beautiful color effects, such as halos of various types around
sun or moon, vertical light pillars, circumzenithal arcs, and
parhelia--"sun-dogs"--or paraselenæ--"moon-dogs"--which are luminous
spots at equal altitudes with sun or moon--one to the left the other
to the right, at an angular distance of 22°.
The most usual form of halo is that of 22° radius. This is a luminous
ring of light surrounding sun or moon, with the inner edge red and
sharply defined and the spectral colors proceeding outward in order;
red is frequently the only color visible, the remainder of the ring
appearing whitish. Since the halo is produced by refraction of light
by ice-crystals, which exist in clouds of a certain type gathering at
high altitudes, it is always a very good indicator of an approaching
storm.
Coronas are luminous rings showing the spectral colors in the
reverse order, that is, with the inner edge blue instead of red.
They are usually of very small radius, scarcely two degrees, closely
surrounding sun or moon and are produced--not by refraction--but
by _diffraction_ or a bending aside of the rays as they pass
between--without entering--very small drops of water in clouds. As in
the case of refraction, the red rays are turned from their course the
least and the violet rays the most.
Many of these phenomena--halos, luminous spots, vertical pillars and
arcs of light may, at times, be seen simultaneously, when clouds of
ice-crystals are forming around the sun or moon. They then present a
very complex and beautiful outline of luminous circles, arches and
pillars that have a mysterious and almost startling appearance when
the cause is not clearly understood.
We have found then that sunlight is made up of rays of many
different wave-lengths and colors and that the atmosphere acts upon
these rays in various ways. It reflects them or turns them back
on their course; it refracts them as they pass through the gases
of which the atmosphere consists, or through the water-vapor and
ice-crystals suspended in it, thus sifting out and dispersing the
rays of different colors and wave-lengths and producing beautiful
color effects; it _diffracts_ them or bends them aside as they pass
between the fine dust particles and small drops of water in the air,
again sifting out the rays of different colors and producing color
effects similar to those produced by refraction; it also scatters
and disperses, through the action of the molecules of oxygen and
nitrogen in the upper strata, the blue and violet rays of shorter
wave-length and thus produces the blue color and brightness of the
sky; it produces beautifully colored auroral streamers and curtains
and rays of light through the electrical discharges resulting when
the rarefied gases in the upper air are bombarded by electrified
particles shot forth from the sun.
It is our atmosphere, then, that we have to thank for all these
beautiful displays of color that delight our eyes and give pleasure
to our existence, as well as for the very fact of our existence upon
a planet that without its presence would be an uninhabitable waste,
covered only with barren rocks.
XXVI
KEEPING TRACK OF THE MOON
Of all celestial objects the nearest and most familiar is our
satellite, the moon. Yet the mistakes and blunders that otherwise
intelligent persons frequently make when they refer to the various
aspects of the moon are quite unbelievable.
Who has not read in classics or in popular fiction of crescent moons
riding high in midnight skies, of full moons rising above western
cliffs or setting beyond eastern lakes? Who has not seen the moon
drawn in impossible positions with horns pointing toward the horizon,
or a twinkling star shining through an apparently transparent moon?
Careful observation of the moon in all its various phases and at
different seasons is the best method to be used in acquiring a
knowledge of the elementary facts regarding the motion of the moon
through the heavens from day to day, but that requires that one be up
often after midnight and in the early hours preceding dawn and so it
is that we feel so hazy in regard to what happens to the moon after
it has passed the full.
A few fundamental rules can be easily acquired, however, and these
will enable us to locate the moon in the right quarter of the
heavens at any time of the day or night when we know its phase and
the approximate position of the sun at the same instant, and thus we
may avoid some of the most obvious blunders that are made in dealing
with the general aspect of the moon at any given time.
As can be verified by direct observation, the moon is always moving
continually eastward. Since it makes a complete revolution around
the earth from new moon back to new moon again in a little less than
thirty days, it passes over about twelve degrees a day (360° divided
by 30), on the average, or one-half a degree an hour, which is about
the angular extent of its own diameter. Therefore every hour the
moon moves eastward a distance equal to its own diameter. This is of
course only approximate as the moon moves more rapidly in some parts
of its orbit than in others.
In addition to its real eastward motion the moon shares the apparent
daily westward motion of all celestial objects which is due to the
daily rotation of the earth on its axis in the opposite direction.
That is, the moon, as well as the sun, stars and planets, rises in
the east and sets in the west daily. On account of its continuous
eastward motion, however, the moon rises later every night, on
the average about fifty minutes, though the amount of this daily
retardation of moon-rise varies from less than half an hour to
considerably over an hour at different seasons of the year and in
different latitudes. In the course of a month then the moon has risen
at all hours of the day and night and set at all hours of the day
and night.
It might seem unnecessary to emphasize the fact that the moon always
rises in the east were it not that the astronomer occasionally meets
the man who insists that he has at times seen the moon rise in the
west.
To be sure the new crescent moon first becomes visible above the
western horizon shortly after sunset though it rises in the east
the morning of the same day shortly after sunrise. As is also true
of the sun the exact point on the horizon where the moon rises or
sets varies from day to day and from season to season. In one month
the moon passes over very nearly the same path through the heavens
that the sun does in one year, for the moon's path is inclined only
five degrees to the ecliptic or apparent path of the sun through the
heavens. It can never pass more than 28-1/2° (23-1/2° + 5°) south of
the celestial equator, nor more than 28-1/2° north of it. It has a
slightly greater range in altitude than the sun, therefore. North of
28-1/2° north latitude it always crosses the meridian south of the
zenith and below 28-1/2° south latitude it crosses the meridian north
of the zenith. In tropical regions the moon sometimes passes north of
the zenith, sometimes south, or again directly through the zenith.
Since the full moon is always diametrically opposite to the sun it
passes over nearly the same part of the heavens that the sun did six
months before. In winter then when the sun is south of the equator
the moon "rides high" at night north of the equator and, vice versa,
in summer when the sun is north of the equator the full moon "rides
low" south of the equator. In winter then we have more hours of
moonlight than we have in summer. This may be of no great advantage
in mid-latitudes but we may imagine what a boon it is to the
inhabitants of the Arctic and Antarctic regions to have the friendly
moon above the horizon during the long winter months when the sun is
never seen for days at a time.
At time of "new" moon the moon lies directly between us and the
sun, but ordinarily passes just to the north or south of the
sun since its orbit is inclined five degrees to the ecliptic or
plane of the earth's orbit. If the moon's path lay exactly in the
ecliptic we would have an eclipse of the sun every month at new
moon and an eclipse of the moon two weeks later at full moon. Now
the moon crosses the ecliptic twice a month, the points of crossing
being called the nodes of its orbit, but only twice a year is the
moon nearly enough in line with the sun at the time it crosses to
cause eclipses. Every year, then, there are two "eclipse seasons,"
separated by intervals of six months, when the moon is in line with
the sun at or close to the point where it crosses the ecliptic; then
and only then can solar and lunar eclipses occur. The solar eclipses,
of course, will occur when the moon is new, that is, when the moon
passes directly between the earth and the sun and throws its shadow
over the earth; and the lunar eclipses two weeks later when the earth
passes between the sun and moon and throws its shadow over the face
of the moon.
Probably there is no astronomical subject that has been more
generally misunderstood than that of solar and lunar eclipses. It
is well to remember that solar eclipses can only occur at time of
new moon and lunar eclipses only at the time of full moon; and at
the time of eclipses, whether lunar or solar, the moon is at or near
its nodes, the points where its orbit crosses the ecliptic. There
are always at least two solar eclipses every year and there may be
as many as five. There are years when there are no lunar eclipses,
though ordinarily both solar and lunar eclipses occur every year,
some partial others total.
The moon shines only by reflected sunlight. It is of itself a solid,
dark body with its day surface intensely hot and its night surface
intensely cold, a world of extreme temperatures.
At new moon all of the night side of the moon is turned toward us, at
full moon all of the day side. At other phases we see part of the day
side and part of the night side and the illuminated side of the moon
is always the side that is towards the sun. Failure to observe this
simple rule leads to many grievous blunders in depicting the moon.
At the time of new moon the moon, moving continually eastward, passes
north or south of the sun from west to east except when it passes
directly in front of the sun, causing eclipses. A day or so later
the waxing crescent moon or the "new moon," as it is popularly
called, becomes visible low in the west immediately after sunset. The
moon is now east of the sun and will remain east of the sun until the
time of full moon. During the period from new moon to full moon it
will, therefore, rise after the sun and set after the sun. The waxing
crescent moon will not be visible in the morning hours because,
inasmuch as it rises after the sun, it is lost to view in the sun's
brilliant rays. Nevertheless, it follows the sun across the sky and
becomes visible in the west as soon as the sun has disappeared below
the western horizon. The thin illuminated crescent has its horns
or cusps turned _away_ from the point where the sun has set. The
horns of the crescent can never point _toward_ the horizon since the
illuminated side of the moon is always turned toward the sun whether
the sun is above or below our horizon.
As hour by hour and day by day the moon draws farther eastward and
increases its angular distance from the sun, more and more of the
illuminated side becomes visible; the crescent increases in width and
area and the moon appears higher in the western sky each night at
sunset.
Usually about seven and a fraction days after the date of new moon
the moon completes the first quarter of its revolution around
the earth. The period from one phase to the next is variable and
irregular, being sometimes less than seven days and at other times
more than eight days, since the moon does not move at a uniform rate
in different parts of its orbit.
When the moon has completed the first quarter of a revolution it
is ninety degrees east of the sun and presents the phase known as
"half-moon" since half of the surface that is turned toward the earth
is illuminated and half is in darkness. It is said to be "at the
first quarter." The illuminated half is of course the western half
because the sun is to the west of the moon. The half moon is near the
meridian at sunset and sets near midnight. Up to the first quarter,
then, the moon is a crescent in the western sky during the first part
of the night and should never be represented as east of the meridian
or near the meridian at midnight.
After the moon has passed the first quarter and before it is full
more than half of the side turned toward the earth is illuminated and
it is in the "gibbous" phase. It is still the western limb that is
fully illuminated. The moon is now east of the meridian at sunset and
it crosses the meridian before midnight and sets before sunrise. All
who are abroad during the first half of the night find this phase of
the moon more favorable to them than the gibbous phase following full
moon.
The moon now being above the horizon at sunset is visible
continuously from sunset to midnight but sets some time during the
second half of the night, while the full moon shines throughout
the night, rising in the east at sunset and setting in the west at
sunrise.
When the moon is full it is 180° east, or west, of the sun and so
both its eastern and western limbs are perfectly illuminated. After
the full the moon goes through its phases in reverse order, being
first gibbous, then a half-moon once more, and lastly a waning
crescent.
It is now west instead of east of the sun and so it is the eastern
limb that is fully illuminated by the sun. Being west of the sun it
will now rise before the sun and set before the sun, the interval
decreasing each day as the moon draws in toward the sun once more.
The gibbous phase preceding full moon is favorable to all abroad
before midnight but the gibbous phase following full moon is more
favorable to those who are abroad after midnight, for from full moon
to last quarter the moon is below the horizon at sunset, and of
course, is rising later and later each night, while at sunrise it is
still above the horizon, appearing each day higher and higher above
the western horizon at sunrise as it approaches the third or last
quarter.
When it has reached this point it is once more a half-moon, though
now it is the eastern half instead of the western half of the disk
that is fully illuminated. The moon is 90° west of the sun at third
quarter and from this phase to the phase of new moon it is a crescent
once more, but now a waning instead of a waxing crescent. It appears
east of the meridian before sunrise and as the crescent grows thinner
it draws nearer and nearer to the eastern horizon and the rising sun.
As with the waxing crescent moon the horns are turned away from the
horizon. The waning crescent moon is always to be looked for east
of the meridian and to be associated with the rising sun, while the
waxing crescent moon is to be looked for west of the meridian and
associated with the setting sun. Neither the waxing nor the waning
crescent moon will be visible during the midnight hours.
As the waning crescent moon grows thinner and draws in closer to
the sun each successive night, its time of rising precedes that of
the sun by an ever-decreasing interval until finally the crescent
disappears from view in the eastern sky; the next day we see no
crescent either in the eastern or western skies--the moon is once
more in conjunction with the sun and "new." One revolution of the
moon about the earth with respect to the sun has been completed and
a day or so later we may look for a new crescent moon in the western
sky after sunset.
XXVII
THE MOTIONS OF THE HEAVENLY BODIES
About three hundred and twenty years ago Giordano Bruno was burned
at the stake for his audacity in believing in the existence of other
worlds. A few decades later the famous astronomer Galileo was forced
to publicly recant his belief that the earth moved. Yet the truth
could not long be suppressed by such means, and since those dark
days man's advance in knowledge has been so rapid that it seems
to us today in this wonderful age of scientific discovery almost
inconceivable that man ever believed that the earth, a tiny planet
of a vast solar system, was "the hub of the universe," the fixed and
immovable center about which revolved all the heavenly bodies. Very
reluctantly, however, and with bitter feeling, but in the light of
overwhelming evidence man finally gave up his long-cherished idea of
terrestrial importance, and when finally forced to move his fixed
center of the universe, he moved it only so far as the comparatively
nearby sun.
This center he then regarded as fixed in space and also held to
his belief that the stars, set in an imaginary celestial sphere,
were immovable in space as well, and all at the same distance from
the sun. So, scarcely two hundred years ago we find that the
astronomer Bradley was endeavoring to measure this common distance
of the "fixed stars." Though he failed in this attempt he made the
important discovery that the observed positions of the stars are not
their true positions, owing to the fact that the velocity of light is
not infinite but takes a definite finite interval of time to travel
a given distance. As a result the stars always appear displaced in
the direction of the earth's motion around the sun, the amount of
the displacement depending upon the velocity of the earth in its
orbit and the velocity of light. This "aberration of light," as it is
called, furnished additional proof that the earth revolves about the
sun and was one more nail driven into the coffin of the old Ptolemaic
theory that the earth was the center of the universe. Bradley also
discovered that the positions of the stars were affected by the
wabbling of the earth's axis, called its "nutation."
Although in the days of Bradley neither the methods of observation
nor the instruments were sufficiently accurate to show the minute
shifts in the positions of the stars that reveal the individual
motions of the stars and the distances of those nearest to us, yet
the discovery of the two large displacements in the positions of all
the stars, due to the aberration of light and the nodding of the
earth's axis were of the greatest value, for they were a necessary
step in the direction of the precise measurements of modern times. It
is only through measurements of the greatest refinement and accuracy
that it is possible to detect the motions and distances of the stars
and to discover the wonderful truths about the nature and structure
of the universe that they are revealing to us today.
After unsuccessful attempts extending over several centuries the
distance of one of the nearest stars, the faint 61 Cygni, as it is
catalogued, was finally determined by the astronomer Bessel in the
year 1838.
This star is about ten light-years distant from the earth, which
places it about six hundred and thirty thousand times farther away
from us than the sun; that is, we would have to travel six hundred
and thirty thousand times the distance from the earth to the sun
to reach this very close stellar neighbor, 61 Cygni. The _nearest_
of all the stars, Alpha Centauri, is over two hundred and seventy
thousand times the distance from the earth to the sun. It is,
therefore, little wonder that the early astronomers believed that the
stars were fixed in space since even the nearest is so far away that,
viewed from opposite points in the earth's orbit, its apparent change
in position due to our actual change in position of 186,000,000
miles, amounts to only one and a half seconds of arc. Two stars
separated by _one hundred and sixty times_ this angular distance
might possibly be glimpsed as two distinct stars by a person with
good eyesight, though to most of us they would appear as one star.
Upon the measurement of such minute angles depended a knowledge of
the distances of the nearest stars.
It is to Sir William Herschel that we owe the discovery, more than a
hundred years ago, of the motion of the sun through the universe.
From the consideration of a long series of observations of the
positions of the stars this famous astronomer discovered that the
stars in the direction of the constellation Hercules were separated
by much greater angular distances than the stars diametrically
opposite in the heavens. In other words, the stars were spreading
apart in one portion of the heavens and crowding together in the
opposite direction and he rightly interpreted this to mean that the
sun was moving in the direction of the constellation of Hercules.
It was not until the spectroscope was applied to the study of the
heavens in the latter part of the nineteenth century that the amount
of this motion of the sun was found to be about twelve and a half
miles per second, or four times the distance from the earth to the
sun in a year.
It is to Sir William Herschel that we owe also the discovery of
binary systems of stars in which two stars swing around a point
between them called their center of gravity.
[Illustration: SPIRAL NEBULA IN CANES VENATICI
Taken with 60-inch Reflector of the Mt. Wilson Observatory]
Our first conception of the immensity and grandeur of the universe
dates from the time of the older Herschel only a century or so
ago. The mysterious nebulæ and star clusters were then discovered,
the wonders of the Milky Way were explored, and a new planet and
satellites in our own solar system were discovered. It was found that
the sun and the stars as well as the planets were in motion. Neither
sun nor earth could be regarded any longer as a fixed point in the
universe.
With the application of the spectroscope to the study of the heavens
toward the end of the nineteenth century the key to a treasure-house
of knowledge was placed in the hands of the astronomers of modern
times and as a result we are now learning more, in a few decades,
about the wonders and mysteries of the heavens than was granted to
man to learn in centuries of earlier endeavor. Yet it is the feeling
of the astronomer of today that he is only standing on the threshold
of knowledge and that the greatest of all discoveries, that of the
nature of matter and of time and space is yet to be made.
It is the spectroscope that tells us so many wonderful facts about
the motions of the stars, nebulæ and star clusters. It tells us also
practically all we know about the physical condition of our own sun
and of the other suns of the universe, their temperature and age, and
the peculiarities of their atmospheres.
Some of the most important astronomical discoveries that have been
made in the past few years have to do with the distribution and
velocities of the heavenly bodies as revealed by the spectroscope.
It has been found, with the aid of the spectroscope, that the most
slowly moving of all stars are the extremely hot bluish Orion stars
with an average velocity of eight miles per second, while the
most rapidly moving stars are the deep-red stars with an average
velocity of twenty-one miles per second, and there is in all cases a
relationship existing between the color, or spectrum, of a star and
its velocity. The reason for this connection between the two still
remains undiscovered.
The spectroscope has also told us some astonishing facts in recent
years about the velocities of the spiral nebulæ.
It is now known that these mysterious objects are moving with the
tremendous average velocity of _four hundred and eighty miles per
second_, which exceeds the average velocity of the stars fully
twenty-five fold. They possess, moreover, internal motions of
rotation that are almost as high as their velocities through space.
It is now generally believed that spiral nebulæ are far distant
objects of enormous size and mass, exterior to our own system of
stars and similar to it in form.
In place of the universe of the "fixed stars" and the immovable sun
or earth of a few centuries ago we find that modern astronomical
discovery is substituting a universe of inconceivable grandeur and
immensity in a state of ceaseless flux and change.
Our earth--an atom spinning about on its axis and revolving rapidly
around a huge sun that is equal in volume to more than a million
earths--is carried onward with this sun through a vast universe of
suns.