Another of the high functions of the moon is that by which she subserves the wants of the navigator, and enables him to track his course over the pathless ocean. Of the two co-ordinates, Latitude and Longitude, that are needful to determine the position of a ship at sea (or of any standpoint upon the earth’s surface) the first is easily found, inasmuch as it is always equal to the altitude of the celestial pole at the place of observation. But the determination of the longitude has always been a difficult problem, and one upon which a vast amount of ingenuity has been expended. When it was first attacked it was soon discovered that the moon was the object of all others by which it could be most accurately and, all things considered, most readily determined. We must premise that the longitude of one place from another is in effect the difference between the local times at the two places, so that when we say that a place or a ship is, for instance, seven hours, twenty-four minutes, ten seconds, west of Greenwich, we mean that the time-o’-day at the place or ship is seven hours twenty-four minutes ten seconds earlier than that at Greenwich. Hence, finding the longitude at sea or at any place and moment means finding what time it is at Greenwich at that moment. Of course this could be most easily done if we could set a timekeeper at Greenwich and rely upon its keeping time during a long sea voyage; and this plan appeared so feasible that our Government long ago offered a prize of £20,000 for a timekeeper which would perform to a stated degree of accuracy after a certain sea voyage. One John Harrison did make such a timekeeper, that actually satisfied the conditions, and obtained the prize: and chronometers are now largely used for longitude, their construction having been brought to great perfection, especially in England, owing to a continuance (in a less liberal degree, however,) of Government inducement. But chronometers are not entirely to be relied on, even where several are carried, which in other than Government ships is rarely the case: recourse must be had to the heavenly bodies for check upon the timekeeper. And the moon is, as we have said, the body that best serves the requirements of the problem.
The lunar method for longitude amounts practically to this. The stars are fixed; the sun, moon, and planets move amongst them; the sun and planets with very slow rates of apparent motion, the moon with a very rapid one. If, then, it be predicted that at a certain instant of Greenwich time the moon will be a certain distance from a fixed star, and if the mariner at sea observes when the moon has that exact distance, he will know the Greenwich time at the instant of his observation.[20] The moon thus becomes to him as the hand of a timepiece, whereof the stars are the hour and minute marks, the whole being, as it were, set to Greenwich time. Then if he knows (which he does by other observations easily obtained) the local time at his ship, he can take the difference between the Greenwich time and his time, which difference is in fact his longitude from Greenwich. The requisite predictions of the distance of the moon from several fixed stars near her are given to the utmost exactness for every three hours of every day and night (when the moon can possibly be seen) in the navigator’s vade mecum, the “Nautical Almanac,” and from these given distances the navigator can, by a simple process of differencing, obtain the distance, and hence the Greenwich time, for any intermediate instant at which he may chance to make his observation. Whenever he can see the moon he can obtain Greenwich time. Of course the whole value of this method depends upon the exactitude of the predicted distances corresponding to the given Greenwich times. These distances are obtained by tables of the moon’s motions, which must be found from observations. The motions in question are of an intricacy almost past comprehension, on account of the disturbing forces to which the moon is subjected by the sun and planets. The powers of the profoundest mathematicians, from Newton downwards, have been severely exercised in efforts to group them into a theory, and represent them by tables capable of furnishing the requisite exact predictions of lunar positions for nautical purposes. Accurate observations of the moon’s place night after night have, from the dawn of this lunar method for longitude, been in urgent request by mathematicians for the purposes specified, and it was solely to procure these observations that the Observatory at Greenwich was established, and mainly for their continued prosecution (and for the stellar observations necessary for their utilization) that it is sustained. For two centuries the moon has been unremittingly observed at Greenwich, and the tables at present used for making the “Nautical Almanac” (those formed by Prof. Hansen) depend upon the observations there obtained. The work still goes on, for even now the degree of exactitude is not what is desired, and astronomers are looking forward with some interest to new lunar tables which were left complete by the late M. Delaunay, formerly the head of astronomy in France, based upon a theory which he evolved. This use of the moon is the grandest of all in respect of the results to which it has led.
Then, too, regarding the moon as a timekeeper, we must not forget the service that it renders in furnishing a division of time intermediate between the day—which is measured by the earth’s rotation—and the year, which is defined by the earth’s orbital revolution. Notwithstanding the survival of lunar reckoning in our religious services, we, in our time and country, scarcely need a moon to mark our months; but we must not forget that with many ancient people the moon was, and with some is still, the chief timekeeper, the calendars of such people being lunar ones, and all their events being reckoned and dated by “moons.” To us, however, the moon is of great service in this department by enabling us to fix dates to many historical events, the times of occurrence of which are uncertain, by reason of defective records or by dependence upon such uncertain data as “lives of emperors,” years of this or that king’s reign, or generations of one or another family. The moon now and then clears up a mystery, or decides a disputed point in chronology, by furnishing the accurate date of an ancient eclipse, which was a phenomenon that always inspired awe and secured for itself careful record. The chronologer is continually applying to the astronomer for the date and place of visibility of some total eclipse, of which he has found an imperfect record, veritable as to the fact, but dated only by reference to some year of a so-and-so’s reign, or by some battle or other historical occurrence. The eclipses that occurred near the time are then examined, and when one is found that tallies with recorded conditions in other respects (such as the time of day and the place of observation), its indisputable date becomes a starting-point from which the chronologer works backwards and forwards in safety. There is one famous eclipse—that predicted by Thales six centuries before Christ, which put an end to the battle between the Medes and Lydians by the terror its darkness created in both armies—which is most intimately associated with ancient chronology, and has been used to rectify a proximate date (the first year of Cyrus of Babylon) which forms the foundation of all Scripture chronology. Sacred and profane history alike are continually receiving assistance from the accurate dates which the moon, by having caused eclipses of the sun, enables the astronomer to fix beyond cavil or doubt.
The mention of eclipses reminds us, too, of the use which the moon has been in increasing, through them, our knowledge of the physical condition of the sun. If the moon had never intervened to cut off the blinding glare of the solar disc, we should have been to this day left to assume that the sun is all-contained by the dazzling globe that we ordinarily see. But, thanks to the moon’s intervention, we now know that the sun is by no means the mere naked sphere we should have suspected. Eclipses have taught us that it is surrounded by an envelope of glowing gases, and that it has a vast vaporous surrounding, beyond its glowing atmosphere, which appears to be composed of matter streaming away from the sun into surrounding space. With these discoveries still in their infancy, it is impossible to foresee the knowledge to which they will eventually lead, but they can hardly be barren of fruit, and whatever they ultimately teach will be so much insight gained into the sublimest problem that human science has before it—the determination of the source and maintaining power of the light and heat and vivifying agency of the sun. In according our thankful reflections to the moon for these revelations, we must not forget that, should there be inhabitants upon our neighbouring worlds, Mercury, Venus, and Mars, which have no satellites, they, the supposed inhabitants, can gain no such knowledge upon the surroundings of the ruler of the solar system. On the other hand, any rational being who may be supposed to dwell upon Saturn or Jupiter, would, through the intervention of their numerous moons, have, in the latter case especially, far more abundant opportunities of acquiring the knowledge in question than we have.
Finally, there is a use of the moon which touches us, author and reader, very closely. It has taught us of a world in a condition totally different from our own; of a planet without water, without air, without the essentials to life development, but rather with the conditions for life destruction; a planet left by the Creator—for wise purposes that we cannot fully know—as it were but half-formed, with all the igneous foundations fresh from the cosmical fire, and with its rough-cast surface in its original state, its fire and mould-marks exposed to our view. From these we have essayed to resolve some of the processes of formation, and thus to learn something of the cosmical agencies that are called forth in the purely igneous era of a planet’s history. We trust that we, on our part, have shown that the study of the moon may be a benefit not merely to the astronomer, but to the geologist; for we behold in it a mighty “medal of creation” doubtless formed of the same material and struck with the same die that moulded our earth; but while the dust of countless ages and the action of powerful disintegrating and denuding elements have eroded and obliterated the earthly impression, the superscriptions on the lunar surface have remained with their pristine clearness unsullied, every vestige sharp and bright as when it left the Almighty Maker’s hands. The moon serves no second-rate or insignificant service when it teaches us of the variety of creative design in the worlds of our system, and exalts our estimation of this peopled globe of ours by showing us that all the planetary worlds have not been deemed worthy to become the habitations of intelligent beings.
Reflections upon the uses of the moon not unnaturally lead our thoughts to some matters that may be regarded as abuses. These mainly take the form of superstitions, erroneous beliefs in the moon’s influence over terrestrial conditions, and occasionally of erroneous ideas upon the moon’s functions as a luminary. The first-mentioned are almost beneath notice, for they include such mythical suspicions as that the moon influences human sanity and other affections of mind and body; that the moon’s rays have a decomposing effect upon organic matter; that they produce blindness by shining upon a sleeper’s eyes; that the moon determines the hours of human death, which is supposed to occur with the change of the tide, etc. All such, having no foundation on fact, are put beyond our consideration. The third matter we have mentioned may also be dismissed in a very few words. The erroneous ideas upon the moon’s functions as a luminary, to which we allude, are those which are manifested by poets and painters, and even historians, who do not hesitate to bring the moon upon a scene in any form and at any time they please without reference to actual lunar circumstances. It is no uncommon thing to see, in a picture representing an evening scene, a moon introduced which can only be seen in the morning—a waning moon instead of a waxing one; and astronomical critics have, indeed, caught artists so far tripping as to put a moon in a picture representing some event that occurred upon a date when the moon was new, and therefore invisible. Writers take the same liberties very frequently. A newspaper correspondent, during the Franco-Prussian war, described the full moon as shining upon a scene of desolation on a particular night, when really there was no moon to be seen. One of the most flagrant cases of this kind, however, occurs in Wolfe’s ballad on “The death of Sir John Moore,” where it is written that the hero was buried “By the struggling moonbeam’s misty light.” But the interment actually took place at a time when the moon was out of sight. We mention these abuses of the moon in the hope of promoting a better observance of the moon’s luminary office. They who wish to bring the moon upon a scene, not knowing ipso facto that it was there, should first take the advice of Nick Bottom in the “Midsummer Night’s Dream,” and make sure of their object by consulting an almanac.
The second of the specified abuses to which the moon is subject refers to its supposed influence on the weather; and in the extent to which it goes this is one of the most deeply rooted of popular errors. That there is an infinitesimal influence exerted by the moon on our atmosphere will be seen from the evidence we have to offer, but it is of a character and extent vastly different from what is commonly believed. The popular error is shown in its most absurd form when the mere aspect of the moon, the mere transition from one phase of illumination to another, is asserted to be productive of a change of weather; as if the gradual passage from first quarter to second quarter, or from that to third, could of itself upset an existing condition of the atmosphere; or as if the conjunction of the moon with the sun could invert the order of the winds, generate clouds, and pour down rains. A moment’s reasoning ought to show that the supposed cause and the observed effect have no necessary connection. In our climate the weather may be said to change at least every three days, and the moon changes—to retain the popular term—every seven days; so that the probability of a coincidence of these changes is very great indeed: when it occurs, the moon is sure to be credited with causing it. But a theory of this kind is of no use unless it can be shown to apply in every case; and, moreover, the change must always be in the same direction: to suppose that the moon can turn a fine day to a wet one, and a wet day to a fine morrow indiscriminately, is to make our satellite blow hot and cold with the same mouth, and so to reduce the supposition to an absurdity. If any marked connection existed between the state of the air and the aspect of the moon, it must inevitably have forced itself unsought upon the attention of meteorologists. In the weekly return of Births, Deaths, and Marriages, issued by the Registrar-General, a table is given, showing all the meteorological elements at Greenwich for every day of the year, and a column is set apart for noting the changes and positions of the moon. These reports extend backwards nearly a quarter of a century. Here, then, is a repertory of data that ought to reveal at a glance any such connection, and would certainly have done so had it existed. But no constant relation between the moon columns and those containing the instrument readings has ever been traced. Our meteorological observatories furnish continuous and unbroken records of atmospheric variations, extending over long series of years: these afford still more abundant means for testing the validity of the lunar hypothesis. The collation has frequently been made for special points in the inquiry, and certainly some connection has been found to obtain between certain positions of the moon in her orbit and certain instrumental averages; but so small are the effects traceable to lunar influence, that they are almost inappreciable among the grosser irregularities that arise from other and as yet unexplained causes.
The lunar influences upon our atmosphere most likely to be detected are those of a tidal character, and those due to the radiation of the heat which the moon receives from the sun. The first would be shown by the barometer, which may be called an “atmospheric tide gauge.” Some years ago Sir Edward Sabine instituted a series of observations at St. Helena, to determine the variations of barometric indications from hour to hour of the lunar day. The greatest differences were found to occur between the times when the moon was on the meridian, and when it was six hours away from the meridian; in other words, between atmospheric high tide and low tide. But the average of these differences amounted only to the four-hundredth part of an inch on the instrument’s scale; a quantity that no weather observer would heed, that none but the best barometers would show, and that can have no perceptible effect on weather changes. The distance of the moon from the earth varies, as is well known, in consequence of the elliptical form of her orbit: this variation ought also to produce an effect upon the instrument’s indications; but Colonel Sabine’s analysis showed that it was next to insensible; the mean reading at apogee differing from that at perigee by only the two-thousandth part of an inch. Schubler, a German meteorologist, had arrived at similarly negative results some years previously. Hence it appears that the great index of the weather is not sensibly affected by the state of the moon: the conclusion to be drawn with regard to the weather itself is obvious enough. As regards the heat received from the moon, we know, from the recent experiments of Lord Rosse in England, and Marie Davy in France, elsewhere alluded to, that a degree of warmth appreciable to the highly sensitive thermopile is exerted by the moon upon the earth near to the time of full moon, when the sun’s rays have been pouring their unmitigated heat upon the lunar surface continuously for fourteen days. And as it is improbable that the whole of the heat sent earthwards from the moon reaches the earth’s surface, we must infer that a considerable amount is absorbed in the higher atmosphere, and does work in evaporating the lighter clouds and thinning the denser ones. The effect of this upon the earth is to facilitate the radiation of its heat into space, and so to cool the lower atmospheric strata. And this effect has been shown to be a veritable one by an exhaustive tabulation of temperature records from various observatories, which was undertaken by Mr. Park Harrison. The general conclusion from these was, that the temperature at the earth’s surface is lower by about 2½ degrees at moon’s last quarter than at first quarter; the paradoxical result being what would naturally follow from the foregoing consideration. The tendency of the full moon to clear the sky has been remarked by several distinguished authorities, to wit, Sir John Herschel, Humboldt, and Arago; and in general the clearing may be accepted as a meteorological fact, though in one case of close examination it has been negatived. It cannot be doubted that a full moon sometimes shows a night to be clear that would in the absence of the moon be called cloudy.
When close comparisons are made between the moon’s positions and records of rain-fall and wind-direction, dim indications of relation exhibit themselves, which may be the feeble consequences of the change of temperature just spoken of; but in every case where an effect has been traced it has been of the most insignificant kind, and no apparent connexion has been recognized between one effect and another. Certainly there is nothing that can support the extensive popular belief in lunar influence on weather, and nothing that can modify the conviction that this belief as at present maintained is an absurd delusion. Yet its acceptance is so general, and runs through such varied grades of society, that we have felt it our duty to dwell upon it to the extent that we have done.