In this manner, Machines may be made to shew the true Motion of the Moon about the Earth, or of any Planet about the Sun; by making the elliptical Plates of the same Excentricities, in proportion to the Radius, as the Orbits of the Planets are whose Motions they represent: and so, their different Equations in different parts of their Orbits may be made plain to sight; and clearer Ideas of these Motions and Equations acquired in half an hour, than could be gained from reading half a day about such Motions and Equations.
The improved Celestial Globe.
[PLATE III]. Fig. III.
438. The Improved Celestial Globe. On the North Pole of the Axis, above the Hour Circle, is fixed an Arch MKH of 231⁄2 Degrees; and at the end H is fixed an upright pin HG, which stands directly over the North Pole of the Ecliptic, and perpendicular to that part of the surface of the Globe. On this pin are two moveable Collets at D and H, to which are fixed the quadrantal Wires N and O, having two little Balls on their ends for the Sun and Moon, as in the Figure. The Collet D is fixed to the circular Plate F whereon the 291⁄2 days of the Moon’s age are engraven, beginning just under the Sun’s Wire N; and as this Wire is moved round the Globe, the Plate F turns round with it. These Wires are easily turned if the Screw G be slackened; and when they are set to their proper places, the Screw serves to fix them there so, as in turning the Ball of the Globe, the Wires with the Sun and Moon go round with it; and these two little Balls rise and set at the same times, and on the same points of the Horizon, for the day to which they are rectified, as the Sun and Moon do in the Heavens.
Because the Moon keeps not her course in the Ecliptic (as the Sun appears to do) but has a Declination of 51⁄3 Degrees on each side from it in every Lunation § [317], her Ball may be screwed as many Degrees to either side of the Ecliptic as her Latitude or Declination from the Ecliptic amounts to at any given time; and for this purpose S is a small piece of pasteboard, of which the curved edge S is to be set upon the Globe at right Angles to the Ecliptic, and the dark line over S to stand upright upon it. From this line, on the convex edge, are drawn the 51⁄3 Degrees of the Moon’s Latitude on both sides of the Ecliptic; and when this piece is set upright on the Globe, it’s graduated edge reaches to the Moon on the Wire O, by which means she is easily adjusted to her Latitude found by an Ephemeris.
The Horizon is supported by two semicircular Arches, because Pillars would stop the progress of the Balls when they go below the Horizon in an oblique sphere.
To rectify it.
To rectify the Globe. Elevate the Pole to the Latitude of the Place; then bring the Sun’s place in the Ecliptic for the given day to the brasen Meridian, and set the Hour Index to XII at noon, that is, to the upper XII on the Hour Circle; keeping the Globe in that situation, slacken the Screw G, and set the Sun directly over his place on the Meridian; which done, set the Moon’s Wire under the number that expresses her age for that day on the Plate F, and she will then stand over her place in the Ecliptic, and shew what Constellation she is in. Lastly, fasten the Screw G, and laying the curved edge of the pasteboard S over the Ecliptic below the Moon, adjust the Moon to her Latitude over the graduated edge of the pasteboard; and the Globe will be rectified.
It’s use.
Having thus rectified the Globe, turn it round, and observe on what points of the Horizon the Sun and Moon Balls rise and set, for these agree with the points of the Compass on which the Sun and Moon rise and set in the Heavens on the given day; and the Hour Index shews the times of their rising and setting; and likewise the time of the Moon’s passing over the Meridian.
This simple Apparatus shews all the varieties that can happen in the rising and setting of the Sun and Moon; and makes the forementioned Phenomena of the Harvest Moon ([Chap. xvi.]) plain to the Eye. It is also very useful in reading Lectures on the Globes, because a large company can see this Sun and Moon going round, rising above and setting below the Horizon at different times, according to the seasons of the year; and making their appulses to different fixed Stars. But, in the usual way, where there is only the places of the Sun and Moon in the Ecliptic to keep the Eye upon, they are easily lost sight of, unless covered with Patches.