THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH AND MAN.
Apart from the recondite functions of the moon considered as one of the
interdependent members of the solar family, into which it would be
beyond our purpose to inquire, there are certain means by which it
subserves human interests and ministers to the wants of civilized man to
which we deem it desirable to call attention, especially as some of them
are not so self-apparent as to have attracted popular attention.
The most generally appreciated because the most evident of the uses of
the moon is that of a luminary. Popular regard for it is usually
confined to its service in that character, and in that character poets
and painters have never tired in their efforts to glorify it. And
obviously this service as a “lesser light” is sufficiently prominent to
excite our warmest admiration. But moonlight is, from the very
conditions of its production, of such a changeable and fugitive nature,
and it affords after all so partial and imperfect an alleviation of
night’s darkness, that we are fain to regard the light-giving office of
the moon as one of secondary importance. Far more valuable to mankind in
general, so estimable as to lead us to place it foremost in our category
of lunar offices, is the duty which the moon performs in the character
of a sanitary agent. We can conceive no direful consequences that would
follow from a withdrawal of the moon’s mere light; but it is easy to
imagine what highly dangerous results would ensue if the moon ceased to
produce the tides of the ocean. Motion and activity in the elements of
the terraqueous globe appear to be among the prime conditions in
creation. Rest and stagnation are fraught with mischief. While the sun
keeps the atmosphere in constant and healthy circulation through the
agency of the winds, the moon performs an analogous service to the
waters of the sea and the rivers that flow into them. It is as the chief
producer of the tides—for we must not forget that the sun exercises
_its_ tidal influences, though in much lesser degree—that we ought to
place the highest value on the services of the moon: but for its aid as
a mighty scavenger, our shores, where rivers terminate, would become
stagnant deltas of fatal corruption. Twice (to speak generally) a day,
however, the organic matter which rivers deposit in a decomposing state
at their embouchures is swept away by the tidal wave; and thus, thanks
to the moon, a source of direful pestilence is prevented from arising.
Rivers themselves are providentially cleansed by the same means, where
they are polluted by bordering towns and cities which, from the nature
of things, are sure to arise on river banks; and it seems to be also in
the nature of things that the river traversing a city must become its
main sewer. The foul additions may be carried down by the stream in its
natural course towards the ocean, but where the river is large there
will be a decrease in velocity of the current near the mouth or where it
joins the sea, thus causing partial stagnation and consequent deposition
of the deleterient matters. All this, however, is removed, and its
inconceivable evils are averted by our mighty and ever active “sanitary
commissioner,” the moon. We can scarcely doubt that a healthy influence
of less obvious degree is exerted in the wide ocean itself; but,
considering merely human interests, we cannot suppress the conviction
that man is more widely and immediately benefited by this purifying
office of the moon than by any other.
But the sanitary service is not the only one that the moon performs
through the agency of the tides. There is the work of tidal transport to
be considered. Upon tidal rivers and on certain coasts, notwithstanding
wind and the use of steam, a very large proportion of the heavy
merchandize is transported by that slow but powerful “tug” the
flood-tide; and a similar service, for which, however, the moon is not
to be entirely credited, is done by the down-flow of the ebb-tide. Large
ships and heavily-laden rafts and barges are quietly taken in tow by
this unobtrusive prime mover, and moved from the river’s mouth to the
far-up city, and from wharf to wharf along its banks; and a vast amount
of mechanical work is thus gratuitously performed which, if it had to be
provided by artificial means, would represent an amount of money value
which for such a city as London would have to be counted by thousands,
possibly millions, of pounds yearly. For this service we owe the moon
the gratitude that we ought to feel for a direct pecuniary benefactor.
In the existing state of civilization and prosperity, we do not,
however, utilize the power of the tides nearly to the extent of their
capabilities. Our coal mines, rich with “the light of other days”—for
coal was long ago declared by Stevenson to be “bottled sunshine”—at
present furnish us with so abundant a supply of power-generating
material that in our eagerness to use it upon all possible occasions we
are losing sight, or putting out of mind, many other valuable prime
movers, and amongst them that of the rise and fall of the waters, which
can be immediately converted into any form of mechanical power by the
aid of tide-mills. Such mills may be found in existence here and there,
but for the present they are generally out-rivalled by the steam-engine
with all its conveniences and adaptabilities; and hence they have not
shared the benefits of that inventive ingenuity which has achieved such
wonders of mechanical appliance while steam has been in the ascendant.
But it must be remembered that in our extravagant use of coal we are
drawing from a bank into which nothing is being paid. We are consuming
an exhaustive store, and the time must come when it will be needful to
look around in quest of “powers that may be.” Then an impetus may be
given to the application of the tides to mechanical purposes as a prime
mover.[19] For the people of the British Islands the problem would have
an especial importance, viewing the extent of our seaboard and the
number of our tidal rivers. The source of motion that offers itself is
of almost incalculable extent. There is not merely the onward flowing
motion of streams to be utilized, but also the _lift_ of water, which,
if small in extent, is stupendous in amount; and within certain limits
it matters little to the mechanician whether the “foot-pounds” of work
placed at his disposal are in the form of a great mass lifted to a small
height or a small mass lifted to a great height. There is no reason
either why the utilization of the tides should be confined to rivers.
The sea-side might well become the circle of manufacturing industry, and
the millions of tons of water lifted several feet twice daily on our
shores might be converted, even by schemes already proposed, to furnish
the prime movement of thousands of factories. And we must not forget how
completely modern science has demonstrated the inter-convertibility of
all kinds of force, and thus opened the way for the introduction of
systems of transporting power that, in such a state of things as we are
for the moment considering, might be of immense benefit. Gravity, for
instance, can be converted into electricity; and electricity gives us
that wonderful power of transmitting _force_ without transmitting (or
even moving) _matter_, which power we use in the telegraph, where we
generate a force at one end of a wire and _use_ it to ring bells or
deflect needles at the other end, which may be thousands of miles away.
What we do with the slight amount of force needful for telegraphy is
capable of being done with any greater amount. A tide-mill might convert
its mechanical energy by an electro-magnetic engine, and in the form of
electricity its force could be conveyed inland by proper wires and there
reconverted back to mechanical or moving power. True, there would be a
considerable loss of power, but that power would cost nothing for its
first production. Another means ready to hand for transporting power is
by compressed air, which has already done good service; another is the
system so admirably worked out by Sir W. Armstrong, of transmitting
water-power through the agency of an “accumulator,” now so generally
used at our Docks and elsewhere, for working cranes and such other uses.
And as the whole duty of the engineer is to _convert_ the forces of
nature, there is a rich field open for his invention, and upon which he
may one day have to enter, in adapting the pulling force of the moon to
his fellow man’s mechanical wants through the intermediation of the
tides.
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.