| XXX | (496) | XXX |
49 6 ASTRO N O M Y. towards the 20th degree of Aquarius, according to the latitude of any given place, the times of the moon’s riobservations of Bianchini. She fhews all the pheno- ling and fetting, together with her amplitude, are Ihewn mena defcribed in Ghap. I. to that place as well as the fun’s; and^ all the various Next, without the orbit of Venus, is the Earth, phenomena of the harveft-moon are made obvious to (No, 4.), which turns round its axis, to any fixed point fightat a great diliance, in 23 hours 56 minutes 4 feconds^ The moon’s orbit (No. 9.) is inclined to the ecliptic, of.mean folar time; but from the fun to the fun again, (No. 11.), one half being above, and the other below in 24 hours of the fame time. No. 6. is a fydereal it. The nodes, or points at o and o, lie in the plane dial plate under the earth, and No. 7. a folar dial-plate of the ecliptic, as before defcribed, and fhift backon the cover of the machine. The index of the former ward through ail its fines and degrees in 184 years. The fhews fydereaf.. and of the latter, folar time; and hence degrees of the moon’s latitude to the higheft at NL the former index gains one entire revolution on the latter (north latitude) and loweft at SL, (fouth latitude), are every year, as 365 folar or natural days contain 366 fy- engraven both ways from her nodes at o and o; and as dereal days, or apparent revolutions of the liars. In the moon rifes and falls iq her orbit according to its inthe time that the earth makes 3654 diurnal rotations on clination, her latitude and diftance from her nodes are its axis, it goes once round the fun in the plane of the Ihewn for every day, having firft rectified her orbit fo as eclipticand always keeps oppofite to a moving index to fet the nodes to their proper places in the ecliptic; (No. 10.) which fhews the fun’s daily change of place, and then, as they come about at different, and almoft and aifo the days of the months. oppofite times of the year, and then point towards the The earth is half covered with a black cap, for divi- fun, all the eclipfes may be fhewn for hundreds of years, ding the apparently enlightened half next the fun from (without any new reftification), by turning the machinery the other half, which, when turned away from him, is backward for time paft, or forward for time to come. in the dark. The edge of the cap reprefents the circle At 17 degrees diftance from each node, on both fides, bounding light and darknefs, and ihews at what time the is engraved a fmall fun; and at 12 degrees diftance, a fun rifes and'fets to all places throughout the year. The fmall moon ; which fhew the limits of folar and lunar earth’s axis inclines 23-I- degrees from the axis of the ec- eclipfes: and when, at any change, the moon falls beliptic, the nordi pole inclines toward the beginning of tween either of thefe funs and the node, the fun will be Cancer, and keeps its parallelifm throughout its annual eclipfed on the day pointed to by the annual index, courfe; fo that in fummer the northern parts of the earth (No. 10.); and as the moon has then north or fouth laincline towards the fun, and in winter from him; by titude, one may eafily judge whether that eclipfe will be which means, the different lengths of days and nights, vifible in the northern or fouthern hemifphere j-efpecialiy and the caufe of the various feafons, are demonftrated as the earth’s axis inclines toward the fun or from him to fight. at that time. And when, at any full, the moon falls There is a broad horizon, to the upper fide of which between either of the little moons and node, fhe will is fixed a meridian femicircle in the north and fouth be eclipfed, and the annual index fhews the day of, that points, graduated on both lides from the horizon to 90° eclipfe. There is a circle of 294 equal parts (No. 8.) in the zenith or vertical point. The edge of the horizon on the cover of the machine, on’which an index fhews is graduated from the call and weft to the fouth and the days of the moon’s age. north points, and within thefe divifions are the points of There are two femicircles (Plate XLVIII. fig. 1.) fixthe corhpafs. From the lower fide of this thin horizon- ed th an elliptical ring, which being put like a cap upon plate ftand out four final! wires, to which is fixed a twi- the earth, and the forked part F upon the moon, fhews light-circle :8 degrees from the graduated fide of the the titfes as the earth turns round within them, and they horizon all round. This horizon may be put upop the are led round it by the moon. When the different earth, (when the cap is taken away), and re&ified to the places come to the femicircle AaEbB, they have tides of latitude, of any place; and then, by a fmall wife called flood ; and when they. come, to the femicircie CZiD, they the fdar ray,, which may be put on fo as to proceed di- have tides of ebb; the index on the hour-circle (No. 7. refdy from the fun’s centre towards the earth’s, but to Plate XLVII.) fhewing the times of thefe phenomena. cpme no farther than almoft to touch the horizon. The There is a jointed wire, of which one end being put beginning of twilight, time of fun-rifing, with his am- into a hole in the upright ftem that holds the earth’s cap, plitude, meridian altitude, time of fetting, amplitude and the wire laid into a fmall forked piece which may be then, and end of twilight, are fhewn for every day of occafionally put upon Venus or Mercury, fhews the dithe year, at that place to which the horizon is re&ified. re<5t and retrograde motions of thefe two planets, with The Moon (No. 5.) goes round the earth, from be- their ftaiionary times and places, as feen from the earth. tween it and any fixed point at a great diftance, in 27 The whole machinery is turned by a winch or handle, ■days 7 hours 43 minutes, or through all the figns and de- (No. 12.), and is fo eafily moved, that a clock might grees of her orbit, which-is called her periodical revolu- turn it without any danger of flopping. tion-. but.fire goes round from the fun to the fim. again, To give a plate of the whe J-work of this machine, or from change to change, in 29 days 12 hours 45 mi- would anfwer no purpofe, becaufe many of the wheels nutes, which is her fynodical revolution', and in that lie fo behind others as to hide them from fight in any view whatever. tirqe fire exhibits all the phafes already defcribed. This When the above mentioned horizon is refilled to the The Cometarium, (Plate XLVIII. fig. 2.)curious
