Universe itself; with the sun, moon, and stars to move round it, and to attend upon it. But, because, in the view of later astronomy, the Solar System is a system or assemblage of planets (the Earth being one) which compose the system, the sun being its centre; therefore this system is sometimes called the Planetary System, rather than the Solar System. CHAP. XXXV. ABOUT AN ORRERY OR PLANETARIUM. FROM the same reference to the planets, more immediately than to the sun, the philosophical machine of which I am going to show you a picture, (and of which the design is to exhibit the several bodies, and the movements of the bodies, belonging to the Solar System, or the Planetary System,) is sometimes called a Planetarium, as it is also sometimes called an Orrery. On the upper plate of this Orrery or Planetarium (which answers to the ecliptic) are placed, in two opposite but corresponding circles, the days of the month, and the signs of the ecliptic, with their respective characters. By this plate we may set the planetary balls so as to be in their respective places on the ecliptic for any day in the year. You perceive that through the centre of the plate a strong brass stem passes, on which is a brass ball to represent the sun; and that round this stem are different sockets, to carry the arms by which the several planets are supported. The planets themselves are represented by ivory balls, having the hemispheres which are towards the sun white, and the other black, to exhibit their respective phases. These are movable, and may be taken off or put on as occasion may require. About the primary planets are placed the secondary planets or moons; and the motion, requisite to produce the various phenomena of the appearance of the whole, is effected by turning the handle, which communicates with a train of wheel-work, concealed in the circular brass box under the upper plate. A small lamp, instead of the ball, is sometimes placed on the centre stem; and this has a pleasing and instructive effect, because it actually enlightens the part of each planet that is turned in its direction. |