But if Jupiter be in antecedence of the Sun (i. e. rises before him) having brought the place where the Sun is vertical, to the zenith, and put the hour index to 12 at noon, draw a line on the Western side of the horizon; then elevate the globe according to the declination of Jupiter, and turn it about Eastward, until the index points to so many hours distant from noon, as is the difference of right ascension of the Sun and Jupiter. The globe being in this position, draw a line along the Western side of the horizon; then the space contained between this line, and the other last drawn, will comprehend all those places upon the Earth where the Eclipse is visible, between the rising of the Sun, and that of Jupiter.
The DESCRIPTION of the Great Orrery, lately made by Mr. Thomas Wright, Mathematical Instrument-Maker to his late Majesty, and now by Benjamin Cole, his Successor.
The Orrery is an Astronomical Machine, made to represent the motions of the Planets. These machines are made of various sizes, some having more Planets than others; but I shall here confine myself to the description of that above-mentioned.
In the Introduction we gave a short account of the Order, Periods, Distances, and Magnitudes of the Primary Planets; and of the Distances and Periodical Resolutions of the Secondary Planets round their respective Primaries. We shall here explain their Stations, Regradations, Eclipses, Phases, &c. but first let us take a general view of the Orrery.
The Description of the Orrery.
Vide [Frontispiece].
The frame which contains the wheel-work, &c. that regulates the whole Machine, is made of fine ebony, and is near four feet in diameter; the outside thereof is adorned with twelve pilasters, curiously wrought and gilt: Between these pilasters the twelve Signs of the Zodiac are neatly painted, with gilded frames. Above the frame is a broad ring, supported with twelve pillars: This ring represents the Plane of the Ecliptic, upon which there are two scales of degrees, and between those the names and characters of the twelve Signs. Near the outside is a scale of months and days, exactly corresponding to the Sun’s place at noon, each day throughout the year.
Above the ecliptic stands some of the principal circles of the sphere, according to their respective situations in the heavens, viz. N° 10, are the two Colures, divided into degrees, and half degrees; N° 11, is one half of the Equinoctial Circle, making an angle with the ecliptic of 23½ degrees. The Tropic of Cancer, and the Arctic Circle, are each fixed parallel, and at their proper distance from the equinoctial. On the Northern half of the ecliptic is a brass semicircle, moveable upon two points fixed in ♈ and ♎: This semicircle serves as a moveable horizon, to be put to any degree of latitude upon the North part of the meridian. The whole machine is also so contrived, as to be set to any latitude, without in the least affecting any of the inside motions: For this purpose there are two strong hinges (N° 13,) fixed to the bottom frame, upon which the instrument moves, and a strong brass arch, having holes at every degree, thro’ which a strong pin is to be put, according to the elevation. This arch and the two hinges, support the whole machine, when it is lifted up according to any latitude; and the arch at other times lies conveniently under the bottom frame.