection; deterioration; decay; decline. 7 (context archaic English) deviation.

1. n. 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]

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.

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").

Declination can refer to:

• Declension, use of case in linguistics
• Declination, coordinate used in astronomy
• The noun form of the verb Decline
• Magnetic declination, angle that must be added or subtracted in the use of compass for geography
• Grid declination, angle between the compass north and north direction of the map coordinate grid
• Clinamen, a concept in early atomic theory