PLANETARIUM, an astronomical ma ohine, contrived to represent the mo tions, orbits, &c. of the planets, as they really are in nature, or according to the Copernican system. A very remarkable machine of this sort was invented by Iluygens, which is still preserved amolg the curiosities of the University at Ley den. In this planetarium, the five pri mary planets perform their revolutions about the Sun, and the moon performs her revolution about the Earth, in the same time that they are really perform vd in the heavens. Also the orbits of the Moon and planets are represented with their true proportions, excentricity, po sition, and declination from the ecliptic, or orbit of the Earth. So that, by this machine, the situation of the planets, with the conjunctions, oppositions, &c. may be known, not only for the present time, but for any other time, either past or yet to come, as in a perpetual ephemeris. There was exhibited in London, 'viz, in the year 1791, a still much more com plete planetarium of this sort, called "a planetarium, or astronomical machine, which exhibits the most remarkable phe nomena, motions, and revolutions of the universe ; invented, and partly executed, by the celebrated NC Hahn, member of the academy of sciences at Erfurt ; but finished and completed by M. A. de My hos." This is a most stupendous and elaborate machine, consisting of the so lar system in general, with all the orbits and planets in their due proportions and positions ; as also the several particular planetary systems of such as have satel lites, as of the Earth, Jupiter, &c.; the whole kept, in continual motion by a chroncimeter, or grand eight-day clock ; by which all these systems are made per petually to perform all their motions ex actly as in nature, exhibiting at ali times the true and real motions, positions, as pects, phenomena, &c. of all the celestial bodies, even to the very diurnal rotation of the planets, and the unequal motions in their elliptic orbits. A description was published of this most superb ma. chine, and it was purchased and sent as one of the presents to the Emperor of China, in the embassy of Lord Macart nev.
We shall now give a description of one • cf these machines in common use.
k'ig, 1, Plate Planetarium, is an eleva tion of the mechanism of a planetarium ; and fig. 2, a plan of the same. A, (fig. 1,) is a ball of brass representing the sun, supported by a wire screwed to a bridge, b, fixed beneath the board, B B, which supports the whole instrument ; a is the section of an endless screw, which has a small handle on the end of its spin dle to turn it by • it gives motion to a worm-wheel, 60, by; teeth, the arbor of this wheel is a tibe, and goes over the central wire sustaining the Sun, to its upper end is fixed the frame, E E, coy taming the vvimeLwork, and carrying the Earth, ED , and Moon, The plan (fig. 2) is this frame of wheels, the upper plate of the frame being removed, d is the first wheel of sixty-four teeth, fixed fast to the central wire of the sun, and having 'no motion, it works with another of sixty four, on the same arbor, h h, with several others to be hereafter described ; it turns another, j; of sixty-four, on whose arbor, g, the Earth is fixed ; as d is fixed, and the next wheel, with its frame, E E, rolls round it, and is thereby turned upon its own axis ; the wheel, f, which is on the other side, will have no motion on its axis, and the axis of the Earth, fixed to it, will remain parallel to itself, while it des cribes an orbit round the Sun, by the mo tion of the frame, E E. The next wheel,
69, upon the arbor, h, turns a pinion, 14, of fourteen teeth, (not seen in the plan) by the intervention of a wheel, 64, which does not alter its velocity ; the arbor of the pinion is a tube, and fitted upon the central wire ; at its upper end it supports the planet, Mercury, V . The third wheel from the bottom, on the arbor, h, has for ty teeth, and by- the wheel, 56, communi cates motion to a small wheel of twenty four, which has the planet, Venus, , fix ed to its tubttlar arbor. The upper wheel of the arbor, h, has seventy-four teeth, and turns a pinion of six, on a tube, concen tric with g, and with it the moon. There is a small wheel of fourteen teeth between the wheel and pinion, but it does not alter the velocity : k, (fig. 1), is a thin brass ring seen edgeways, which has a wire die metrically across it, on which it turns a san axis, to set it at any given obliquity to the axis, g-, supporting the Eart'.', the wire is fixed into a short tube, which turns stiffly in a hole made in the upper plate of the frame, E E, and thus the circle can be 'turned round, while its plane continues oblique to the axis, g, this ring represents the plane of the Moon's orbit, and is en graved with the different phases of the Moon. The Moon is not fixed to the arm which turns it, but its stem slides up and down in a short tube fixed to the arm, and rests upon the ring, so as to de scribe a parallel plane to it. On the end of the frame, E E, a pillar is erected, to support a small semi-circular piece of brass, m, inclosing the Earth, and show ing the line of light and darkness. Nis a tube screwed last to the board, B B, by a flanch at the lower end ; it fits the out side of the tube of the wheel, 60, beneath the board, and thus steadies the whole frame as it turns round ; upon this tube long arms are fitted, carrying Mars, Ju piter, and the other superior but as there is no wheel-work to turn these, they are omitted in the plate. This instrument is defective in not having the diurnal motion of the Earth upon its axis shown, and the rotation of the Moon's there have been instruments de, which show all these motions, and those of the superior planets with their satellites ; but they are so complicated, that it would far exceed the limits of our plates to describe them.