THE MOON AND HER SURFACE
As the sun has always reigned as king of day, so is the moon queen of
night. Observation of her phases, now waxing, now waning, with her
stately motion always eastward among the stars, began with the earliest
ages. Often when near the full she must have been seen herself eclipsed,
and much more rarely the occurrence of total eclipses of the sun are
certain to have suggested the moon's intervention between earth and sun,
shutting off the sunlight completely, because these eclipses never took
place except when the moon was in the same part of the sky with the sun.
If we watch the nightly march of the moon, we shall find that she
travels over her own breadth in about an hour's time. By using a
telescope on the stars just eastward or to the left of her, she will now
and then be seen to pass between us and a star--on very rare occasions a
planet--extinguishing its light with great suddenness, the most nearly
instantaneous of all phenomena in nature. Draw a line connecting the
cusps, or horns of the lunar crescent, and then a line eastward at right
angles to this, and it will show the direction of the moon's own motion
in its orbit round the earth quite accurately.
As the phase advances, note the inside edge of the advancing crescent:
this will be quite rough and jagged, compared to the outside edge which
is the moon's real contour and relatively very smooth. The position of
the inside curve will change from night to night, and it marks the line
of sunrise on the moon during the fortnight elapsing between new moon
and full; while from full through last quarter and back to new moon,
this advancing line marks the region of sunset on the moon. The general
shape of this line is never a circle but always elliptical, and
astronomers call it the terminator. All along the terminator, sunlight
strikes the lunar surface at a small angle, whether near sunrise or
sunset; so that owing to the mountains and other high masses of the
moon's surface, the terminator is always a more or less jagged and
irregular line.
Onward from new moon toward full the horns of the crescent are always
turned upward or eastward. When the general line of the terminator
becomes a straight line from cusp to cusp, the moon is said to have
reached first quarter or quadrature. Onward toward full the terminator
will be seen to bend the other way, and in about a week's time it will
have merged itself with the moon's limb. The moon is then said to be
full. Afterward the phase phenomena recur in the reverse order, with
third quarter midway between full and new moon again; the phase of the
moon being called gibbous all the way from first quarter to third
quarter, except when exactly full.
As we know that the moon is, like the earth, a nonluminous body, and
shines only by virtue of the sunlight falling upon it, clearly an entire
half of the moon's globe must be perpetually illumined by sunlight. The
varying phases then are due simply to that part of the illuminated
hemisphere which is turned toward us. New moon is entirely invisible
because the sunward hemisphere is turned wholly away from us, while at
full moon we see the lunar disk complete because we are on the same side
of the moon that the sun is and practically in line with both sun and
moon.
If we could visit the moon, we should see the earth in exactly
complementary phase. At new moon here we should be enjoying full earth
there, and full moon here would be coincident with new or dark earth
there. The narrow crescent of new moon here would be the period of
gibbous earth there; and it is the reflection of sunlight from this
gibbous earth which illuminates the part of the moon but faintly seen at
this time, popularly known as the "old moon in the new moon's arms." Its
greater visibility at some times than at others is due to greater
prevalence of clouded area in the reflecting regions of the earth turned
toward the moon, and the higher reflective power of clouds than that
possessed by mere land and water.
As the moon goes all the way round the sky every month, the same as the
sun does in a year, and travels in nearly the same path, clearly it must
also go north and south every month as the sun does. So in midsummer
when the sun runs high upon the meridian, we expect to find full moons
running low, and likewise in midwinter the full moon always runs high,
as almost everyone has sometimes or other noticed.
This eastward or true orbital motion of the moon is responsible for
another relation which soon comes to light when we begin to observe the
moon; and that is the later hour of rising or setting each night. Our
clock time is regulated by the sun, which also is moving eastward about
1 deg. daily, or twice its own breadth. So the moon's eastward gain on the
sun amounts to about 12 degrees daily, and one degree being equal to 4
minutes, the retarded time of moonrise or moonset each day amounts to
very nearly 50 minutes on the average; though sometimes the delay will
be less than a half hour and at other times it will exceed an hour and a
quarter. The season of least retardation of rising of the full moon is
in the autumn, and so the moon that falls in late September or October
is known as the Harvest moon, and the next succeeding full moon is
called the Hunter's moon.
Lunation is a term sometimes given to the moon's period from any
definite phase round to the same phase again. Its length is the true
period of the moon's revolution once around the earth, from the sun all
the way round till it overtakes the sun again. The synodic period is
another name for lunation, and its true length is 29 and one-half days,
or very accurately 29 d. 12 h. 44 m. 2.7 s. as calculated by astronomers
with great exactness from many thousand revolutions of the moon. But if
we want the true period of the moon round the earth as referred to a
star, it is much shorter than this, amounting to only 27 days and nearly
one-third. This is called the moon's sidereal period of revolution,
because it is the time elapsed while she is traveling eastward from a
given star around to coincidence with the same star again.
If we study the moon's path in the sky more critically, we shall find
that it does not quite follow the ecliptic, or the sun's path, but that
twice each month she deviates from the ecliptic, once to the north and
once to the south of it, by roughly ten times her own breadth. More
accurately this angle is 5 deg.8'40", an almost invariable quantity, and it
is therefore known as an astronomical constant, or the inclination of
the moon's orbit to the ecliptic. So the moon's orbit must intersect the
ecliptic, and as both are great circles in the sky, the points of
intersection are known as the moon's nodes, one ascending and the other
descending, and the nodes are 180 degrees apart.
The figure of the moon's orbit is not circular, although it deviates
only slightly from that form. But like the paths of all other satellites
round their primary planets, and of the planets themselves round the
sun, the moon's orbit is also an ellipse. The distance of the moon's
center from the earth's center is therefore perpetually changing; the
point of nearest approach is called perigee, and that of farthest
recession, apogee.
The moon's distance from the earth is easier and simpler to be
ascertained than that of any other heavenly body, because it is the
nearest. An outline of the method of finding this distance is not
difficult to present; and it resembles in every particular the method a
surveyor uses to find the distance of some inaccessible point which he
cannot measure directly. Up and down a stream, for example, he measures
the length of a line, and from each end of it he measures the angle
between the other end of the line and the object on the opposite side of
the stream whose distance he wishes to find out. Then he applies the
science of trigonometry to these three measures, two of angles and one
the length of the side or base included between them, and a few minutes'
calculation gives the distance of the inaccessible object from either
end of the base line.
Now in like manner, to transfer the process to the sky, let the two ends
of the base be represented by two astronomical observatories, for
example, Greenwich in the northern hemisphere and Cape Town in the
southern. The base line is the chord or straight line through the earth
connecting the two observatories, and we know the length of this line
pretty accurately, because we know the size of the earth. The angles
measured are somewhat different from those in the terrestrial example,
but the process amounts to the same thing because the astronomers at the
two observatories measure the angular distance of the center of the moon
from the zenith, each using his own zenith at the same time; and the
same science of trigonometry enables them to figure out the length of
any side of the triangles involved. The side which belongs to both
triangles is the distance from the center of the earth to the center of
the moon, and the average of many hundred measures of this gives 238,800
miles, or about ten times the distance round the equator of the earth.
We have said that the orbit in which the moon travels round the earth is
practically a circle, but the earth's center is found not at the center
of this orbit, but set to one side, or eccentrically, so that the
distance spanning the centers of the two bodies is sometimes as small as
221,610 miles at perigee, and 252,970 miles at apogee. The moon's speed
in this orbit averages rather more than half a mile every second of
time--more accurately 3,350 feet a second, or 2,290 miles per hour.
Once the moon's distance is known, its size or diameter is easy to
ascertain. An angular measure is necessary, of course, that of the angle
which the disk of the moon fills as seen from the earth. There are many
types of astronomical instruments with which this angle can be measured,
and its value is something more than half a degree (31'7"). The moon's
actual diameter figures out from this 2,163 miles; and it would
therefore require nearly fifty moons merged in one to make a ball the
size of the earth.
Still, no other planet has a satellite as large in proportion to its
primary as the moon is in relation to the earth. But the materials that
compose the moon have less than two-thirds the average density of those
that make up the earth, so that eighty-one moons fused together would be
necessary to equal the mass or weight of the earth. If we figure out the
force of attraction of the moon for bodies on its surface, we find it
equals about one-sixth that of the earth. Athletes could perform some
astounding feats there--miracles of high jump and hammer-throw.
Our interest in the moon's physical characteristics never wanes. Her
nearness to us has always fascinated astronomer and layman alike. Early
users of the telescope were readily led into error regarding the general
characteristics of the lunar surface; and it is easy to see why they
thought the smooth level planes must be seas, and gave them names to
that effect which persist to-day, as Mare Crisium, Mare Serenitatis and
so on. We may be sure that no water exists on the moon's surface,
although some astronomers think that solid water, as ice or snow, may
still exist there at a temperature too low for appreciable evaporation.
Perhaps water, seas, and oceans were once there, but their secular
dissemination and loss as vapor have gone on through the millions of
millions of years till even the moon's atmosphere appears to have
vanished completely. At least there is much better evidence of absence
of atmosphere on the moon than of its presence--not enough at any rate
to equal a thousandth part of the barometric pressure that we have at
the earth's surface. Frequent observations of stars passing behind the
moon in occultation have satisfied astronomers on this point.
We often say of the brilliant full moon, it is as bright as day. The
photometer or instrument for accurate comparison of lights, their amount
and intensity, tells a different story. Indeed, if the entire dome of
the sky were filled with full moons, we should be receiving only
one-eighth of the light the sun gives us, and it would require more than
600,000 average full moons to equal the light radiation of the sun. Heat
from the moon, however, is quite different. Early attempts to measure it
detected none at all, but with modern instruments there is little
trouble in detecting heat from the moon, though measurement of it is not
easy.
Much of the moon's heat is sun heat, directly reflected from the moon,
as sunlight is, but most of it is due to radiation of solar heat
previously absorbed by the materials of the lunar surface. The actual
temperature of the moon's surface suffers great variation. A fortnight's
perpetual shining of the sun upon the lunar rocks would certainly heat
them above the temperature of boiling water, if the moon had an
atmosphere to conserve and store this heat; but the entire absence of
such an air blanket probably permits the sun's heat to be radiated away
nearly as fast as it is received, leaving the temperature at the surface
always very low.
What physical influences the moon really has upon the earth must be very
slight, barring the tides. But there is little hope of getting people
generally to take that view, because the moon appears to be the planet
of the people, and opinion that the moon controls the weather, for
instance, amounts with them to practical certainty. More than likely all
these notions are but legitimate survivals of superstition and
astrology. In addition to the tides, our magnetic observatories reveal
slight disturbances with the swinging of the moon from apogee to perigee
and back; but long series of weather observations have been faithfully
interrogated, with negative or contradictory results. If one believes
that the moon's changes affect the weather, it is easy to remember
coincidences, and pass over the many times when no change has taken
place. The moon changes pretty frequently anyhow. As Young well puts it:
"A change of the moon necessarily occurs about once a week.... _All_
changes, of the weather for instance, must therefore occur within three
and three-fourth days of a change of the moon, and fifty per cent of
them ought to occur within forty-six hours of a change, even if there
were no causal connection whatever."
When we turn to the strongly diversified surface of the moon itself, we
find much to rivet the attention, even with slender optical aid.
Everyone wants to know how near the telescope, the biggest possible
telescope, brings the moon to us. That will depend on many things, first
of all on the magnifying power of the eyepiece employed on the
telescope, and eyepieces are changed on telescopes just as they are on
microscopes, though not for the same reasons. The theoretical limit of
the power of a telescope is usually considered as 100 for each inch of
diameter or aperture of the object glass.
A 40-inch telescope, as that of the Yerkes Observatory, the largest
refracting telescope in existence, should bear a magnifying power not to
exceed 4,000. But this limit is practically never reached, one-half of
it or fifty to the inch of aperture being a good working limit of power,
even under exceptional conditions of steadiness of atmosphere. If we
reduce the effective distance of the moon from 240,000 miles to 100
miles, that is about the utmost that can be expected. But even at that
distance we can make out only landscape details, nothing whatever like
buildings or the works of intelligence.
The larger relations of light and shade, so obvious to the naked eye on
the moon, vanish on looking at it with the telescope, but we are at once
captivated by the novel character of the surface and the seemingly great
variety of detail that is clearly visible. As soon as the new moon comes
out in the west, one may begin to gaze with interest and watch the
terminator or sunrise line gradually steal over the roughened surface,
bringing new and striking craters into view each night. Around the time
of quarter moon, or a little past it, is one of the best times for
telescopic views of the moon, because the huge craters, Tycho and
Copernicus, are then in fine illumination. Close to the phase of full
moon is never a good time, because there are no shadows of the rough
surface then, and its entire structure seems to be quite flat and
uninteresting, except for the streaks or rills which radiate from Tycho
in every direction, and are the only lunar features that are best seen
near full.
In a broad, general way, the moon's surface, if compared with the
earth's, differs in having no water. Our extensive oceans are replaced
there by smooth, level plains which were at first thought to be seas and
so named. There are ten or twelve of them in all. Then we find mountain
ranges, so numerous on the earth, relatively few on the moon. Those that
exist are named, in part, for terrestrial mountain ranges, as the Alps,
Caucasus, and the Apennines.
But the nearly circular crater, a relatively rare formation on the
earth, is seen dotted all over the moon in every size, from a fraction
of a mile in diameter up to sixty, seventy, and in extreme cases a
hundred miles. No mere description of plains and mountains and craters
affords an adequate idea of the moon's surface as it actually is; a
telescopic view is necessary, or some of the modern photographs which
give an even better notion of the moon than any telescopic view. Many of
the lunar craters are without doubt volcanic in origin, others seem to
be ruins of molten lakes. Many thousands of the smaller ones appear as
if formed by a violent pelting of the surface when semi-plastic, perhaps
by enormous showers of meteoric matter. More than 30,000 craters cover
the half of the lunar surface visible from the earth, and hundreds of
them are named for philosophers and astronomers.
Measurement of the height of lunar mountains has been made in numerous
instances, especially when their shadows fall on plains or surfaces that
are nearly level, so that the length of the shadow can be measured. In
general, the height of lunar peaks is greater than that of terrestrial
peaks, owing probably to the lesser surface gravity on the moon. About
forty lunar peaks are higher than Mont Blanc.
Most astronomers regard it as certain that no changes ever take place on
the moon; probably no very conspicuous changes ever do. Some, however,
have made out a fair case for comparatively recent changes in surface
detail. Extreme caution is necessary in drawing conclusions, because the
varying changes of illumination from one phase to another are themselves
sufficient to cause the appearance of change. At intervals of a double
lunation, equal to fifty-nine days, one and one-half hours, the
terminator goes very nearly through the same objects, so that the
circumstances of illumination are comparable. In Mare Serenitatis the
little crater named Linne was announced to have disappeared about a half
century ago; subsequently it became visible again and other minor
changes were reported, perhaps due to falling in of the walls of the
crater.
If one were to visit the moon, he must needs take air and water along
with him, as well as other sustenance. No atmosphere means no diffused
light; we could see nothing unless the sun's direct rays were shining
upon it. Anyone stepping into the shadow of a lunar crag would become
wholly invisible. No sound, however loud, could be heard; sound in fact
would become impossible. A rock might roll down the wall of a lunar
crater, but there would be no noise; though we should know what had
happened by the tremor produced. So slight is gravity there that a good
ball player might bat a baseball half a mile or more. Looking upward,
all the stars would be appreciably brighter than here, and visible
perpetually in the daytime as well as at night.
If one were to go to the opposite side of the moon, he would lose sight
of the earth until he came back to the side which is always turned
toward the earth. Even then the earth would never rise and set at any
given place, as the moon does to us, but would remain all the time at
about the same height above the lunar horizon. The earth would go
through all the phases that the moon shows to us here, full earth
occurring there when it is new moon here. Our globe would appear to be
nearly four times broader than the moon seems to us. Its white polar
caps of ice and snow, its dark oceans, and the vast cloud areas would be
very conspicuous. Faint stars, the zodiacal light, and the filmy solar
corona would be visible, probably even close up to the sun's edge; but
although his rays might shine upon the lunar rocks without intermission
for a fortnight, probably they would still be too cold to touch with
safety. On the side of the moon turned away from the sun, the
temperature of the moon's surface would fall to that of space, or many
hundred degrees below zero.