DECLINATION
, in Astronomy, is the distance of the sun, star, planet, &c, from the equinoctial, either northward or southward; being the same with latitude in geography, or distance from the equator.
Declination is either real or apparent, according as the real or apparent place of the point or object is considered.
The declination of any point S is an arch of the meridian SE, contained between the given point and the equinoctical EQ. The declination of a star &c, is found by knowing or observ- ing the latitude of the place, i. e. the height of the pole, and then the meridian altitude of the star, &c; hence the difference between the co-latitude and the altitude of the star &c, is the declination, viz, the difference between EH the co-latitude, and SH the altitude, is ES the declination. For ex. Tycho found at Uranibourg the meridian
| altitude of Cauda Leonis, viz, | HS= | 50° | 59′ | 00″ |
| the co-latitude is | HE= | 34 | 5 | 45 |
| rem. declin. north | ES= | 16 | 53 | 15 |
To find the Sun's Declination at any time; having given his place in the ecliptic; the rule is, as radius is to the sine of the sun's longitude, so is the sine of the greatest declination, or obliquity of the ecliptic, to his present declination sought.
In constructing tables of declination of the sun, planets, and stars, regard should be had to refraction, aberration, nutation, and parallax.
By comparing ancient observations with the modern, it appears that the declination of the fixed stars is variable; and that differently in different stars; for in some it increases, and in others decreases, and that in different quantities.
Circles of Declination, are great circles of the sphere passing through the poles of the world, on which the declination is measured; and consequently are the same as meridians in geography.
Parallax of Declination, is an arch of the circle of declination, by which the parallax in altitude increases or diminishes the declination of a star.
Parallels of Declination, are lesser circles parallel to the equinoctial. The tropic of Cancer is a parallel of declination at 23° 28′ distance from the equinoctial northward; and the tropic of Capricorn is the parallel of declination as far distant southward.
Refraction of the Declination, an arch of the circle of declination, by which the declination of a star is increased or diminished by means of the refraction.
Declination of the Compass, or Needle, is its deviation from the true meridian. See Variation.
Declination of a Vertical Plane, or Wall, in Dialling, is an arch of the horizon, comprehended either between the plane and the prime vertical circle, when it is counted from the east or west; or between the plane and the meridian, if it be accounted from the north or south.
