Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/549

 459 ASTRO N O M Y. fuppofe from eight in the morning till four in the aftermean ftate, is a 365th part of a circle. Hence, in 365 noon, about which hours the end of the fhaclow fhould days the earth turns 366 times round its axis; and without all the circles ; watch the times in the therefore, as a turn of the earth on its axis completes fall forenoon, when the extremity of the fhortening fhadow a fydereal day, there mull; be one fydereal day more in juft the ievei al circles, and there make marks. £ year than the number of folar days, be tire number Then,touches in the afternoon of the fame day, watch the vhat it will, on the earth, or any other pH nut One lengthening fhadow, and where its end touches the feveturn being loft with reipedl to the number of iolar days ral circles in going over them, make marks alfb. Laftin a year, by the planets going round the fun ; juft as ly, with a pair of compaffes, exaftly the middle it would be loft to a traveller, who, in going round the point between the two marks onfind any circle, and draw a earth, would lofe one day by following the apparent ftraight line from the centre to that which line diurnal motion of the fun; and coofe<]uently would will be covered at noon by the fhadowpoint; of a fmall upright, reckon one day lefs at his return (let him take what which fhould be put in'the place of the pin. The time he would to go round the earth) than thofe who wire, for drawing feveral circles is, that in cafe one part remained all the while at the place from which they fet reafon day fhould prove clear, and the other part fomeout. So, if there were two earths revolving equably ofwhatthecloudy, you mifs the time when the point of the on their axes, and if one remained at until the other fhadow fhouldiftouch you may perhaps catch travelled round the fun from /I to A again,_//;«/ earth it in touching another. oneThecircle, time for drawing a mewhich kept its place at A would have its iolar and fyde- ridian line, in this manner, is beft about the fummer folftice; rial days always of the fame length ; and fo would have becaufe the fun changes his declination one folar day more than the other at its return. Hence, altitude fafteft in the longeft.days. floweft, and his if the earth turned but once round its axis in a year, If the cafement of a window on which the fun fhines and if that turn was made the fame way as the earth at noon, be quite upright, you may draw a line along goes round, the fun, there would be continual day on the edge of its fhadow on the floor, when the fhadow of one: fide of the earth, and continual night on the other. t;he pin is exadlly on the meridian line of the board ; and as the motion of the fhadow of the cafement will be much more fenfible on the floor, than that of the fhadow Chap. XI. Of the Equation of Time. of the pin on the board, you may,know to a few feThe earth’s motion on its axis being perfedtly uni- conds-when it touches the meridian line on the floor; form, and equal at all times of the year, the -fydereal and fo regulate your clock for the day of obfervation by days are always precifely of an equal length; and io that line and any good equation table.. would the fclar or natural days be, if the earth’s orbit As the equation of time, or difference between the fhewn by a well-regulated clock and a true fun-dial, were a perfedt circle, and its axis perpendicular- to its time orbit. But the earth’s diurnal motion on an incli- depends upon two caufes, namely, tire obliquity of the Bed axis, and its annual motion in an elliptic orbit; ecliptic, and the unequal motion of the earth in it, we eaufe the funs apparent motion in the heavens to be fhall firft explain the effedts of thefe caufes Separately unequal: For fometimes he revolves from the meridian confidered, and then the united effedts refulting from to the meridian again in fomewhat lefs than 24 hours, their combination. fhewn by a well-regulated clock ; and at other times in 1 he earth’s motion on its axis being perfedtly equable, , fomewhat more: So that the time fhewn by an equal or always at the fame rate, and the plane of the equator going clock and a true fun-dial is never the fame but on being perpendicular to its axis, it is evident, that in equal the 1 rth of’April, the 16th of June, the 31ft of Auguft,. times equal portions of the equator pals over the meriand the 24th of December. The clock, if -it goes equal- dian ; and fo would equal portions of the ecliptic, if it ly and true all the year round, will be before.the fun were parallel to or coincident with the equator. But, from the 24th of December till the 15th of April; from as the ecliptic is oblique to the equator, the equable, that time till the 16th of June the fun will be before the motion of the earth carries unequal portions of ti e eclock; from the 16th o( June till the 31ft of Auguft, cliptic over the meridian, in equal times, the difference the clock will be again before the fun; and from thence being proportionate to the obliquity ; and, as fome parts . to the -24th of December the.fun will be fafter than the. of tite ecliptic are much more oblique than others, thofd clock. differences are unequal among themfelves. . Therefore, The eafieft and mqft expeditious way of drawing a me- if two funs fhould ftart either from the beginning of Aries ridian line is this : Make four or five concentric circles,, or Libra, and continue to move through-equal arcs in eabout a quarter of an inch from one another, on a flat : qual times, one in the equator, and the other in the eboard, about a foot in breadth ; and let the outmoft cir- cliptic, the equatoreal fun would always run to the mecle be but little lefs than the board will contain.. Fix a ridian in 24 hours time, as-meafured by a well-regulapin perpendicularly in the centre, arid of fuch a length ; ted clock ; but the fun in the ecliptic would return to that its whole fhado w may fall within ..the innermoft cir- the meridian fometimes fooner, and fometimes later cle, for at leaft four, hours in the middle of the day.. than the equatoreal fun ; and only at the fame moments The pin ought to'be about an eighth part of an inch i with him on four days of; the year; namely, the 20th thick, and to have a round blunt point. The board i of March, when the fun enters Aries; the 21ft of June, beingjet.exactly level in a place where the fun fhines, when he enters Cancer; the 23d of September, when he. enters s