(Luna, D),** in astronomy, one of the heavenly bodies, usually ranked among the planets; but with more propriety accounted a satellite, or secondary planet.

Among the ancients, the moon was an object of prime regard.—By the Hebrews she was more regarded than the sun, and they were more inclined to worship her as a deity. The new moons, or first days of every month, were kept as festivals among them, which were celebrated with sound of trumpets, entertainments, and sacrifice. (See Numb. xxviii. 11. x. 16. 1 Sam. xx. 5—18.) People were not obliged on these days to rest. The feasts of new moons were a miniature representation of the feast of trumpets, which was held on the first of the month Tifri, which was the beginning of the civil year. The Jews not being acquainted with the physical cause of eclipses, looked upon them, whether of sun or moon, as signs of the divine displeasure. The Grecians looked upon the moon as favourable to marriage; and the full moons or the times of conjunction of sun and moon, were held the most lucky seasons for celebrating marriages; because they imagined the moon to have great influence over generation. The full moon was held favourable for any undertakings by the Spartans: And no motive could induce them to enter upon an expedition, march an army, or attack an enemy, till the full of the moon. The moon was supposed both by Greeks and Romans to preside over child-birth.—The patricians at Rome wore a crescent on their shoes, to distinguish them from the other orders of men. This crescent was called Lunula. Some say it was of ivory, others that it was worked upon the shoe, and others that it was only a particular kind of fibula or buckle.

As all the other planets move primarily round the sun, so does the moon round the earth: her orbit is an ellipse, in which she is retained by the force of gravity; performing her revolution round the earth, from change to change, in 29 days, 12 hours, and 44 minutes, and round the sun with it every year: she goes round her orbit in 27 days, 7 hours, 43 minutes, moving about 2790 miles every hour; and turns round her axis exactly in the time that she goes round the earth, which is the reason of her keeping always the same side towards us; and that her day and night taken together are as long as our lunar month.

The mean distance of the moon from the earth is 60½ semi-diameters of the earth; which is equivalent to 240,000 miles. The mean eccentricity of her orbit is ¼ of her mean distance, or in miles 13,000, which makes a considerable variation in that mean distance. —Her diameter is to that of the earth as 100 to 365, as 11 to 400½, or 2180 miles: its mean apparent diameter is 31 minutes 16½, and that of the sun 32 minutes 12 seconds. Its mean diameter, as seen from the sun, is 6 seconds.

The moon's surface contains 14,808,750 square miles, and its solidity 5,428,246,000 cubical ones. The density of the moon's body is to that of the earth as 48,911 to 39,214; to that of the sun, as 48,911 to 10,000; its quantity of matter to that of the earth, nearly as 1 to 39.15; the force of gravity on its surface is to that on the surface of the earth as 139.2 to 407.8; and the moon's bulk to that of the earth as ¾ to 1. The moon has scarce any difference of seasons; because her axis is almost perpendicular to the ecliptic.

The different appearances of the moon are very numerous; sometimes she is increasing, then waning; sometimes horned, then semicircular; sometimes globous, then full and round. Sometimes, again, she illumines us the whole night; sometimes only a part of it; sometimes she is found in the southern hemisphere, sometimes in the northern; all which variations having been first observed by Endymion, an ancient Grecian who watched her motions, she was fabled to have fallen in love with him. The source of most of these appearances is, that the moon is a dark, opaque, and spherical body, and only shines with the light she receives from the sun; whence only that half turned... turned towards him, at any instant, can be illuminated, the opposite half remaining in its native darkness. The face of the moon visible on our earth, is that part of her body turned towards the earth; whence, according to the various positions of the moon, with regard to the sun and earth, we observe different degrees of illumination; sometimes a large and sometimes a less portion of the enlightened surface being visible.—But for a particular account of the nature, phenomena, &c. of this secondary but interesting planet, see Astronomy-Index, at Moon.

New Observations on the Atmosphere, Twilight, &c. of the Moon. M. Schroeter of the Royal Society of Gottingen has lately published a very curious and elaborate work in German, intitled Selenotopographische Fragmente, &c. or Selenotopographical Fragments, intended to promote a more accurate knowledge of the Moon's surface. The several maps of the moon*, which have been delineated by Hevelius, Riccioli, Cassini, and Mayer, are well known to every person conversant with astronomical subjects. It is evident that these delineations can give only a very general idea of the spots, together with their relative position on the lunar disk; and as, with respect to us, the appearance of these must vary according to the direction in which the rays of the sun fall on them, the moon's surface will not exactly correspond with the representation of it laid down in the map, except when it happens to be illuminated under the same angle as when this map was drawn. This consideration induced the author to apply himself to the invention of a more accurate mode of describing these phenomena than had hitherto been attempted. For this purpose, having provided himself with a telescope seven feet in length, constructed by Dr Herschel, he resolved, repeatedly, and under various angles of illumination, to observe and delineate very small portions of the lunar disk; in order that, by comparing his different drawings of the same objects, he might compile an accurate topographical description of the moon's surface: but, in this manner, to form a complete lunar atlas, was an undertaking too extensive for a single person. He therefore found himself obliged to prescribe more narrow limits to his design, and confined his plan to the delineation of the several portions of the moon's surface under one angle only of illumination, and this a very small one, that he might obtain more distinct and accurate observations and drawings of the shadows; intending at the same time to examine such parts as appeared either more remarkable or less distinct than the rest, by repeated observations under various angles of illumination: And the present volume contains the result of his observations, with respect to the northern parts of the lunar disk.

The author observes, that, through a telescope which magnifies a thousand times, a lunar object of 190 feet in surface appears like a very small point; and that, to be distinguishable with respect to shape, it must not be less than 800 feet in extent. He tells us, that for his observations he preferred those times when the sun's rays fell on the moon under the least angle; that he carefully and repeatedly examined every object that could be distinguished, and either actually measured its apparent diameter and the length of its shadow, or compared these dimensions with others which he had already measured; and that he never used magnifiers of greater power than what was absolutely necessary to render the object distinct. In order to facilitate the delineation, he applied to his telescope a projecting micrometer, divided into small squares, which, by means of a brass rod, could be placed at any distance from the eye, and always be kept parallel to the line of the moon's horns. His maps or drawings are orthographical projections; and his scale is so constructed, that 20 seconds of the moon's disk correspond with half an English inch on the map; thus the space of 4 seconds is represented in the compass of a decimal line, and, according to M. Schroeter's computation, answers to a German mile or 3807 toises. The inconveniences and inaccuracy of the common method of measuring the lunar mountains, induced him to contrive others capable of greater exactness and more general application: these he varied as the circumstances of the case required; but they are all trigonometrical calculations of the height of the mountain, or the depth of the cavity from the angle of illumination and from the length of the shadow.

If, as some have supposed, a great part of the moon's surface be volcanic, it is natural to expect that the marks of eruptions should from time to time be discernible. A single instance of this kind occurred to our author: ever since the 27th of August 1788, he had constantly seen a cavity, or, as he terms it, a volcanic crater, in the spot Hevelius, which he had never before perceived, though he had often examined this part of the moon with the utmost attention, and in the most favourable circumstances. According to his conjectures, this phenomenon must have commenced between the 24th of October 1787 and the 27th of August 1788.

He observed some alterations in the appearance of lunar objects, which, though too considerable to be attributed to the variation of light, were not sufficiently permanent to be considered as the effect of volcanoes. These he ascribes to meteors; for though he does not suppose the moon to be surrounded with air, exactly like that which invests our globe, he thinks it probable that it may have an atmosphere of some kind, in which some of the elements of bodies, decomposed on its surface, may be suspended; and that some of the lunar mountains may emit nebulous vapours, not unlike the smoke of our volcanoes, which obscure and disguise the objects seen through them.

In regard to those bright points, which have been seen on the moon's surface during eclipses, and at other times on her unenlightened part, and which some have supposed to be burning volcanoes; Schroeter, after the most attentive examination of them, imagines that most of them must be ascribed to the light reflected from the earth to the dark part of the moon's disk, which returns it from the tops of its mountains, under various angles, and with different degrees of brightness. Some of these phenomena he suspects to be no more than optical illusions, arising from igneous meteors floating in our atmosphere, which happen to fall within the field of the telescope.

But the most interesting part of this work consists of the author's "Remarks on the Formation and physical Constitution of the Moon's Surface and Atmosphere," The surface of the moon appears to be much more unequal than that of our earth; and these inequalities have great variety both in form and magnitude. There are large irregular plains, on which are observed long and narrow strata of hills running in a serpentine direction: some of the mountains form extensive chains; others, which are in general the highest, stand alone, and are of a conical shape: some have craters; others form a circular ring including a plain; and in the centre of many of these plains, as well as in the middle of some of the craters, other mountains are found, which have likewise their craters. These mountains are various with respect to colour, some being much darker than others.

The most lofty mountain on the surface of our globe is supposed to be Chimborazo, which is not 20,000 feet in height; but there are many in the moon which are much higher; that which is distinguished by the name of Leibnitz, is not less than 25,000 feet. This elevation will appear more extraordinary, if compared with the moon's diameter, of which it is $\frac{1}{4}$th; whereas Chimborazo is not above $\frac{1}{32}$th of that of the earth: thus considered, the lunar mountains are near five times as high as any on our globe.

The craters of the moon are circular, and surrounded with an annular bank of hills: they are remarkable for their width, many of them being from 4 to 15 geographical miles in diameter; some are not deeper than the level of the moon's surface; others are 9000, 12,000, and 15,000 feet in depth; that of one, which our author calls Bernoulli, is above 18,000 feet. The height of the annular bank is seldom equal to the depth of the crater which it surrounds; but the quantity of matter in the one appears to be in general nearly equal to the capacity of the other. The principal mountains and cavities seem to be connected by a series of others of less magnitude; and sometimes by hilly strata, which, like the radii of a circle, may be traced to a common centre; this is generally either a mountain or crater, though not of the greatest height or depth. These hilly strata, which, through smaller telescopes, appear like veins on the moon's surface, have often been mistaken for torrents of lava; none of which, M. Schroeter says, he could ever discover.

From all the preceding circumstances, the author concludes, that whatever may have been the cause of the inequalities of the moon's surface, it must not only have operated with great violence, but also have met with great resistance; which inclines him to think, that the substance of this planet must originally have been very hard and refractory. He is of opinion that these mountains and cavities must have been produced in consequence of some great revolution occasioned by the action of a force directed from the centre towards the surface, and in this respect similar to that which gave birth to our volcanoes; but he observes, that we have no reason to suppose it absolutely volcanic, nor that it originated from fire. In some places, this force has only elevated the surface, and thus formed hills and mountains; in others, the ground has yielded to its violence, and has either been thrown up as a bank round the crater thus formed, or else falling into other cavities, has in part filled them up; after having exerted its greatest violence in these mountainous accumulations, it has diffused itself in various directions, and produced the hilly strata which are observed to diverge from them, like the radii of a circle from the centre. In support of this hypothesis, it is alleged, that the largest craters have the least depth, and that in the deepest there is the most equal proportion between the capacity of the crater and the volume of the annular bank around it: but beside the grand revolution here supposed, M. Schroeter is of opinion that there have been others of later date, and less extent; to these he ascribes the formation of secondary mountains, which arise either from the middle of the craters of the primary, or from the centre of a plain surrounded by a circle of hills; many of these have also craters, and, like the primary mountains, are connected by a series of cavities and hilly strata, that mark the progress of the cause by which they were produced. The new crater discovered by our author in the spot Hevelius, together with other circumstances here enumerated, seem to indicate that the surface of the moon is far from being permanently settled and quiescent.

The author's observations confirm the opinion that the cavities visible on the lunar surface do not contain water: hence he concludes, that there can be no extensive seas and oceans, like those which cover a great part of the earth; but he allows that there may be springs and small rivers. The question, whether the moon be inhabited? is not omitted by M. Schroeter, who observes, that though it be not adapted to beings organized as we are, this is no proof that it may not be peopled with intelligent agents, endowed with bodily constitutions suitable to the nature and economy of the planet for which they are destined.

With regard to a lunar atmosphere, the existence of which has been a subject of much dispute*, our author adduces a variety of proofs in support of the affirmative side of the question. He also makes a number of observations on several of its relative properties, compared with the same in our atmosphere; such as its greater dryness, rarity, and clearness, which, however, do not prevent its refracting the solar rays, having pointed out the circumstance, that the mountains in the dark hemisphere of the moon, near its luminous border, which are of sufficient height to receive the light of the sun, are the more feebly illuminated the more distant they are from that border: from which proofs of a refracting atmosphere, he also deduced the probability of the existence of a faint twilight, though his long series of observations had not yet fully evinced it.—He had, however, ascertained the existence of a twilight on Venus; and as one fortunate discovery often leads to another, he had no sooner succeeded in his observations on that planet, than he was induced to direct his attention, for a similar purpose, to the moon. In doing this, he applied the calculations and inferences he there made to some appearances he had already noticed on this satellite. It occurred to him, that if in fact there were a twilight on the moon, as there is on Venus and our earth, it could not, considering the greater rarity of its atmosphere, be so considerable; that the vastness of it, allowing for the brightness of the luminous part of the moon, the strong light that is thence thrown upon the field of the telescope, and in some measure the reflected light of our earth, could only be traced on the limb, particularly... cularly at the cusps; and even this only at the time when our own twilight is not strong, but the air very clear, and when the moon, in one of its least phases, is in a high altitude, either in the spring, following the sun two days after a new moon; or in the autumn, preceding the sun in the morning, with the same aspect: in a word, that the projection of this twilight will be the greater and more perceptible the more falcated the phase, and the higher the moon above the horizon, and out of our own twilight.

All the requisite circumstances do not often coincide. M. Schroeter, however, was so fortunate as to be favoured with a combination of them on the 24th of February 1792: And the observation proved in every respect so complete, and the inferences deducible from it appeared to him so new and interesting, that he could not withhold the immediate communication of it from the public. His observations concerning both the Moon and Venus have been accordingly detailed in a paper sent to the Royal Society of London, and inserted in their Transactions for 1792; from which the following respecting the Moon are extracted.

"On the above mentioned evening, at 5h 40', two days and 12 hours after the new moon, when in consequence of the libration the western border of the grey surface of the Mare Crisium was 1° 20' distant from the western limb of the moon, the air being perfectly clear, I prepared my seven-feet reflector, magnifying 74 times, in order to observe the first clearing up of the dark hemisphere, which was illuminated only by the light of our earth, and more especially to ascertain whether in fact this hemisphere, which, as is well known, is always somewhat more luminous at the limb than in the middle, would emerge out of our twilight at many parts at once, or first only at the two cusps. Both these points appeared now, most distinctly and decidedly, tapering in a very sharp, faint, scarce anywhere interrupted, prolongation; each of them exhibiting, with the greatest precision, its farthest extremity faintly illuminated by the solar rays, before any part of the dark hemisphere could be distinguished. But this dark hemisphere began soon after to clear up at once at its border, though immediately only at the cusps, where, but more particularly at their points, this border displayed, on both at the same time, a luminous margin, above a minute in breadth, of a very pale grey light, which, compared with that of the farthest extremities of the cusps themselves, was of a very different colour, and relatively as faint as the twilight I discovered on the dark hemisphere of Venus, and that of our own earth, when compared with the light immediately derived from the sun. This light, however, faded away so gradually towards the east, as to render the border on that side perfectly undefined, the termination losing itself imperceptibly in the colour of the sky.

"I examined this light with all possible care, and found it of the same extent at both points, and fading away at both in the same gradual proportion. But also, with the same caution, explored whether I could distinguish any part of the limb of the moon farther towards the east; since, if this crepuscular light had been the effect of the light reflected from our globe, it would undoubtedly have appeared more sensibly at the parts most remote from the glare of the illuminated hemisphere. But, with the greatest exertion of my visual powers, I could not discover any part of the, as yet, wholly darkened hemisphere, except one single speck, being the summit of the mountainous ridge Leibnitz, which was then strongly illuminated by the solar light; and indeed eight minutes elapsed before the remainder of the limb became visible; when not only separate parts of it, but the whole, displayed itself at once.

"This alone gave me certain hopes of an ample recompence, and satisfied me that the principles I had laid down in my Selenotop. Fragm. § 525. et seq., concerning the atmospheres of the planets, and especially of the moon, are founded on truth. But a similar observation made on the 6th, after seven o'clock, afforded me several collateral circumstances, which strongly corroborate what I have there advanced on this subject. The whole limb of the dark hemisphere, illuminated only by the reflected light of our globe, appeared now so clear and distinct, that I could very readily discern not only the large but also the smaller spots, and among these Plato, Aristarchus, Menelaus, Manilius, Copernicus, &c. and even the small speck to the north-west of Aristarchus, marked b, Tab. XXVII. fig. 1. of the Fragments. I could apply the usual power, magnifying 16x times; and had full leisure, and the means, to examine every thing carefully and repeatedly, and to take very accurate measurements.

"Although a just idea of so delicate a phenomenon as this crepuscular light cannot possibly be conveyed by a drawing, but must be gathered from actual inspection, I have, nevertheless, attempted a delineation of it, and of the southern and eastern cusps, fig. 1, and 2, as deduced from my measurements, especially at the southern cusp, in hopes thereby to render what I have farther to say concerning this observation the more intelligible.

"The southern cusp (fig. 1.) extended from a to c, with a gradually fading but still resplendent solar light, of its usual pale yellow colour, and terminated at c with a mountain. That this was really the point of the cusp, appears not only from the general construction of the falcated segment, which was sufficiently narrow even at its beginning a, near which it was somewhat disfigured at b by a high mountain, but also from the narrowness of its luminous curve at d e and f, the breadth of which seldom exceeded 1/2", and had a sensible interruption so near as d. This curve was throughout, from a to c, except where the glare of the solar rays spread some degree of light, bordered with the pale ash-colour of the dark hemisphere, glimmering with the faint light reflected from our earth; out of which, however, rose the higher mountains g, h, i, c, which were now already illuminated by the sun; and farther on, not less than 30 lines, or according to my usual projection, two minutes distant from the point c, was seen another mountain l, which belonged to the high ridge Leibnitz, and also received its light immediately from the sun.

"There can hence be no doubt of the termination of the cusp being at c; and this being well ascertained, I now distinguished with the greatest certainty the twilight extending from c to l. The most remarkable circumstances attending this light were, that it Moon.

was broadest and brightest at c, and that it dwindled away and contracted towards k, where it lost itself in the faint glimmering of our terrestrial light; and that at the northern cusp (fig. 2.), at which there do not appear to be so many mountains and inequalities as at the southern, this light exhibited the same pyramidal form, and was of equal length, and alike fading in intensity and colour, as that at the southern.

"This light, compared with that of the thinnest and least bright part of the cusp d e, was as faint as the pale ash coloured spots in the luminous hemisphere, when opposed to the bright ones. But this is still better illustrated by a comparison between the high mountain l (fig. 1.) which now already appeared illuminated by the solar light and the spot Aristarchus, which shone moderately merely with the light reflected from our globe. The said mountain had, comparatively with the thin luminous arc d e f of the bright hemisphere, and the mountains g, h, i, c, a very pale, fading, but yet brighter light than Aristarchus, as indeed might have been expected from what I said in my Selenotop. Fragm.; but this reflected light upon Aristarchus was, however, sensibly brighter than the glimmering light from c to k. And, reflecting the still fainter terrestrial light which bordered the luminous curve from c to k (fig. 1. and 2.), I cannot give a better idea of it than by observing, that the light at the extremities of both the cusps appeared of a pyramidal form, similar to, but though gradually fading, and very undefined, yet brighter than that of our zodiacal light, when, in the months of March and April, it blends itself, comparatively with the remaining colour of the sky, with the terrestrial light, terminating in a very sharp point.

"The undefined and gradually fading appearance of this light was the cause that, though I had recourse to a dark projection table, I could not, however, take any accurate measurements of it. I found nevertheless, by repeated comparisons, that the length of this pyramidal glimmering light, in which I could perceive no sensible inequality at the limb of the moon, amounted to about \( \frac{3}{4} \) of the distance between the two mountains c, l (fig. 1.) which shone with the solar light. Comparing also this southern twilight with the northern, it appeared of the same length; and, on measuring the distance c l, I found it repeatedly = 30 lines = \( \frac{3}{4} \); so that the length of the twilight must have amounted to 20 lines = \( \frac{1}{2} \). Its greatest breadth at c could, on the other hand, because of the extent and greater density of its light, be easily ascertained by means of the immediate application of the projection table. This measurement gave at most \( \frac{3}{4} \) of a line, or full \( \frac{1}{2} \).

"Although I be positively certain of this very remarkable appearance at both cusps, and of its perfect similarity, in all my observations, I could not, however, trace any vestige of a like crepuscular light at any other part of the terminating border; nor could I on the very next evening, being the 25th, and also on the 26th of February, perceive, even at the cusps, any of the twilight I expected to see there; the very thin, faint, luminous line, which did indeed appear on the 26th, at the southern cusp between a and b, (fig. 3.), being undoubtedly the effect of the immediate solar light, probably illuminating some prominent, flat area, as yet situated in the dark hemisphere.

"Thus far the observations: and now for the application of them.

"I need hardly insist upon the proofs, that the very faint pyramidal glimmering light, observed on the 24th of February at the extremities of both cusps, could by no means be the immediate effect of the solar light, all the circumstances of the observations militating uniformly and decidedly against this supposition, which, were it true, would oblige us to admit a most inaccountable diminution of light, and thence also a density of the lunar atmosphere, that ought to exceed even the density of ours; a fact absolutely contradicted by all the lunar observations hitherto made. This light, indeed, was so very faint, that it disappeared at 7h 20', when the moon approached the horizon; whilst, on the other hand, Aristarchus, which had no light but what it received from the earth, was still very distinguishable; and the summit of Leibnitz l, fig. 1. (which, though far within the dark hemisphere, was, however, illuminated by the immediate solar rays) displayed a degree of brightness, which, although when compared with that of the cusp d e f, it appeared very faint and dwindling, equalled, however, that of our Peak of Teneriffe. Nor can it be conceived why this glimmering light broke off so suddenly at both the cusps, without a progressive diminution. It can hardly be supposed, that similar, grey, prominent, flat areas, of the same form and dimensions, and comparatively of a faint light, which, whilst in the dark hemisphere, they derive immediately from the sun, exist on all parts of the moon; more especially as, at the places observed, the limb happened to exhibit throughout an exact spherical form, without the least sensible inequality; and as in both the bordering regions of the northern and southern hemispheres, especially in the latter, no such grey prominent planes are anywhere discernible. It may then be asked, why did this faint glimmering light appear at both cusps, along equal arcs of the limb, of equal length and breadth, and of the same pyramidal form? and why did its farther extremity blend itself with the terrestrial light of the dark hemisphere, which, according to a great number of my selenotographic observations, is by no means the case, even with those grey prominent areas, which, being at some distance on the dark side of the terminating border, are nevertheless illuminated immediately by the sun?

"These, therefore, could certainly not derive their light immediately from the sun; whence this appearance, like the similar ones on the planet Venus, can only be ascribed to the solar rays reflected by the atmosphere of the moon upon those planes, producing on them a very faint, gradually diminishing, glimmering light, which at last loses itself in the reflected terrestrial light, in the same manner as our twilight blends itself with the light of the moon. Every circumstance of the above observation seems to me to confirm this supposition; and hence the observation itself, which, though single, was however a most fortunate and complete one, must appear of no small degree of importance, since it not only confirms the observations and inferences on the long contested lunar atmosphere. atmosphere contained in my Selenotop. Fragm. but also furnishes us with many more lights concerning the atmosphere of planets in general than had been afforded us by all those observations collectively."

This, and the mathematical certainty that the phenomenon is in fact nothing but a real twilight in the lunar atmosphere, he farther evinces by a series of theoretical deductions and calculations, which do not admit of being here stated. Among other results, it appears, that the lower and more dense part of the lunar atmosphere, that part, namely, which has the power of reflecting this bright crepuscular light, is only 1356 Paris feet in height; and hence it will easily be explained how, according to the different librations of the moon, ridges of mountains, even of a moderate height, situated at or near the terminating border, may partially interrupt, or at times wholly prevent, this crepuscular light, either at one or the other cusp, and sometimes at both.

"I cannot hence (says our author) but consider the discovery I here announce as a very fortunate one, both as it appears to me decisive, and as it may induce future observers to direct their attention to this phenomenon. Admitting the validity of this new observation, which I think cannot well be called in question, I proceed now to deduce from it the following inferences.

"1. It confirms, to a degree of evidence, all the selenotopographic observations I have been so successful as to make on the various and alternate changes of particular parts of the lunar atmosphere. If the inferior and more dense part of this atmosphere be in fact of sufficient density to reflect a twilight over a zone of the dark hemisphere 2° 34', or 10½ geographical miles in breadth, which shall in intensity exceed the light reflected upon its dark hemisphere by the almost wholly illuminated disk of our earth; and if, by an incidental computation, this dense part be found to measure 1356 feet in perpendicular height, it may, according to the strictest analogy, be asserted, that the upper, and gradually more rarified strata, must, at least, reach above the highest mountains in the moon. And this will appear the more evident, if we reflect, that notwithstanding the inferior degree of gravitation on the surface of the moon, which Newton has estimated at somewhat less than one-sixth of that on our earth, the lower part of its atmosphere is nevertheless of so considerable a density. This considerable density will, therefore, fully account for the diminution of light observed at the cusps, and on the high ridges Leibnitz and Doerfel, when illuminated in the dark hemisphere; as also for the several obscurations and returning serenity, the eruptions, and other changes, I have frequently observed in the lunar atmosphere. This observation also implies:

"2. That the atmosphere of the moon is, notwithstanding this considerable density, much rarer than that of our earth. And this indeed is sufficiently confirmed by all our other lunar observations. I think I may assert, with the greatest confidence, that the clearer part of our twilight, when the sun is 4° below our horizon, and when we can conveniently read and write by the light we receive from it, surpasses considerably in intensity the light which the almost wholly illuminated disk of our earth reflects upon the dark hemisphere of the moon 2½ days before and after the new moon. But should we even admit an equal degree of intensity, it will, however, appear from computation, that our inferior atmosphere, which reflects as strong a light over 4° as that of the moon does over 2° 34' of their respective circumferences, must be at least eight times higher than that of the moon.

"3. The striking diminution of light I noticed in my twelve years observations on Venus, likewise indicates, that the atmosphere of that planet, which is in many respects similar to ours, is much denser than that of the moon; and this will be still farther corroborated, if we compare together the several measurements and computations made concerning the twilights of different planets. There is no doubt but that the faintest twilight of Venus, as seen either before or after the rising and setting of the sun across our twilight, is much brighter than that of the moon; and it appears, moreover, from computation, that the denser part of the atmosphere of Venus measures at least 15000 Paris feet in height, and spreads its twilight 67 geographical miles into the dark hemisphere, whilst the denser part of the lunar atmosphere, whose height does not exceed 1356 feet, produces a faint twilight not above 10½ geographical miles in breadth. Thus, as my successful observations on the twilight of Venus led me to the discovery of that of the moon, so did these latter reciprocally confirm the former; and thus, whichever way we contemplate the subject, must we be struck with the coincidence that prevails throughout.

"4. But if the lunar atmosphere be comparatively so rare, it follows, that the inflection of light produced by it cannot be very considerable; and hence does the computation of M. du Sejour, according to which the inflection of the solar rays which touch the moon amounts to no more than 4½', receive an additional degree of authenticity *. Besides which,

"5. As the true extent of the brightest lunar twilight amounts to 2° 34', the obliquity of the ecliptic Agren, in the moon only to 1° 29'; the inclination of the orbit of the moon, on the contrary, to 5° 15', and its synodic period, during which it performs a revolution round its axis is = 29d 12h; it follows, that its brightest twilight, to where it loses itself in the light reflected by the almost fully illuminated disk of our earth, must, at least at its nodes, last 5h 3', and that it will be still longer at other parts of the orbit, according to the situation of the nodes.

"6. And lastly, it being a well known fact† that the fixed stars, as they approach the moon, diminish in splendor at the most only a very few seconds before their occultations, it was natural for me, after the successful observations I had made on the twilight of the moon, to pay particular attention to this circumstance. On the 25th of February, at 6h P.M., the sky being very clear, the limb of the dark part of the moon appeared uncommonly distinct; and only a few seconds of a degree from its edge was seen a telescopic star of about the 10th or 12th magnitude. I counted full 20° before its occultation, and 18½ of these, without the least perceptible diminution of light. The star, however, began now gradually to fade, and after the remaining 1½', during which I observed it with all possible attention, it vanished in an instant. This observation agrees perfectly with the above computations.

L12 putations. Although it be proved that the inferior dense part of the lunar atmosphere reflects a stronger light than that which the dark hemisphere receives from an almost fully illuminated disk of our earth; and although, considering the inferiority of gravitation on the surface of the moon, there be no doubt that this dense part, together with the superior gradually more rarified regions of its atmosphere, must extend far above its highest mountains; it is yet a fact, that the breadth of this observed twilight, to where it loses itself in our reflected terrestrial light, does not measure more than $2^\circ 34'$; it is therefore highly probable, that its greatest extent, in the most favourable places near our new moon, can never exceed the double of the above arc, or $5^\circ 8'$; and hence we can only infer a perpendicular height of an atmosphere, capable of inflicting the solar rays, which at most measures $5376$ feet: nor is it very likely that, unless accidental and hitherto unknown circumstances should occasionally condense different parts of this atmosphere, these upper strata should materially affect the distinctness of a star seen through it.

"But admitting the height of the atmosphere, which may affect the brightness of a fixed star, not to be less than $5376$ feet, this will amount to an arc of only $0^\circ 94''$, or not quite one second; and as the moon describes an arc of $1''$ in $2''$ of time, it follows, that in general the fading of a star, which approaches to an occultation, cannot last quite $2''$ in time; that if the appulse be at a part of a limb of the moon where a ridge of mountains interferes, the gradual obscuration will last a shorter time; and that it may, under some circumstances of this nature, be even instantaneous."

To the foregoing observations, M. Schroeter subjoins the following account of an occultation of Jupiter by the moon when near its full, which occurred to him by mere accident on the 7th of April 1792.

"The sky being very serene, and Jupiter uncommonly bright, I prepared my seven-feet reflector, magnifying 74 times, in hopes that the strong light and distinctness it afforded would enable me to compare the appearances of this phenomenon with the results which I had deduced from my late observations on the height and density of the atmosphere of the moon.

"Fig. 4 represents the situation of Jupiter's four satellites, as they appeared, most distinctly, two of them to the westward, the second about one, and the first near two of Jupiter's diameters distant from its limb; and the two others to the eastward, the third about seven and the fourth near eight of the same diameters, distant from the said limb.

"Fig. 5 shows Jupiter with its belts, and of a somewhat spheroidal form, as it now appeared to me, and as distinctly as I had ever seen it. The equatorial belt, from $a$ to $d$, was very apparent. It consisted properly of two zones, $a$ and $c$, of a brownish-grey colour, with a more luminous interval $b$ between them. At $e$ and $f$ were two comparatively well-defined stripes, which I had noticed for many years back, but which now crossed the whole disk; and the polar regions appeared again, from $g$ and $h$, more dim and grey than the bright part of the planet. But what particularly struck me, were two nebulous undefined spots, $i$ and $k$, which were sensibly darker than the principal zone $d$; and at $l$ a still more remarkable, circular, tho' imperfectly defined spot, somewhat brighter than the luminous interval between the zones, and perfectly similar to the remarkable luminous spot which I had observed in 1786 and 1787 on the same part of Jupiter, and which then led me to some very unexpected inferences concerning the atmosphere of that planet.*

* Bystr. zu den vierten

"The favourable circumstances led me to the following accurate observation, which I was certain would prove instructive to me. At $10^h 40' 50''$ I saw and Tab. I the spot $i$ at about the middle of its parallel; and im. fig. 6 immediately after began the occultation; than which a more distinct and beautiful one was perhaps never seen.

"Immersion. The western, preceding, first satellite, disappeared behind the sharp bright limb of the moon, at $10^h 43' 12''$. The second satellite disappeared, without becoming at all indistinct, exactly at $10^h 45' 19''$. The western limb of Jupiter came in contact, most distinctly, with the eastern limb of the moon, at $10^h 46' 32''$. Jupiter's eastern limb disappeared, as distinctly, at $10^h 48' 20''$. This immersion took place, as represented in fig. 6, to the eastward of Arcturus, at about the 25th degree of north latitude.

"The third satellite disappeared, after having been for about one or two seconds faint and indistinct, at $10^h 58' 57''$. The fourth satellite, which appeared the least of them all, became undiscernible near the limb, and vanished at about $11^h 2' 16''$.

"Emergence. The two preceding first and second satellites were here likewise of use in determining precisely the emergence of both the limbs of Jupiter from the dark hemisphere of the moon.

"The first appearance of Jupiter's western limb was very distinct at $11^h 43' 54''$. Emergence of the eastern limb, as distinct, at $11^h 45' 39''$. This emergence took place, as represented in fig. 7, to the north-eastward of Seneca (B, Tab. VIII. of the Frag.), at about the 23rd degree of north latitude.

"The emergence of the next, or third satellite, was not observed.

"That of the fourth was distinct at $11^h 59' 1''$. This observation gave me the more satisfaction, as it singularly contributed to confirm the discovery I had been so fortunate as to make of the twilight in the moon, and the height and density of the lower stratum of its atmosphere.

"Experience has sufficiently proved, that a stronger will ever obscure a fainter light; and it follows hence, that the light of a bright star approaching the moon, when full or nearly so, will lose something of its lustre: but little can be inferred in favour of an atmosphere either of the Moon or of Mars, from the observation of Cassini; in which, as Dr Herschel has illustrated by some observations of his own*, a star in Aquarius, of the fifth magnitude, and as yet six minutes distant from Mars, diminished in light when both were seen in the same field of the telescope. A mere apparent diminution of light, occasioned by the glare of a larger luminous object, when seen at the same time with a smaller..." smaller one in the field of the telescope, is one thing; and another thing is a real indistinctness of the small luminous body, which increases in proportion as they approach nearer to each other.

"It was very natural for Jupiter to diminish in brightness when it approached so near to the moon, then almost at its full, as to be seen at the same time in the field of the telescope, which was in fact the circumstance of this observation; but I could not observe any progressive variation of light in the eastern and western, equally luminous, disks, proportional to their distances from the limb of the moon, much less a real indistinctness; and this neither when the limbs of the two planets were nearly in contact, nor when Jupiter was partly, or about one half, covered by the moon.

"It was a sight truly gratifying to an eye accustomed to the light of the moon, or in general to similar observations, to behold how Jupiter, at its immersion as well as emersion, being half or more than half covered by the moon, exhibited its belts and other parts as distinctly close to the limb of the moon as it does at some distance from it; and had I not already succeeded in my numerous observations on the atmosphere of the moon, and very recently in those which enabled me to determine its twilight, I should perhaps have adopted the doubts the ancient astronomers entertained concerning the existence of a lunar atmosphere; and this the rather, as when Jupiter in its immersion was so far covered, that the luminous spot, fig. 5, was close to the moon, I could plainly distinguish this spot, although it be in itself by no means very perceptible.

"Such, however, must have been the appearances, according to my new observations and measurements of the twilight of the moon: for if it be proved, that the extent of this twilight, to where it loses itself in the light reflected from the almost wholly illuminated disk of our earth, amounts to no more than an arc of $2^\circ 3' 1"$ of the circumference of the moon, and if it be hence demonstrable, that its greatest dilatation does barely amount to $5^\circ 8'$, and the perpendicular height of that part of the lower more condensed stratum of its atmosphere, which is capable of reflecting the solar rays, and of producing some other, perhaps more remarkable, obscurities in the stars seen through it, does not exceed 5000 Paris feet, and hence cannot reach above one second of a degree above the limb of the moon; we need not wonder that so small a magnitude, which loses itself in the inequalities of the limb, many parts of which are known to be considerably mountainous, should not become sensible, especially at the approach of a body of so large a diameter as Jupiter, and when so small a magnifying power is applied. And thus may I with confidence assert a perfect coincidence between this and my many other observations.

"The appearance, fig. 8, when Jupiter, at the emersion, the objects being particularly sharp and distinct, came forth from behind the moon, which now covered no more than one quarter of its diameter, was truly splendid and satisfactory: and I must here particularly mention the circumstance, that the part of the moon's dark hemisphere, between its bright terminating edge $mn$ and its outward limb, bordering upon the emerging planet $op$, was particularly opaque, and hence produced a very striking effect.

"I omit entering here upon any farther considerations; and shall conclude with observing, that, after the occultation was completely ended, the luminous spot had at $12^\circ 1'$ so far advanced in its parallel $de$, as to have reached to within $\frac{1}{3}$, or at most $\frac{1}{4}$, of its whole length of the western limb: and that on the 27th of March, five days after a new moon, I observed an occultation of a very distinct though telescopic star, by the dark hemisphere of the moon; in which, agreeably to the above observation, not the least gradual diminution of light or indistinctness could be perceived, the star being seen to vanish on a sudden."

Influence of the Moon on the Human Body, the Weather, &c. The vulgar doctrine concerning the influence of the moon on the changes of weather is very ancient, and has gained credit among the learned without sufficient examination; but it seems now to be pretty generally exploded by philosophers, as equally destitute of all foundation in physical theory, and unsupported by any plausible analogy. The common opinion is, that the lunar influence is exerted at the syzygies and quadratures, and for three days before and after each of those epochs. There are 24 days, therefore, in each synodic month, over which the moon at this rate is supposed to preside; and as the whole consists but of 29 days $2\frac{3}{4}$ hours, only $5\frac{1}{2}$ days are exempt from her pretended dominion. Hence, though the changes of the weather should happen to have no connection whatever with the moon's aspects, and they should be distributed in an equal proportion through the whole synodic month, yet any one who shall predict that a change shall happen on some one of the 24 days assigned, rather than in any of the remaining $5\frac{1}{2}$, will always have the chances 24 to $5\frac{1}{2}$ in his favour. Men, therefore, easily deceive themselves, especially in so unsettled a climate as ours. Moreover, the writers who treat of the signs of the weather, derive their prognostics from circumstances which neither argue any real influence of the moon as a cause, nor any belief of such an influence, but are merely indications of the state of the air at the time of observation: such are, the shape of the horns, the degree and colour of the light, and the number and quality of the luminous circles which sometimes surround the moon, and the circumstances attending their disappearance. (See the Astronomia of Aratus, and the Scholiast of Theon.) The vulgar soon began to consider these things as causes, which had been proposed to them only as signs; and the notion of the moon's influence on all terrestrial things was confirmed by her manifest effect upon the ocean. See, on this subject, Phil. Trans. vol. lxv. part 2 p. 178, &c.

The famous Dr Mead was a believer in the influence of the sun and moon on the human body, and published a book to this purpose, intitled De Imperio Solis ac Lunae in Corpore humano: but this opinion has been exploded by most philosophers as equally unreasonable in itself, and contrary to fact. As the most accurate and sensible barometer is not affected by the various positions of the moon, it is not thought likely that the human body should be affected by them. Several learned and ingenious men, however, still consider Dr Mead's doctrine as far from being unfounded.

Harvest-Moon. It is remarkable, that the moon, during the week in which she is full in harvest, rises sooner... Moon sooner after sun-setting than she does in any other full moon week in the year. By doing so, she affords an immediate supply of light after sun-set, which is very beneficial to the farmers for reaping and gathering in the fruits of the earth; and therefore they distinguish this full moon from all the others in the year, by calling it the harvest-moon. For an account of which, see Astronomy, p. 379.

Moon-Eyes, in the manege. A horse is said to have moon-eyes when the weakness of his eyes increases or decreases according to the course of the moon; so that in the wane of the moon his eyes are muddy and troubled, and at new moon they clear up; but still he is in danger of losing his eye-sight quite.

Moon-Stone, a genus of siliceous earths, of a clear white colour approaching to that of milk. When looked at in a certain position, it reflects a strong light like mother-of-pearl; in others, it shows spots of a carnation colour. It is found in pieces with obtuse angles, sometimes of a quadrangular figure. When broken, it appears evidently foliated. According to Werner it agrees in hardness and most other respects with felt-spar. He tells us, likewise, that it is probably the androdamas of Pliny, the common giralde of the Italians, and the water opal of Ceylon. Sometimes, he tells us, it is clasped with the opal, and sometimes with the cat's eye. According to M. Magellan, this stone is of the chalcedony or pseudo-opal kind: it reflects a whitish light, with some various shades of few intermixed colours on a bluish bottom, like the face of the moon when high enough not to appear reddish by the interposition of earthly vapours. The iris, or rainbow-stone, seems to be no other than a moon-stone in which the yellow, purple, and blue rays are most conspicuously reflected. When looked at, it appears of a reddish brown; but on holding it in the light of the sun, we discover the figure of a rainbow. There are, however, several other stones which have the same appearance in the sun's light.

Moon-Wort in botany. See Lunaria.