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The distance of any heavenly body from the equinoctial, measured upon the meridian, is called its declination.
Therefore, the sun's declination, north or south, at any time, is the same as the latitude of any place to which he is then vertical, which is never more than 23's degrees.
Therefore all parallels of declination on the celestial globe are the very same as parallels of latitude on the terrestrial. Stars may
have north latitude and south declination, and vice versa.
That which is called longitude on the terrestrial globe, is called right ascension on the celestial; namely, the sun or star's distance from that meridian which passes through the first point of Aries, counted on the equinoctial.
Astronomers also speak of oblique ascension and descension, by which mean the distance of that point of the equinoctial from the first point of Aries, which in an oblique sphere rises and sets at the same time that the sun or star rises or sets.
Ascensional difference is the difference betwixt right and oblique ascension. The sun's ascensional difference turned into time, is just so much as he rises before or after six o'clock.
The celestial signs and constellations on the surface of the celestial globe, are represented by a variety of human and other figures, to which the stars that are either in or near them, are referred.
The several systems of stars, which are applied to
those images, are called constellations. Twelve of these are represented on the ecliptic circle, and extend both northward and southward from it. So
many of those stars as fall within the limits of 8 degrees on both sides the ecliptic circle, together with such parts of their images as are contained within the aforesaid bounds, constitute a kind of broad hoop, belt, or girdle, which is called the zodiac.
The names and the respective characters of the twelve signs of the ecliptic, may be learned by inspection on the surface of the broad paper circle, and the constellations from the globe itself,
The zodiac is represented by eight circles parallel to the ecliptic, on each side thereof; these circles are
one degree distant from each other, so that the whole ; breadth of the zodiac is sixteen degrees.
Amongst those parallels, the latitude of the planets is reckoned; and in their apparent motion they never exceed the limits of the zodiac.
On each side of the zodiac, as we observed, other constellations are distinguished ; those on the north side are called northern, and those on the south side of it, southern constellations.
OF THE PRECESSION OF THE EQUINOXES.
All those stars which compose these constellations, are supposed to increase their longitude continually ; upon which supposition, the whole starry firmament has a slow motion from west to east; insomuch that the first star in the constellation of Aries, which
peared in the vernal intersection of the equator and ecliptic in the time of Meton, the Athenian, upwards of 1900 years ago, is now removed about 30 degrees from it.
This change of the stars in longitude, which has now become sufficiently apparent, is owing to a small retrograde motion of the equinoctial points, of about 50 seconds in a year, which is occasioned by the attraction of the sun and moon upon the protuberant matter about the equator. The same cause also oc( casions a small deviation in the parallelism of the
earth's axis, by which it is continually directed towards different points in the heavens, and makes a complete revolution round the ecliptic, in about 25,920 years. The former of these motions is called the precession of the equinoxes, the latter the nutation of the earth's axis. In consequence of this shifting of the equinoctial points, an alteration has taken place in the signs of the ecliptic; those stars, which, in the infancy of astronomy were in Aries, being now got into Taurus, those of Taurus into Gemini, &c. so that the stars which rose and set at any particular season of the year, in the times of Hesiod, Eudoxes, and Virgil, will not at present answer the description given of them by those writers.
PROBLEM I. To represent the motion of the equinoctial points backwards, or in antecedentia, upon the celestial globe.
Elevate the north pole, so that its axis may be
perpendicular to the plane of the broad paper circle, and the equator will then be in the same plane; let these represent the ecliptic, and then the poles of the globe will also represent those of the ecliptic; the ecliptic line upon the globe will, at the same time represent the equator, inclined in an angle of 23; degrees, to the broad paper circle now called the ecliptic, and cutting it in two points, which are called the equinoctial intersections.
Now if you turn the globe slowly round upon its axis, from east to west, while it is in this position, these points of intersection will move round the same way; and the inclination of the circle, which in shewing this motion represents the equinoctial, will not be altered by such a revolution of the intersecting or equinoctial points. This motion is called the precession of equinoxes, because it carries the equinoctial points backwards amongst the fixed stars.
The poles of the world seem to describe a circle from east to west, round the poles of the ecliptic, arising from the precession of the equinox. It is a very slow motion, for the equinoctial points take up 72 years to move one degree, and therefore they are 25,920 years in describing 360 degrees, or completing a revolution.
This motion of the poles is easily represented by the above described position of the globe, in which, if the reader remembers, the broad paper circle represents the ecliptic, and the axis of the globe being perpendicular thereto, represents the axis of the ecliptic; and the two points where the circular lines
meet will represent the poles of the world, whence, as the globe is slowly turned from east to west, these points will revolve the same way about the poles of ' the globe, which are here supposed to represent the poles of the ecliptic. The axis of the world may revolve as above, although its situation, with respect to the ecliptic, be not altered; for the points here supposed to represent the poles of the world, will always keep the same distance from the broad paper circle, which represents the ecliptic in this situation of the globe. *
From the different degrees of brightness in the stars, some appear to be greater than others, or nearer to us: on our celestial globe they are distinguished into seven different magnitudes.
* A globe is sometimes mounted with an apparatus peculiarly for this purpose, and is the best adapted to convey a sufficient idea of this curious phenomenon, and which I have already more clearly explained in page 174.