This machine may likewise be used to exhibit the falsity of the Ptolemaic system, which places the Earth in the centre, and supposes the sun and all the planets to revolve around it. For this purpose, the ball representing the Sun is removed, and placed on the wire or pillar which supports the Earth, and the ball representing the Earth is placed in the centre. It will then be observed, that the planets Mercury and Venus, being both within the orbit of the sun, cannot at any time be seen to go behind it, whereas, in the heavens we as often see them go behind as before the sun. Again, it shows that as the planets move in circular orbits about the central earth, they ought at all times to appear of the same magnitude; while, on the contrary, we observe their apparent magnitudes in the heavens to be very variable; Mars, for example, appearing sometimes nearly as large as Jupiter, and at other times only like a small fixed star. Again, it is here shown that the planets may be seen at all distances from the sun; for example, when the sun is setting, Mercury and Venus, according to this arrangement, might be seen, not only in the south but even in the eastern quarter of the heavens—a phenomenon which was never yet observed in any age; Mercury never appearing beyond 27° of the Sun, nor Venus beyond 48°. In short, according to the system thus represented, it is seen, that the motions of the planets should all be regular, and uniformly the same in every part of their orbits, and that they should all move the same way, namely from west to east; whereas, in the heavens, they are seen to move with variable velocities, sometimes appearing stationary, and sometimes moving from east to west, and from west to east. All which circumstances plainly prove that the Ptolemaic cannot be the true system of the universe.
A Planetarium, such as that now described, might be constructed with brass wheel-work, for about 5 guineas. The brass wheel-work of one which I long since constructed cost about 3 guineas, and the other parts of the apparatus about 2 guineas more. The following are the prices of some instruments of this kind as made by Messrs. Jones, 30, Lower Holborn, London. ‘An Orrery, showing the motions of the Earth, Moon, and inferior planets, Mercury and Venus, by wheel-work, the board on which the instrument moves being 13 inches diameter, £4: 14s. 6d.’ ‘A Planetarium showing the motions of all the primary planets by wheel-work with 1½ inch or 3 inch papered globes,—according to the wheel-work and the neatness of the stands, from £7: 17s. 6d. to £10: 10s.’ ‘Ditto, with wheel-work to show the parallelism of the Earth’s axis, the motions of the Moon, her phases, &c., £18: 18s.’ ‘Ditto, with wheel-work, to show the earth’s diurnal motion, on a brass stand in mahogany case, £22: 1s.’ ‘A small Tellurian, showing the motion of the Earth and Moon, &c., £1: 8s.’
HENDERSON’S PLANETARIUM.
The following is a description of the most complete and accurate planetarium I have yet seen. The calculations occupied more than eight months. For this article I am indebted to my learned and ingenious friend Dr. Henderson, F.R.A.S., who is known to many of my readers by his excellent astronomical writings.
figure 96.
Section of the wheel-work of a Planetarium for shewing with the utmost degree of accuracy the mean tropical revolutions of the planets round the sun, calculated by E. Henderson, LL.D. &c.
In the above section the dark horizontal lines represent the wheel-work of the Planetarium, and the annexed numerals, the numbers of teeth in the given wheel. The machine has three axes or arbors, indicated by the letters A, B, C.—Axis ‘C,’ the ‘Yearly axis,’ is assumed to make one revolution in 365.242,236 days, or, in 365 days 5h 48m 49.19s and is furnished with wheels 17, 44, 54, 36, 140, 96, 127, 86, which wheels are all firmly riveted to said axis, and consequently they turn round with it in the same time. Axle ‘B’ is a fixture; it consists of a steel rod, on which a system of pairs of wheels revolve; thus wheels 40 and 77 are made fast together by being riveted on the same collet represented by the thick dark space between them, as also of the rest: the several wheels on this axis may be written down thus; 40/77, 49/129, 20/94, 79/81, 30, 27/50, 41/65, 59/65, 96, 77/47, 67/42. On axis A a system of wheels, furnished with tubes revolve, and these tubes carry horizontal arms, supporting perpendicular stems with the planets. The wheels on this axis are 173, 117/190, 111, 119, 122/130, 123/127, 83, 239, 96, 128, 72. From the following short description the nature of their several actions will, it is presumed, be readily understood—viz.,
MERCURY’S PERIOD.