SYSTEM (Astronomy). This term is applied to every theory of the disposition and internal arrangements of the solar system, or of the material creation generally. Thus we have the system of Ptolemy, of Copernicus, &c. Perhaps a short description of the distinctive characters of the different systems may be useful in a work of reference.
l'Idemaie.—The earth is an absolutely fixed centre, and the planets revolve in circles about centres which themselves revolve round the earth.
Copernican.—The sun is a centre, round which the planets revolve. Some of the machinery of the Ptolemaic system is retained.
Tychonia—The sun is a centre of motion to all the planets, which revolve round it, while the sun and, planetary orbits arc carried together round the earth as a fixed centre.
Senti-Tychenic.—The sun is a centre of motion to Mercury and Venus, as in the Tychonie, and the motions of the other planets are as in the Ptolemaic system.
Neletonian.—There is no fixed centre, the sun only approximating to that character from its greater magnitude. The orbits of the planets are approximately represented by ellipses, exactly by ellipses of which the elements vary.
The Newtonian system is frequently called Copernican, from its rejecting what Copernicus rejected; but it is far from receiving all that Copernicus received. The introduction of the ellipse is due to Kepler. We have not included the system of Des Cartes [Yount:Es), because it has reference to physical causes, and contains no peculiarity of arrangement. [PTOLEMAIC SYSTEM; BRAME (TYCHO), COPERNICUS, and NEWTON, in BIOG. Div.; PRINCIPIA ; GRAVITATION; SOLAR SYS TEM, &C.] The term system is also applied to the subdivisions of the solar system : thus we have the terrestrial, Jovial, Saturnian, Uraniau systems.