ASCENSION, in astronomy, the rising of' the sun or star, or any part of the equi noctial with it, above the horizon : is ei their right or oblique.
Right ascension is that degree of the equator, reckoned from the beginning of Aries, which rises with the sun, or a star, in a right sphere. It is found by the fol lowing proportion. As the radius to the cosine of the sun or star's greatest decli nation, so is the tangent of the distance from Aries to Libra to the tangent of right ascension.
Oblique ascension is that degree and minute of the equinoctial, counting from the beginning of Aries, which rises with the centre of the sun, or a star, or which comes to the horizon at the same time as the sun or star in an oblique sphere. ,In order to find the oblique ascension, we must first find the ascensions] difference.
The arch of right ascension coincides with the right ascension itself, and is the same in all parts of the globe. The arch of oblique ascension coincides with the oblique ascension, and changes according to the latitude of the places.
The sun's right ascension in time is use ful to the practical astronomer in regular observatories, who adjusts his clock by sideral time. It serves also for convert ing apparent into sideral time ; as e. g. that of an eclipse ofJupiter's satellites, in order to know at what time it may be ex pected to happen by his clocks. For this purpose the sun's right ascension at the preceding noon, together with the in crease of right ascension from noon, must be added to the apparent time Of the phenomenon set down in the ephemeris. The sun's right ascension in time serves also for computing the apparent time of a known star's passing the meridian : thus, subtract the sun's right ascension in time at noon from the star's right ascension in time, the remainder is the apparent time of the star's passing the meridian nearly; from which the proportional part of the daily increase of the sun's right ascen sion from this apparent time from noon being subtracted, leaves the correct time of the star's passing the meridian. The sun's right ascension in time is also useful for computing the time of the moon and planets passing the meridian. The prac tical method of finding the right ascen sion of a body from that of a fixed star, by a clock adjusted to sideral time, is this : let the clock begin its motion from Oh 0' 0" at the instant the first point of Aries is on the meridian ; then, when any star comes to the meridian, the clock would shew the apparent right ascension of the star, the right ascension being esti mated in time at the rate of 15° an hour, provided the clock was subject to no error ; because it would then shew at any time how far the first point of Aries was from the meridian. But as the clock is
liable to err, we most be able at any time to ascertain its error, or the difference between the right ascension shewn by the clock, and the right ascension of that point of the equator which is at that time on the meridian. To do this we must, when a star, whose righ ascension is known, passes the meridian, compare its apparent right ascension with the right ascension shewn by the clock, and the difference will shew the error of the clock. E. g. Let the apparent right ascension of Aldebaran be 23' .50", at the time when its transit over the meri dian is observed by the clock ; and sup pose the time shewn by the clock to be 23' 52", then there is an error of 2" in the clock, as it gives the right ascen sion of the star 2" more than it ought. If the clock be compared with several stars, and the mean error taken, we shall have more accurately the error at the mean time of all the observations. These observations being repeated every day, will give the rate of the clock's going, or shew how much it gains or loses. The error of the clock andthe rate of its going being thus ascertained, if the time of the transit of any body be observed, and the error of the clock at the time be applied, we shall have the right ascension of the body. This is the method by which the right ascension of the sun, moon, and pla nets, are regularly found in observatories. To find the right ascensions mechanically by the globe, see GLOBES, the use of. The arch ofright ascension is that portion of the equator intercepted between the be ginning of Aries and the point of the equator which is in the meridian : or it is the number of degrees contained in it. This coincides with the right ascension itself. The right ascension is the same in all parts of the globe. We sometimes also say, the right ascension of a point of the ecliptic, or any other point of the heavens. The right ascension of the mid heaven is often used by astronomers, particularly in calculating eclipses by the nonagesimal degree ; and it denotes the right ascension of that point of the equa tor which is in the meridian, and is equal to the sum of the sun's right ascension and the horary angle or true time, re duced to degrees, or to the sum the mean longitude of the sun and mean time.