File:EB1 Plate LXXXVII Fig. 1 Armillary Sphere.png

English: A diagram of an armillary sphere from Plate LXXXVII from the Encyclopaedia Britannica, 1st ed., pp. 675–6:

The Deſcription and Uſe of the Armillary Sphere.
[See Plate LXXXVII. Fig. 1.]

The exterior parts of this machine are, a compages of braſs rings, which repreſent the principal circles of the heaven, viz. 1. The equinoctial A A, which is divided into 360 degrees (beginning at its interſection with the ecliptic in Aries) for ſhewing the ſun's right aſcenſion in degrees; and alſo into 24 hours, for ſhewing his right aſcenſion in time. 2. The ecliptic B B, which is divided into 12 ſigns, and each ſign into 30 degrees, and alſo into the months and days of the year; in ſuch a manner, that the degree or point of the ecliptic in which the ſun is, on any given day, ſtands over that day in the circle of months. 3. The tropic of Cancer C C, touching the ecliptic at the beginning of Cancer in e, and the tropic of Capricorn D D, touching the ecliptic at the beginning of Capricorn in f; each 23½ degrees from the equinoctial circle. 4. The arctic circle E, and the antarctic circle F, each 23½ degrees from its reſpective pole at N and S. 5. The equinoctial colure G G, paſſing through the north and ſouth poles of the heaven at N and S, and through the equinoctial points Aries and Libra, in the ecliptic. 6. The ſolſtitial colure H H, paſſing through the poles of the heaven, and through the ſolſtitial points Cancer and Capricorn, in the ecliptic. Each quarter of the former of theſe colures is divided into 90 degrees, from the equinoctial to the poles of the world, for ſhewing the declination of the ſun, moon, and ſtars; and each quarter of the latter, from the ecliptic at e and f, to its poles b and d, for ſhewing the latitude of the ſtars.

In the north pole of the ecliptic is a nut b, to which is fixed one end of a quadrantal wire, and to the other end a ſmall ſun Y, which is carried round the ecliptic B B, by turning the nut; and in the ſouth pole of the ecliptic is a pin d, on which is another quadrantal wire, with a ſmall moon Z upon it, which may be moved round by hand: but there is a particular contrivance for cauſing the moon to move in an orbit which croſſes the ecliptic at an angle of 5⅓ degrees, in two oppoſite points called the moon's nodes; and alſo for ſhifting theſe points backward in the ecliptic, as the moon's nodes ſhift in the heaven.

Within theſe circular rings is a ſmall terreſtrial globe I, fixt on an axis K K, which extends from the north and ſouth poles of the globe at n and s, to thoſe of the celeſtial ſphere at N and S. On this axis is fixt the flat celeſtial meridian L L, which may be ſet directly over the meridian of any place on the globe, and then turned round with the globe, ſo as to keep over the ſame meridian upon it. This flat meridian is graduated the ſame way as the braſs meridian of a common globe, and its uſe is much the ſame. To this globe is fitted the moveable horizon M M, ſo as to turn upon two ſtrong wires proceeding from its eaſt and weſt points to the globe, and entering the globe at the oppoſite points of its equator, which is a moveable braſs ring let into the globe in a groove all around its equator. The globe may be turned by hand within this ring, ſo as to place any given meridian upon it, directly under the celeſtial meridian L L. The horizon is divided into 360 degrees all around its outermoft edge, within which are the points of the compaſs, for ſhewing the amplitude of the ſun and moon, both in degrees and points. The celeſtial meridian L L paſſes through two notches in the north and ſouth points of the horizon, as in a common globe: but here, if the globe be turned round, the horizon and meridian turn with it. At the ſouth pole of the ſphere is a circle of 24 hours, fixt to the rings, and on the axis is an index which goes round that circle, if the globe be turned round its its axis.

The whole fabric is ſupported on a pedeſtal N, and may be elevated or depreſſed upon the joint O, to any number of degrees from 0 to 90, by means of the arc P, which is fixed in the ſtrong braſs arm Q, and ſlides in the upright piece R, in which is a ſcrew at r, to fix it at any proper elevation.

In the box T are two wheels (as in Dr Long's ſphere) and two pinions, whoſe axes come out at V and U; either of which may be tumed by the ſmall winch W. When the winch is put upon the axis V, and turn backward, the terreſtrial globe, with its horizon and celeſtial meridian, keep at reſt; and the whole ſphere of circles turns round from eaſt, by ſouth, to weſt, carrying the ſun Y, and moon Z, round the ſame way, and cauſing them to riſe above and ſet below the horizon. But when the winch is put upon the axis U, and turned forward, the ſphere with the ſun and moon keep at reſt; and the earth, with its horizon and meridian, turn round from weſt, by ſouth, to eaſt; and bring the ſame points of the horizon to the ſun and moon, to which theſe bodies came when the earth kept at reft, and they were carried round it; ſhewing that they riſe and ſet in the ſame points of the horizon, and at the ſame times in the hour circle, whether the motion be in the earth or in the heaven. If the earthly globe be turned, the hour-index goes round its hour-circle; but if the ſphere be turned, the hour-circle goes round below the index.

And ſo, by this conſtruction, the machine is equally fitted to ſhew either the real motion of the earth, or the apparent motion of the heaven.

To rectify the ſphere for uſe, firſt ſlacken the ſcrew r in the upright ſtem R, and taking hold of the arm Q, move it up or down until the given degree of latitude for any place be at the ſide of the ſtem R; and then the axis of the ſphere will be properly elevated, ſo as to ſtand parallel to the axis of the world, if the machine be ſet north and ſourth by a ſmall compaſs: this done, count the latitude from the north pole, upon the celeſtial meridian L L, down towards the north notch of the horizon, and ſet the horizon to that latitude; then, turn the nut b until the ſun Y comes to the given day of the year in the ecliptic, and the ſun will be at its proper place for that day: find the place of the moon's aſcending node, and alſo the place of the moon, by an Ephemeria, and ſet them right accordingly: laſtly, turn the winch W, until either the ſun comes to the meridian L L, or until the meridian comes to the ſun (according as you want the ſphere or earth to move) and ſet the hour-index to the XII, marked noon, and the whole machine will be rectified.———Then turn the winch, and obſerve when the ſun or moon riſe and ſet in the horizon, and the hour-index will ſhew the times thereof for the given day.