THEORY, NOTIONS, PERTURBATIONS, INEQUALITY, ke. The term pLasiet was first used as one of distinction between the stars which preserve their places, or seem to do so, and those which have a sea sible motion, and, as Is now known, about the sun. The etymology would oblige us to Include comets—many of the stars, which have small "melons of their own, or proper motions, they are ealled—ell those double revolve about each other—and all the satel lites which revolve about other planets. As discovery proceeds, it is likely that every body In the universe will be found to be planetary. The word however has changed its meaning, and is used to stand for a heavenly body which makes successive revolutions about our sun. It thus includes—the old planets, Mercury, Venus, Mars, Jupiter, Saturn (excluding the sun and the moon, the most easily discovered planets of all)—the planet of Copernicus and Galileo, on which we live —the new planets or ASTEROIDS, of which there are now (November, I6i0) sixty-two ; and the periodic comets of Halley, Eucke, Biala, Brursen, D'Arrest, and probably many others.
The plan of this work leaves us little to say under so general a term as Planet, and that little consists meetly of references, and explana tions of isolated points. The order of discoveries connected with the subject, so far as it is matter of history, must be looked fur under ASTRONOMY. It is impossible to separate the history of one part of astronomy from that of another : the fixed stars, on which all celestial measurements depend, must accompany the planets in every account of the latter ; the nuithematical and physical considerations which give rise to our power of predicting the motions and places of the planets, apply equally to those of the 'neon and comets.
By a Planetary Theory was once meant any hypothesis which serves to explain the motions of a planet, as actually perceived. Thus there was one theory of Mercury, another of Venue, &c., each (without connection with the rest) expounding the nature and 'magnitude of all the suppositions which must be superadded to that of the simple circular motion round the sun, in order to enable the theorist to predict the planet's place at any given future time. At present, by the theory of a planet is meant the deduction of its motion from the theory of gravitation. Given the places of all the bodies of the solar system at any one moment, together with their velocities and the directions of their motions, the assumptions of the theory of gravita tion are found sufficient to deduce tables which almost rival observa tion Reel' in the accuracy with which predictions can be made; for an instance, see Moos. When tables are constructed with such fundamental data only, they are called theoretical : but if some of those details which should have been deduced from the theory of gravitation have been deduced from observation, the tables are then partly empirical. For the treatment of the theory of gravita
tion as a question of physics, see Arrnacriox : for the deduc tion of the character of the main inequalities of the planetary motions, and most points connected with the detailed results of that theory, WO GRAVITATION : for the results which are particularly connected with the secular inequalities, and the reasons which we have to infer the permanence of the whole system, unless acted on by some new and external cause, see SOLAR SYSTEM. See also the general considerations under the word TIIEOR tr.
The Planetary Theory, it must be remembered, in the sense in which those words are now understood, explains only the motions of the planets round the sun. A spectator situated on the last-named body would find It sufficient to predict the place among the stars, of every planet at every time : or a terrestrial astronomer might assign by it the places of the pLaneta as seen from the sun, which are called hefiseemarie places. But our own circumstances, as being ourselves revolving about the sun, came the apparent planetary motions to vary most materially from the real ones, and the geocentric places (or places as seen from the earth) to differ literally tote clf19 from the helio centric places. Thus, as seen from the sun, a planet moves from west to east, always: as seen from the earth, It sometimes moves from east to west. As to this point, the circumstances of our day arc, in regard to the astronomical education of the world at large, a complete reversal of those which preceded the time of Newton. Formerly, the apparent motions were well known to those who knew anything; the real motions were matter of dispute : now, every well educated boy of fourteen has a very good notion of the real motions, while few, except astronomers, could distinctly explain the apparent ones, and maps of them are quite out of date. To explain these motions here would require us to Introduce the contents of another article ; the whole of what is necessary on this point will be found in TltOcIIOIDAI. CURVES, the preliminary considerations being found under Monroe.
The places of the principal planets are usually given In the almanacs, at Intervals of several days, in a manner which is sufficient to find them in the heavens. In the 'Nautical Almanac' is to be found a Greenwich meridian ephemeris of every planet for every day in the year, in which the places are predicted to the smallest quantities, so that the reduced oletervation of any one day affords an immediate comparison of the theoretical tables with the fact.