Cancer. This can be very distinctly seen by the naked eye on a clear
moonless night in winter or spring as a faint nebulous object,
surrounded by three small stars. The smallest telescope shows it as a
group of stars.
Of all stellar objects, the great nebula of Orion is that which has
most fascinated the astronomers of two centuries. It is distinctly
visible to the naked eye, and may be found without difficulty on any
winter night. The three bright stars forming the sword-belt of Orion
are known to every one who has noticed that constellation. Below this
belt is seen another triplet of stars, not so bright, and lying in a
north and south direction. The middle star of this triplet is the great
nebula. At first the naked eye sees nothing to distinguish it from
other stars, but if closely scanned it will be seen to have a hazy
aspect. A four-inch telescope will show its curious form. Not the least
interesting of its features are the four stars known as the
"Trapezium," which are located in a dark region near its centre. In
fact, the whole nebula is dotted with stars, which add greatly to the
effect produced by its mysterious aspect.
The great nebula of Andromeda is second only to that of Orion in
interest. Like the former, it is distinctly visible to the naked eye,
having the aspect of a faint comet. The most curious feature of this
object is that although the most powerful telescopes do not resolve it
into stars, it appears in the spectroscope as if it were solid matter
shining by its own light.
The above are merely selections from the countless number of objects
which the heavens offer to telescopic study. Many such are described in
astronomical works, but the amateur can gratify his curiosity to almost
any extent by searching them out for himself.
[Illustration with caption: NEBULA IN ORION]
Ever since 1878 a red spot, unlike any before noticed, has generally
been visible on Jupiter. At first it was for several years a very
conspicuous object, but gradually faded away, so that since 1890 it has
been made out only with difficulty. But it is now regarded as a
permanent feature of the planet. There is some reason to believe it was
occasionally seen long before attention was first attracted to it.
Doubtless, when it can be seen at all, practice in observing such
objects is more important than size of telescope.
VI
WHAT THE ASTRONOMERS ARE DOING
In no field of science has human knowledge been more extended in our
time than in that of astronomy. Forty years ago astronomical research
seemed quite barren of results of great interest or value to our race.
The observers of the world were working on a traditional system,
grinding out results in an endless course, without seeing any prospect
of the great generalizations to which they might ultimately lead. Now
this is all changed. A new instrument, the spectroscope, has been
developed, the extent of whose revelations we are just beginning to
learn, although it has been more than thirty years in use. The
application of photography has been so extended that, in some important
branches of astronomical work, the observer simply photographs the
phenomenon which he is to study, and then makes his observation on the
developed negative.
The world of astronomy is one of the busiest that can be found to-day,
and the writer proposes, with the reader's courteous consent, to take
him on a stroll through it and see what is going on. We may begin our
inspection with a body which is, for us, next to the earth, the most
important in the universe. I mean the sun. At the Greenwich Observatory
the sun has for more than twenty years been regularly photographed on
every clear day, with the view of determining the changes going on in
its spots. In recent years these observations have been supplemented by
others, made at stations in India and Mauritius, so that by the
combination of all it is quite exceptional to have an entire day pass
without at least one photograph being taken. On these observations must
mainly rest our knowledge of the curious cycle of change in the solar
spots, which goes through a period of about eleven years, but of which
no one has as yet been able to establish the cause.
This Greenwich system has been extended and improved by an American.
Professor George E. Hale, formerly Director of the Yerkes Observatory,
has devised an instrument for taking photographs of the sun by a single
ray of the spectrum. The light emitted by calcium, the base of lime,
and one of the substances most abundant in the sun, is often selected
to impress the plate.
The Carnegie Institution has recently organized an enterprise for
carrying on the study of the sun under a combination of better
conditions than were ever before enjoyed. The first requirement in such
a case is the ablest and most enthusiastic worker in the field, ready
to devote all his energies to its cultivation. This requirement is
found in the person of Professor Hale himself. The next requirement is
an atmosphere of the greatest transparency, and a situation at a high
elevation above sea-level, so that the passage of light from the sun to
the observer shall be obstructed as little as possible by the mists and
vapors near the earth's surface. This requirement is reached by placing
the observatory on Mount Wilson, near Pasadena, California, where the
climate is found to be the best of any in the United States, and
probably not exceeded by that of any other attainable point in the
world. The third requirement is the best of instruments, specially
devised to meet the requirements. In this respect we may be sure that
nothing attainable by human ingenuity will be found wanting.
Thus provided, Professor Hale has entered upon the task of studying the
sun, and recording from day to day all the changes going on in it,
using specially devised instruments for each purpose in view.
Photography is made use of through almost the entire investigation. A
full description of the work would require an enumeration of technical
details, into which we need not enter at present. Let it, therefore,
suffice to say in a general way that the study of the sun is being
carried on on a scale, and with an energy worthy of the most important
subject that presents itself to the astronomer. Closely associated with
this work is that of Professor Langley and Dr. Abbot, at the
Astro-Physical Observatory of the Smithsonian Institution, who have
recently completed one of the most important works ever carried out on
the light of the sun. They have for years been analyzing those of its
rays which, although entirely invisible to our eyes, are of the same
nature as those of light, and are felt by us as heat. To do this,
Langley invented a sort of artificial eye, which he called a bolometer,
in which the optic nerve is made of an extremely thin strip of metal,
so slight that one can hardly see it, which is traversed by an electric
current. This eye would be so dazzled by the heat radiated from one's
body that, when in use, it must be protected from all such heat by
being enclosed in a case kept at a constant temperature by being
immersed in water. With this eye the two observers have mapped the heat
rays of the sun down to an extent and with a precision which were
before entirely unknown.
The question of possible changes in the sun's radiation, and of the
relation of those changes to human welfare, still eludes our scrutiny.
With all the efforts that have been made, the physicist of to-day has
not yet been able to make anything like an exact determination of the
total amount of heat received from the sun. The largest measurements
are almost double the smallest. This is partly due to the atmosphere
absorbing an unknown and variable fraction of the sun's rays which pass
through it, and partly to the difficulty of distinguishing the heat
radiated by the sun from that radiated by terrestrial objects.
In one recent instance, a change in the sun's radiation has been
noticed in various parts of the world, and is of especial interest
because there seems to be little doubt as to its origin. In the latter
part of 1902 an extraordinary diminution was found in the intensity of
the sun's heat, as measured by the bolometer and other instruments.
This continued through the first part of 1903, with wide variations at
different places, and it was more than a year after the first
diminution before the sun's rays again assumed their ordinary intensity.
This result is now attributed to the eruption of Mount Pelee, during
which an enormous mass of volcanic dust and vapor was projected into
the higher regions of the air, and gradually carried over the entire
earth by winds and currents. Many of our readers may remember that
something yet more striking occurred after the great cataclasm at
Krakatoa in 1883, when, for more than a year, red sunsets and red
twilights of a depth of shade never before observed were seen in every
part of the world.
What we call universology--the knowledge of the structure and extent of
the universe--must begin with a study of the starry heavens as we see
them. There are perhaps one hundred million stars in the sky within the
reach of telescopic vision. This number is too great to allow of all
the stars being studied individually; yet, to form the basis for any
conclusion, we must know the positions and arrangement of as many of
them as we can determine.
To do this the first want is a catalogue giving very precise positions
of as many of the brighter stars as possible. The principal national
observatories, as well as some others, are engaged in supplying this
want. Up to the present time about 200,000 stars visible in our
latitudes have been catalogued on this precise plan, and the work is
still going on. In that part of the sky which we never see, because it
is only visible from the southern hemisphere, the corresponding work is
far from being as extensive. Sir David Gill, astronomer at the Cape of
Good Hope, and also the directors of other southern observatories, are
engaged in pushing it forward as rapidly as the limited facilities at
their disposal will allow.
Next in order comes the work of simply listing as many stars as
possible. Here the most exact positions are not required. It is only
necessary to lay down the position of each star with sufficient
exactness to distinguish it from all its neighbors. About 400,000 stars
were during the last half-century listed in this way at the observatory
of Bonn by Argelander, Schonfeld, and their assistants. This work is
now being carried through the southern hemisphere on a large scale by
Thome, Director of the Cordoba Observatory, in the Argentine Republic.
This was founded thirty years ago by our Dr. B. A. Gould, who turned it
over to Dr. Thome in 1886. The latter has, up to the present time,
fixed and published the positions of nearly half a million stars. This
work of Thome extends to fainter stars than any other yet attempted, so
that, as it goes on, we have more stars listed in a region invisible in
middle northern latitudes than we have for that part of the sky we can
see. Up to the present time three quarto volumes giving the positions
and magnitudes of the stars have appeared. Two or three volumes more,
and, perhaps, ten or fifteen years, will be required to complete the
work.
About twenty years ago it was discovered that, by means of a telescope
especially adapted to this purpose, it was possible to photograph many
more stars than an instrument of the same size would show to the eye.
This discovery was soon applied in various quarters. Sir David Gill,
with characteristic energy, photographed the stars of the southern sky
to the number of nearly half a million. As it was beyond his power to
measure off and compute the positions of the stars from his plates, the
latter were sent to Professor J. C. Kapteyn, of Holland, who undertook
the enormous labor of collecting them into a catalogue, the last volume
of which was published in 1899. One curious result of this enterprise
is that the work of listing the stars is more complete for the southern
hemisphere than for the northern.
Another great photographic work now in progress has to do with the
millions of stars which it is impossible to handle individually.
Fifteen years ago an association of observatories in both hemispheres
undertook to make a photographic chart of the sky on the largest scale.
Some portions of this work are now approaching completion, but in
others it is still in a backward state, owing to the failure of several
South American observatories to carry out their part of the programme.
When it is all done we shall have a picture of the sky, the study of
which may require the labor of a whole generation of astronomers.
Quite independently of this work, the Harvard University, under the
direction of Professor Pickering, keeps up the work of photographing
the sky on a surprising scale. On this plan we do not have to leave it
to posterity to learn whether there is any change in the heavens, for
one result of the enterprise has been the discovery of thirteen of the
new stars which now and then blaze out in the heavens at points where
none were before known. Professor Pickering's work has been continually
enlarged and improved until about 150,000 photographic plates, showing
from time to time the places of countless millions of stars among their
fellows are now stored at the Harvard Observatory. Not less remarkable
than this wealth of material has been the development of skill in
working it up. Some idea of the work will be obtained by reflecting
that, thirty years ago, careful study of the heavens by astronomers
devoting their lives to the task had resulted in the discovery of some
two or three hundred stars, varying in their light. Now, at Harvard,
through keen eyes studying and comparing successive photographs not
only of isolated stars, but of clusters and agglomerations of stars in
the Milky Way and elsewhere, discoveries of such objects numbering
hundreds have been made, and the work is going on with ever-increasing
speed. Indeed, the number of variable stars now known is such that
their study as individual objects no longer suffices, and they must
hereafter be treated statistically with reference to their distribution
in space, and their relations to one another, as a census classifies
the entire population without taking any account of individuals.
The works just mentioned are concerned with the stars. But the heavenly
spaces contain nebulae as well as stars; and photography can now be
even more successful in picturing them than the stars. A few years ago
the late lamented Keeler, at the Lick Observatory, undertook to see
what could be done by pointing the Crossley reflecting telescope at the
sky and putting a sensitive photographic plate in the focus. He was
surprised to find that a great number of nebulae, the existence of
which had never before been suspected, were impressed on the plate. Up
to the present time the positions of about 8000 of these objects have
been listed. Keeler found that there were probably 200,000 nebulae in
the heavens capable of being photographed with the Crossley reflector.
But the work of taking these photographs is so great, and the number of
reflecting telescopes which can be applied to it so small, that no one
has ventured to seriously commence it. It is worthy of remark that only
a very small fraction of these objects which can be photographed are
visible to the eye, even with the most powerful telescope.
This demonstration of what the reflecting telescope can do may be
regarded as one of the most important discoveries of our time as to the
capabilities of astronomical instruments. It has long been known that
the image formed in the focus of the best refracting telescope is
affected by an imperfection arising from the different action of the
glasses on rays of light of different colors. Hence, the image of a
star can never be seen or photographed with such an instrument, as an
actual point, but only as a small, diffused mass. This difficulty is
avoided in the reflecting telescope; but a new difficulty is found in
the bending of the mirror under the influence of its own weight.
Devices for overcoming this had been so far from successful that, when
Mr. Crossley presented his instrument to the Lick Observatory, it was
feared that little of importance could be done with it. But as often
happens in human affairs outside the field of astronomy, when ingenious
and able men devote their attention to the careful study of a problem,
it was found that new results could be reached. Thus it was that,
before a great while, what was supposed to be an inferior instrument
proved not only to have qualities not before suspected, but to be the
means of making an important addition to the methods of astronomical
investigation.
In order that our knowledge of the position of a star may be complete,
we must know its distance. This can be measured only through the star's
parallax--that is to say, the slight change in its direction produced
by the swing of our earth around its orbit. But so vast is the distance
in question that this change is immeasurably small, except for,
perhaps, a few hundred stars, and even for these few its measurement
almost baffles the skill of the most expert astronomer. Progress in
this direction is therefore very slow, and there are probably not yet a
hundred stars of which the parallax has been ascertained with any
approach to certainty. Dr. Chase is now completing an important work of
this kind at the Yale Observatory.
To the most refined telescopic observations, as well as to the naked
eye, the stars seem all alike, except that they differ greatly in
brightness, and somewhat in color. But when their light is analyzed by
the spectroscope, it is found that scarcely any two are exactly alike.
An important part of the work of the astro-physical observatories,
especially that of Harvard, consists in photographing the spectra of
thousands of stars, and studying the peculiarities thus brought out. At
Harvard a large portion of this work is done as part of the work of the
Henry Draper Memorial, established by his widow in memory of the
eminent investigator of New York, who died twenty years ago.
By a comparison of the spectra of stars Sir William Huggins has
developed the idea that these bodies, like human beings, have a life
history. They are nebulae in infancy, while the progress to old age is
marked by a constant increase in the density of their substance. Their
temperature also changes in a way analogous to the vigor of the human
being. During a certain time the star continually grows hotter and
hotter. But an end to this must come, and it cools off in old age. What
the age of a star may be is hard even to guess. It is many millions of
years, perhaps hundreds, possibly even thousands, of millions.
Some attempt at giving the magnitude is included in every considerable
list of stars. The work of determining the magnitudes with the greatest
precision is so laborious that it must go on rather slowly. It is being
pursued on a large scale at the Harvard Observatory, as well as in that
of Potsdam, Germany.
We come now to the question of changes in the appearance of bright
stars. It seems pretty certain that more than one per cent of these
bodies fluctuate to a greater or less extent in their light.
Observations of these fluctuations, in the case of at least the
brighter stars, may be carried on without any instrument more expensive
than a good opera-glass--in fact, in the case of stars visible to the
naked eye, with no instrument at all.
As a general rule, the light of these stars goes through its changes in
a regular period, which is sometimes as short as a few hours, but
generally several days, frequently a large fraction of a year or even
eighteen months. Observations of these stars are made to determine the
length of the period and the law of variation of the brightness. Any
person with a good eye and skill in making estimates can make the
observations if he will devote sufficient pains to training himself;
but they require a degree of care and assiduity which is not to be
expected of any one but an enthusiast on the subject. One of the most
successful observers of the present time is Mr. W. A. Roberts, a
resident of South Africa, whom the Boer war did not prevent from
keeping up a watch of the southern sky, which has resulted in greatly
increasing our knowledge of variable stars. There are also quite a
number of astronomers in Europe and America who make this particular
study their specialty.
During the past fifteen years the art of measuring the speed with which
a star is approaching us or receding from us has been brought to a
wonderful degree of perfection. The instrument with which this was
first done was the spectroscope; it is now replaced with another of the
same general kind, called the spectrograph. The latter differs from the
other only in that the spectrum of the star is photographed, and the
observer makes his measures on the negative. This method was first
extensively applied at the Potsdam Observatory in Germany, and has
lately become one of the specialties of the Lick Observatory, where
Professor Campbell has brought it to its present degree of perfection.
The Yerkes Observatory is also beginning work in the same line, where
Professor Frost is already rivalling the Lick Observatory in the
precision of his measures.
Let us now go back to our own little colony and see what is being done
to advance our knowledge of the solar system. This consists of planets,
on one of which we dwell, moons revolving around them, comets, and
meteoric bodies. The principal national observatories keep up a more or
less orderly system of observations of the positions of the planets and
their satellites in order to determine the laws of their motion. As in
the case of the stars, it is necessary to continue these observations
through long periods of time in order that everything possible to learn
may be discovered.
Our own moon is one of the enigmas of the mathematical astronomer.
Observations show that she is deviating from her predicted place, and
that this deviation continues to increase. True, it is not very great
when measured by an ordinary standard. The time at which the moon's
shadow passed a given point near Norfolk during the total eclipse of
May 29, 1900, was only about seven seconds different from the time
given in the Astronomical Ephemeris. The path of the shadow along the
earth was not out of place by more than one or two miles But, small
though these deviations are, they show that something is wrong, and no
one has as yet found out what it is. Worse yet, the deviation is
increasing rapidly. The observers of the total eclipse in August, 1905,
were surprised to find that it began twenty seconds before the
predicted time. The mathematical problems involved in correcting this
error are of such complexity that it is only now and then that a
mathematician turns up anywhere in the world who is both able and bold
enough to attack them.
There now seems little doubt that Jupiter is a miniature sun, only not
hot enough at its surface to shine by its own light The point in which
it most resembles the sun is that its equatorial regions rotate in less
time than do the regions near the poles. This shows that what we see is
not a solid body. But none of the careful observers have yet succeeded
in determining the law of this difference of rotation.
Twelve years ago a suspicion which had long been entertained that the
earth's axis of rotation varied a little from time to time was verified
by Chandler. The result of this is a slight change in the latitude of
all places on the earth's surface, which admits of being determined by
precise observations. The National Geodetic Association has established
four observatories on the same parallel of latitude--one at
Gaithersburg, Maryland, another on the Pacific coast, a third in Japan,
and a fourth in Italy--to study these variations by continuous
observations from night to night. This work is now going forward on a
well-devised plan.