PLANETARIUM. An astronomical machine of the same nature as the orrery, designed to exhibit the orbits, motions, and phenomena of the planets in the solar system. In a machine of this kind, which was constructed by Huggens, and is presetved in the University of Leyden, the revolution of the primary planets about the sun, and that of the moon round the earth, are performed in the exact time that they are actually performed in nature. The orbits of the moon and planets are here represented with their true proportions, eccentricities, positions, and declinations from the ecliptic ; and by this machine, as by a per petual ephemeris, the situations, conjunctions, oppositions, &c. of the planets for any time may be accurately determined. • Dr. Desaguliers constructed a very complete planetarium, which he has described in his Course of Experi mental Philosophy, published in 1734 ; but the most stupendous, superb, and elaborate planetarium ever constructed, was that which was publicly shown in London in 1791, and afterwards purchased by government to be sent out with Lord Macartney, in 1793, as a present to the emperor of " the Celestial empire."
It exhibits all the bodies, both primary and secondary, of the solar system, with their orbits in their due proportions and positions, and all performing their annual and diurnal motions exactly as in nature, exhibiting, at all times, the true and real motions, position; aspects, phenomena, and even the inequalities of their motions in elliptical orbits. As engravings of planetariums have, how ever, been exhibited in all the Cyclopedia' and works of mechanical science, and as every mechanic well understands that their motions are regulated by a numerous train of wheels, which it would be extremely tedious to detail, pre cisely in the same manner as in horological machines (the hands or indexes of which, instead of bearing planets at their extremities point out the time), we shall content ourselves with referring the reader to the article ASTRONOMY in the Oxford Encyclopedia, for a very full and interesting account, illustrated by engravings, of several admirable machines of this kind.