ORRERY. There are four planetary machines to which distinct name have been given, corresponding to the phenomena they are intended to exhibit, and which, when combined so as to derive their motions from a common origin, constitute what is now generally understood by an orrery. These machines are the Planetarium, the Tellurian, the Lunarian, and the Satellite machine. The planetarium is a mechanical representation of the orbital motions of the planets about the sun, either in circles or ellipses, and with angular velocities either uniform or variable, according to the quality of the instrument. The tellurian and luuarian, when connected in their improved form, exhibit the motion of the moon about the earth and that of the earth about the sun, and the principal phenomena which accompany the changes in their relative positions, such as the succession of day and night and the variable length of both according to the season of the year, the eclipses of the sun and moon, the variations in the moon's latitude, velocity, and distance from the earth, the progressive motion of her apogee, and the retrogradation of her nodes, &c. The satellite machine is chiefly intended to represent the motions of Jupiter's satellites about their primary, combined with the motions of the Litter about the sun.
The origin of the term "orrery " is thus given by Mr. Desaguliers, in his ' Course of Experimental Philosophy,' 4to, London, 1734, L, p. 431. After stating his belief that Mr. George Graham, about the year 1700, first invented a movement for exhibiting the motion of the earth about the sun at the same time that the moon revolved round the earth, he remarks," This machine being in the hands of an instrument maker to be sent with some of his own instruments to Prince Eugene, he copied it, and made the first for the late Earl of Orrery, and then several others with additions of his own. Sir Richard Steele, who knew nothing of Mr. Graham's machine, in one of his lucubrations, thinking to do justice to the first encourager, as well as to the inventor, of such a curious instrument, called it an orrery, and gave 31r. J. Rowley the praise due to Mr. Graham." In the; latter part of the 17th century Huygherur and Roemer em ployed themselves in the construction of planetary machines in con formity with the Copernican doctrine. The one invented by Huyghens, who first introduced a systematic method of calculating with precisioh the necessary wheel-work for these machines, received from its author the name of the Automaton. It derived its motion from a spring
regulated by a balance, the pendulum not having been then introduced as a regulating agent, and served for many years as a pattern in the construction of orreries, as did the instrument of Roemer, called by him a Planetarium, in the construction of machines intended chiefly to exhibit the orbital motions of the planets. Roemer had also invented a satellite machine prior to the year 1679, the original or a copy of which was presented by him in that year to the English astronomer Flamsteoch Both his machines are described in his ' Barris Astronomim,' printed in 1735. We may also mention the Planetarium of the Royal Institution of London, constructed about the year 1801, on a plan suggested by Dr. Young and the Rev. W. Pearson, and described in the paper ' On Planetary Machines,' in the Edinburgh Enyclopredia; by the latter gentleman.
The chief part of every orrery is the mechanism composing the planetarium, by means of which the paths of the planets about the sun and their relative periodic times are exhibited with more or less approach to truth : and this mechanism, with the method of com puting it, being once understood, it will be easy to extend the same principles to the more complex cases in which the satellites revolve about their primaries at the same time that the latter rotate upon their axes and revolve about the sun, as well as to those in which the parallelism of the planets' axes and the changes in the positions of their orbits, &c. are sought to be represented. For this reason we shall confine ourselves to the method of computing the wheel-work, which will give the relative periods with any required degree of accuracy, and to the explanation of a very ingenious contrivance by means of which a true elliptic orbit may be produced. Before, however, proceeding to this, we would caution the purchasers of these expensive toys (those exhibited in the shops of mathematical instrument-makers vary in price from sixteen to forty guineas) against a defect which was at one time not uncommon and may still exist, and which, while it rendered them worse than useless, showed how little knowledge their contrivers could have possessed of the science they are intended to illustrate. We allude to the substitution of the synodic for the sidereal periods, whereby each planetary ball was made to revolve about the sun in the time which ought to have elapsed between two consecutive conjunctions of such planet with the earth.