Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/582

 482 ASTRONOMY. goes over the centre of the earth’s diflc as feen from the Plate XLVII. fig. 1. : to thofe who live on the circle moon ; and confequently, by defcribing the iongeft line marked 2 in Plate XLVI. fig. 3. the moon curs off two poffible on the earth, continues the longeft upon it; twelfth parts of the fun, as at 2 in Plate XLVI1. namely, at a mean rate, 5 hours 50 minutes ; more, if fig. 1.; to thofe on the circle 3, three parts ; and fo on the moon be at her greateft diflance from the earth, be- to the centre at 12 in Plate XLVI. fig. 3. where the caufe fire then moves flowed:; lefs, if fhe be at her lead fun is centrally eclipfed, as at B in the middle of fig. 1. didance, becaufe of her quicker motion. Plate XLVII. ; under which figure there is a fcale of To make fevefal of the above and other phenomena hours and minutes, to Ihew at a mean ftate how long it plainer, (Plate XLVI. fig. 3.), let S be the fun, E the is from the beginning to the end of a central eclipfe of the earth, M the moon, and AMP the moon’s orbit. the fun on the parallel of London ; and how many diDraw the right line Wc 12 from the wedern fide of the gits are eclipfed at any particular time from the beginning fun at IV, touching the ^wedern fide of the moon at c, at A to the middle at B, or the end at C. Thus, in 16 and the earth at’ 12 : draw alfo the right line Vd 12 from minutes from the beginning, the fun is two digits ethe eadern fide of the fun at V, touching the eadern fide clipfed ; in an hour and five minutes, eight digits ; and of the moon at d, and the earth at 12 : the dark fpace in an hour and 37 minutes, 12 digits. csd included between thofe lines is the moon’s fha- By Plate XLVI. fig. 3. it is plain, that the fun is totally dow, ending in a point at 12, where it touches the or centrally eclipfed but to a fmall part of the earth at any earth ; becaufe in this cafe the moon is fuppofed to time ; becaufe the dark conical ihadow e of the moon change at M in the middle between A the apogee, ’or M falls but on a fmall part of the earth ; and that the farthed point of her orbit from the earth, and P the partial eclipfe is confined at that time to the fpace inperigee, or neared point to it. For, had the point P cluded by the circle aob, of which only one half can be been at M, the moon had been nearer the earth ; and projected in the figure, the other half being fuppofed to her.dark fhadow at e would have covered a fpace upon it be hid by the convexity of the earth : and likewife, about 180 miles broad, and the fun would have been that no part of the fun is eclipfed to the large fpace TT totally darkened, with fome continuance : but had the of the earth, becaufe the moan is not between the fun point A been at M, the moon would have been and any of that part of the earth : and therefore to all farther from_the earth, and her fhadow would have that part the eclipfe is invifible. The earth turns eaftended in a point about e, and therefore the fun would wardonits axis, as from ^ to h, which is the fame way have appeared like a luminous ring all around the moon. that the moon’s Ihadow moves ; but the moon’s motion Draw the right lines JVXdh and VXcg, touching the is much fwifter in her orbit from / to / : and therefore, contrary fides of the fun and moon, and ending on the although eclipfes of the fun are of longer duration on earth at a and/£: draw alfo the right line SXM, account of the earth’s motion on its axis than they would, from the centre of the fun’s dilk, through the moon’s be if that motion was ftopt, yet, in four minutes of time centre, to the earth at 12 ; and fuppofe the two former at moft, the moon’s fwifter motion carries her dark Ihalines JVXdh and VXcg to. revolve on the line SXMi 2 as dow quite over any place that i:s centre touches at the an axis, and their points a and b will defcribe the limits time of greateft obfeuration. The motion of the Ihaof the penumbra TT on the earth’s furface, including dow on tire earth’s diflc is equal to the moon’s motion the large fpace «o^i2tf ; within which the fun appears from the fun, which is about 30X minutes of a degree more or lefs eclipfed, • as the places are more or lefs di- every hour at a mean rate ; but fo much of the moon’s ftant from the verge of the penumbra aob. orbit is equal to 30!- degrees of a great circle on the Draw the right lineyt2 acrofs the fun’s dilk, perpen- earth; and therefore the moon’s fhadow goes 304 dedicular to SXM the axis of the penumbra : then, divide grees, or 1830 geographical miles on the earth in an the line y 12 into twelve equal parts, as in the figure, for hour, or 304 miles in a minute, which is almoft four the twelve digits or equal' parts of the fun’s diameter ; times as fwift as the motion of a cannon ball. and, at equal diftances from the centre of the penumbra As feen from the fun or moon, the earth’s axis apat 12 (on the earth’s furface TT) to its edge aob, draw pears differently inclined every day of the year, on actwelve concentric circles, as marked with the numeral count of keeping its parallelifm throughout its annual figures 1234 <bc. and remember that the moon’s mo^ courfe. In Plate XLVII. fig. 2. let EBON be the tion in her orbit AMP is from weft to eaft, as from x earth at the two equinoxes and the two folftices, NS its to t. Then, axis, iVthe north pole, S the fouth pole, ^.’j^the e* To an obferver on the earth at b, the eaftern limb of quator, T the tropic of Cancer, t the tropic of Caprithe moon at d feems to touch the weftern limb of the fun corn, and ABC the circumference of the earth’s enat JV, when the moon is at, TV/; and the fun’s eclipfe be- lightened diflc as feen from the fun or new moon at thefe gins at b, appearing as, at A in Plate XLV1I. fig. x. at times. The earth’s axis has the pofition NES at the. the left hand ; but, at the fame moment of abfolute time vernal equinox, lying tov/ards the right hand, as feen to an obferver at a in Plate XLVI. fig. 3. {he weftern from the fun or new moon; its poles N and S being edge of the moon at c leaves the eaftern edge of the fun then in the circumference of the diflc ; and the equator at V, and the eclipfe ends, as at the right hand C, Plate all its parallels feem to be ftraight lines, becaufe XLVH. fig, 1. At the very fame inftant, to all thofe and their planes pafs through the oblerver’§ eye looking down who live on the circle marked 1 on the earth E, in Plate upon the earth from the fun or moon diredlly over E', XLVI. fig. 3. the moon M cuts off or darkens a twelfth where the ecliptic EG interfedls the equator A?. At the pari of the fun S,. and ecligfes him one digit, as at t in fummer folftice, the earth’s axis has the pofition NOS •