Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/558

 ASTRO its dark fide is towards her. Hence a new moon anfwers to a full earth, and a full moon to a new earth. The quarters are alfo reveried to each other. . Between the third quarter and change, the moon is frequently vifible in the forenoon, even when the fun Urines; and then (he affords us an opportunity of feeing a very agreeable appearance, where-ever we find a globular ftone above the level of the eye, as fuppofe on the top of a gate. For, if the fun Ihines on the ftone, and ■we place ourfelves fo as the upper part of the fun may juft feem to touch the point of the moon’s lowermoft horn, we ihall then fee the enlightened part of the ftone exa&ly of the fame drape with the moon, horned as (he is, and inclining the fame way to the horizon. The reafon is plain, for the fun enlightens the ftone the fame ■way as he does the moon ; and both being globes, when we put ourfelves into the above fituation, the moon and ftone have tire fame pofition to our eyes, and therefore we muft fee as much of the illuminated part of the one as of the other. The pofition of the moon’s cufps, or a right line touching the points of her horns, is very differently inclined to the horizon at different hours of the fame days ©f her age. Sometimes {he ftands, as it were, upright on her lower horn, and then fuch a line is perpendicular to the horizon: when this happens, (he is in what the aftronomers call the nonagefirual degree, which is the higheft point of the ecliptic above the horizon at that time, and is 90 degrees from both fides of the horizon, where it is then cut by the ecliptic. But this never happens when the moon is on'the meridian, except when (he is at the very beginning of Cancer or Capricorn. The inclination of that part of the ecliptic to the horizon in which the moon is at any time when horned, may be known by the pofition of her horns; for a right line touching their points is perpendicular to the ecliptic. And as the angle that the moon’s orbit makes with the ecliptic can never raife her above, nor deprefs her below the ecliptic, more than two minutes of a degree, as feen from the fun, it can have no fenfible effedt upon the pofition of her horns. Therefore, if a quadrant be held up, fo as one of its edges may feem to toueh the moon’s horns, the graduated fide being kept towards the eye, and as far from the eye as it can be conveniently held, the arc between the plumb-line and that edge of the quadrant which feems to touch the moon’s horns, will ftiew the inclination of that part of the ecliptic to the horizon. And the arc between the other edge of the quadrant and-plumb line will, (hew the inclination of the moon’s horns to the horizon. The moon generally appears as large as the fun ; for the angle vkA, (Plate XL1II. fig. 3.) under which the moon is u.en from the earth, is the fame with the angle LkM, under which the fun is feen from it. And therefore the moon may hide the fun’s whole dilk from usj as fhe fometimes does in folar eclipfes The reafon why (he does not eclipfe the fun at every change (hall be explained afterwards. If the moon were farther from the earth, as at {he could never hide the whole >of the fun from us ; for then fire would appear under the angle NkO, eclipfing’’ only that part of. the fun which lies between iV and 0 : were file

N O M Y. ftill further from the earth, as at X, flie would appear under the fmall angle TkIV, like a fpot on the fun, hiding only the part 'TIV from our fight. The moon turns round her axis in the time that ftte goes round her orbit; which is evident from hence, that a fpe&ator at reft, without the periphery of the moon’s orbit, would fee all her fides turned regularly towards him in that time. She turns round her axis from any ftar to the fame ftar again in 27 days 8 hours; from the fun to the fun again in 29-f days : the former is the length of the fydereal day, and the latter the length of her folar day. A body moving round the fun would have a folar day in every revoiution, without turning on its axis, the fame as if it had kept all the while at reft, and the fun moved round it; but without turning round its axis it could never have one fydereal day, becaufe it would always keep the fame fide towards any given ftar. If the earth had no annual motion, the moon would go round it fo as to.compleat a lunation, a fydereal, and a folar day, all in the fame time. But, becaufe the earth goes forward in its orbit, while the moon goes round the earth in her orbit, the moon muft go as much more than round her orbit from change to change in compleating a folar day, as the earth has gone forward in its orbit during that time, i, e. almoft a. twelfth part of a circle. The moon’s periodical and fynodical revolution may be familiarly reprefented by the motions of the hour and minute-hands of a watch round its dial-plate, which is divided into 12 equal parts or hours, as the ecliptic is divided into 12 figns, and the year into 12 months. Let us fuppofe thefe 12 hours to be 12 figns, the hourhand the fun, and the minute-hand the moon ; then will the former go round once in a year, and tire latter once in a month ; but the moon, or minute-hand, muft gp more than round from any point of the circle where it was laft conjoined with the fun, or hour-hand, to overtake it again : For the hour-hand being in motion, can never be overtaken by the minute-hand at that point from which they ftarted at their laft conjunction. If the earth had no annual motion, the moon’s motion round the earth, and her track in ablolute fpace, would be always the fame. But as the earth and moon move round the fun, the moon’s real path in the heavens is very different from her vifible path round the earth; the latter being in a progreflive circle, and the former in a curve of different degrees of concavity, which would always be the fame in the fame parts of the heavens, if the moon performed a complete number of lunations in a year without any thing over. Let a nail in the end of the axle of a chariot-wheel reprefent the earth, and a pin in the nave the moon ; if the body of the chariot be propped up fo as to keep that wheel from touching the ground,, and the wheel be then turned round by hand, the pin will defcribe a circle both round the nail, and in the fpace it moves through. But if the props be taken away, the horfes put to, and the chariot driven over a piece of ground which is circularly convex, the nail in the axle will defcribe a circular curve, and the pin in the nave wilfftill defcribe a circle round the progreflive nail in the axle, but not in the fpace through which it moves. In this cafe, the curve defcribed.