, an astronomical machine, contrived to represent the motions, orbits, &c, of the planets, as they really are in nature, or according to the Copernican system. The larger sort of them are called Orreries. See Orrery.

A very remarkable machine of this sort was invented by Huygens, and described in his Opusc. Posth. tom. 2. p. 157, edit. Amst. 1728. And it is still preserved among the curiosities of the university at Leyden.

In this Planetarium, the five primary planets perform their revolutions about the sun, and the moon performs her revolution about the earth, in the same time that they are really performed in the heavens. Also the orbits of the moon and planets are represented with their true proportions, eccentricity, position, 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 complete Planetarium of this sort; called “a Planetarium or astronomical machine, which exhibits the most remarkable phenomena, motions, and revolutions of the universe. Invented, and partly executed, by the celebrated M. Phil. Matthew Hahn, member of the academy of sciences at Erfurt. But finished and completed by Mr. Albert de Mylius.” This is a most stupendous and elaborate machine; consisting of the solar 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 satellites, as of the earth, Jupiter, &c; the whole kept in continual motion by a chronometer, or grand eight-day-clock; by which all these systems are made perpetually to perform all their motions exactly as in nature, exhibiting at all times the true and real motions, positions, aspects, 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 machine; and it was purchased and sent as one of the presents to the emperor of China, in the embassy of Lord Macartney, in the year 1793.

But the Planetariums or orreries now most commonly used, do not represent the true times of the celestial motions, but only their proportions; and are not kept in continual motion by a clock, but are only turned round occasionally with the hand, to help to give young beginners an idea only of the planetary system; as also, if constructed with sufficient accuracy, to resolve problems, in a coarse way, relating to the motions of the planets, and of the earth and moon, &c.

Dr. Desaguliers (Exp. Philos. vol. 1, p. 430.) describes a Planetarium of his own contrivance, which is one of the best of the common sort. The machine is contrived to be rectified or set to any latitude; and then by turning the handle of the Planetarium, all the planets perform their revolutions round the sun in proportion to their periodical times, and they carry i<*>dices which shew the longitudes of the planets, by pointing to the divisions graduated on circles for that purpose.

The Planetarium represented in fig. 1, plate xxii. is an instrument contrived by Mr. Wm. Jones, of Holborn, London, mathematical instrument maker, who has paid considerable attention to such machines, to bring them to a great degree of simplicity and perfection. It represents in a general manner, by various parts of its machinery, all the motions and phenomena of the planetary system. This machine consists of, the Sun in the centre, with the Planets in the order of their distance from him, viz. Mercury, Venus, the Earth and Moon, Mars, Jupiter with his moons, and Saturn with his ring and moons; and to it is also occasionally applied an extra long arm for the Georgian Planet and his two moons. To the earth and moon is applied a frame CD, containing only four wheels and two pinions, which serve to preserve the earth's axis in its due parallelism in its motion round the sun, and to give the moon at the same time her due revolution about the earth. These wheels are connected with the wheelwork in the round box below, and the whole is set in motion by the winch H. The arm M that carries round the moon, points out on the plate C her age and phases for any situation in her orbit, upon which they are engraved. In like manner the arm points out her place in the ecliptic B, in signs and degrees, called her geocentric place, that is, as seen from the earth. The moon's orbit is represented by the flat rim A; the two joints of it, upon which it turns, denoting her nodes; and the orbit being made to incline to any required angle. The terrella, or little earth, of this machine, is usually made of a three inch globe papered, &c, for the purpose; and by means of the terminating wire that goes over it, points out the changes of the seasons, and the different lengths of days and nights more conspicuously. By this machine are seen at once all the Planets in motion about the Sun, with the same respective velocities and periods of revolution which they have in the heavens; the wheelwork being calculated to a minute of time, from the latest discoveries. See Mr. Jones's Description of his new portable Orrery.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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