ASCENSION

, in Astronomy, is either Right or Oblique.

Right Ascension of the sun, or of a star, is that degree of the equinoctial, accounted from the beginning of Aries, which rises with them, in a right sphere.— Or, Right Ascension, is that point of the equinoctial, counted as before, which comes to the meridian with the sun or star, or other point of the heavens. And the reason of thus referring it to the meridian, is, because this is always at right angles to the equinoctial; whereas the horizon is so only in a right or direct sphere.—The right ascension, stands opposed to the right descension; and is similar to the longitude of places on the earth. All the fixed stars, &c, which have the same right-ascension, that is, which are at the same distance from the first point of Aries, or, which comes to the same thing, which are in the same meridian, rise at the same time in a right sphere, namely to the people who live at the equator. And if they be not in the same meridian, the difference between their times of rising, or of coming to the meridian of any place, is the precise difference of their right ascension.—But, in an oblique sphere, where the horizon cuts all the meridians obliquely, different points of the same meridian never rise or set together: so that two or more stars on the same meridian, or having the same right ascension, never rise or set at the same time in an oblique sphere; and the more oblique the sphere is, the greater is the interval of time between them.

To find the right ascension of the sun, stars, &c, by Trigonometry, say, As radius is to the cosine of the sun's greatest declination, or obliquity of the ecliptic, so is the tangent of the sun's or star's longitude, to the tangent of the right ascension.

Right Ascension of the Mid-heaven, often used by astronomers, especially in calculating eclipses by means of the nonagesimal degree, is the right ascension of that point of the equator which is in the meridian; and it is equal to the sum of the sun's right ascension and | the horary angle or true time reduced to degrees, or to the sum of the mean longitude and mean time.

Oblique Ascension, is an arch of the equator intercepted between the first point of Aries, and that point of the equator which rises together with the star, &c, in an oblique sphere.—The Oblique Ascension is counted from west to east; and is greater or less, according to the various obliquity of the sphere.—To sind the Oblique Ascension of the sun, see Ascensional and Globe.

The Arch of Oblique Ascension, is an arch of the horizon intercepted between the beginning of Aries, and the point of the equator which rises with a star or planet, in an oblique sphere; and it varies with the latitude of the place.

Refraction of Ascension and Descension. See REFRACTION.