Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/602

 493 ASTRO true motion of the moon about the earth, or of any planet about the fun, by making the elliptical plates of the fame ex<*fntricities, in proportion to the radius, as the orbits of the planets are, vvhofe motions they reprefent; and fo their ‘different equations in different parts of their orbits may be made plain to fight, and clearer ideas of thefe motions and equations acquired in balf an hour, than could be gained from reading half a day about fuch motions and equations. The Improved Celestial Globe, (Plate XLIV. fig. 2.). On the north pole of the axis, above the hourcircle, is fixed an arch MKH of 234 degrees; and at the end H is fixed an upright pin HG, which Itands diredlly over the north pole of the ecliptic, and perpendicular to that part of the furface of the globe. On this pin are two moveable collets at D and H, to which are fixed the quadrantile wires N and 0, having two little balls on their ends for the fun and moon, as in the figure. The collet D is fixed to the circular plate F, whereon the 294 days of the moon’s age are engraven, beginning -juft under the fun’s wire TV; and as this wire is moved round the globe, the plate F turns round wiih it. Thefe •wires are eafily turned, if the ferew G be flackened; and w hen they are fet to their proper places, the ferew ferves to fix them there fo as in turning the ball of the globe, the wires with the fun and moon go round with it; and thefe two little balls rife and fet at the fame times, and «n the fame points of the horizon, for the day to which they are reftified, as the fun and moon do in the heavens. Becaufe the moon keeps not her courfe in the ecliptic, (as the fun appears to do), but has a declination of 5-3degrees on each fide from it in.every lunation, her ball may be ferewed as many degrees to either fide of the ecliptic as her latitude or declination from the ecliptic amounts to at any given time ; and for this purpofe S, Plate LT. fig. 2. (by mijlake omitted to be inferted in the proper plate J is a final! piece of pafteboard, of which the curved edge at 5 is to be fet upon the globe at right angles to the ecliptic, and the dark line over S to Hand upright upon it. From this line, on the convex edge, are drawn the 5y degrees of the moon’s latitude on both fides of the ecliptic; and when this piece is fet upright on the globe, its graduated edge reaches to the moon on the wire 0, by which means fhe is eafil-y adjufted to her latitude found by an ephemeris. The horizon is fupported by two femicircular arches, becaufe pillars would (top the progrefs of the balls when they go below the horizon in an oblique fphere. To reftify this globe. Elevate the pole to the latitude of the place; then bring the fun’s place in the ecliptic for the given day to the brafen meridian,, and fet the hourindex to XII at noon, that is to the upper XII on the hour-circle; keeping the globe in that fituation, flacken the ferew G, and fet the fun diredtly over his place on the meridian; which done, fet the moon’s wire under the number that expreffes her age for that day on the plate F; and fhe will then ftand over her place in the ecliptic, and fhew what conflellatian fne is in. Laftly, ftflen the ferew G, and laying the curved, edge of the pafteboard S over the ecliptic below the moon, adjuft the moon to her latitude over the graduated edge of the pal’ceboard; and the globe will be redlified.

N O M Y. Having thus redtified the globe, turn it round, and obferve on what points of the horizon the fun and moon balls rife and fet, for thefe agree with the points of the compafs on which the fun and moon rife and fet in the heavens on the given day; and the hour-index fhews the times of their rifing and fetting ; and likewife the time of the moon’s pafling over the meridian. This fimple apparatus fhews all the varieties that can happen in the rifing and fetting of the fun and moon; and makes the fore-mentioned phenomena of the harveftmoon plain to the eye. It is alfo very ufeful in reading ledtures on the globes, becaufe a large company can fee this fun and moon go round, rifing above and fetting below the horizon at different times, according to the feafons of the year; and making their appulfes to different filled ftars. But in the ufual way, where there is only the places of the fun and moon in the ecliptic to keep the eye upon, they are eafily loft fight of, unlefs they be covered with patches. The Planetary Globe, (Plate XLIX. fig. 1.) In this machine, a terreftrial globe is fixed on its axis Handing upright on the pedeftal C D E, on which is an hour-circle, having its index fixed on the axis, which turns fomewhat tightly in the pedeftal, fo that the globe may not be liable to fhake; to prevent which, the pedeftal is about two inches thick, and the axis gois quite through it, bearing on a fhoulder. The globe is hung in a graduated brazen meridian, much in the ufual way; and the thin plate N, NE, E is a moveable horizon graduated round the outer edge, for fhewing the bearings and amplitudes of the fun, moon, and planets. The brafen meridian is grooved round the outer edge; and in this groove is a flender femi-circle of brafs, the ends of which are fixed to the the horizon in its north and fouth points : this femi-circle Aides in the groove as the horizon is moved in rectifying it for different latitudes. To the middle of this femi-circle is fixed a pin, which always keeps in the zenith of the horizon, and on this pin the quadrant of altitude q turns ; the lower end of which, in all pofitions, touches the horizon as it is moved round the fame. This quadrant is divided into 90 degrees from the horizon to the zenithal pin on which it is turned, at 90. The great flat circle or plate AB is the„ecliptic,. on the outer edge of which the figns and degrees are laid down; and every fifth degree is drawn through the reft of the furface of this plate towards its center. On this plate are feven grooves, to which feven little balls are adyufted by Aiding wires, fo that they are eafily moved in the grooves, without danger of ftarting them. The ball next the terreftrial globe is the moon, the next without it is Mercury, the next Venus, the next the fun, then Mars, then Jupiter, and daftly Saturn. This plate, or ecliptic, is fupported by four ftrong wires, having their lower ends fixed into the pedeftal, at C Z>, E, the fourth being hid by the globe. The ecliptic is inclined 23-r degrees to the pedeftal, and is therefore properly inclined to the axis of the globe which Hands upright on the pedeftal. To redify this matbine. Set the fun, and all the planetary balls, to their geocentric places in the-ecliptic forany given time, by an ephemeris; then fet the north pointthe.. of