called descriptive astronomy. With them, as we have seen, advantage
is taken of cumulative light-effects on the sensitive plate, and
the telescope's light-gathering and space-penetrating powers are
vastly increased. We are enabled to carry our researches far
beyond the confines of the old visible universe. Extremely faint
objects can be recorded, even down to their minutest details, with
a fidelity unknown to older visual methods. But at present we
intend to consider principally applications of photography in the
astronomy of measurement, rather than the descriptive branch of our
subject. Instead of describing pictures made simply to see what
certain objects look like in the sky, we shall consider negatives
intended for precise measurement, with all that the word precision
implies in celestial science.
Taking up first the photography of stars, we must begin by
mentioning the work of Rutherfurd at New York. More than thirty
years ago he had so far perfected methods of stellar photography
that he was able to secure excellent pictures of stars as faint as
the ninth magnitude. In those days the modern process of dry-plate
photography had not been invented. To-day, plates exposed in the
photographic telescope are made of glass covered with a perfectly
dry film of sensitized gelatine. But in the old wet-plate process
the sensitive film was first wetted with a chemical solution; and
this solution could not be allowed to dry during the exposure.
Consequently, Rutherfurd was limited to exposures a few minutes
in length, while nowadays, as we have said, their duration can be
prolonged at will.
When we add to this the fact that the old plates were far less
sensitive to light than those now available, it is easy to see
what were the difficulties in the way of photographing faint stars
in Rutherfurd's time. Nor did he possess the modern ingenious
device of a combined visual and photographic instrument. He had no
electric controlling apparatus. In fact, the younger generation of
astronomers can form no adequate idea of the patience and personal
skill Rutherfurd must have had at his command. For he certainly did
produce negatives that are but little inferior to the best that can
be made to-day. His only limitation was that he could not obtain
images of stars much below the ninth magnitude.
To understand just what is meant here by the ninth magnitude, it
is necessary to go back in imagination to the time of Hipparchus,
the father of sidereal astronomy. (See page 39.) He adopted the
convenient plan of dividing all the stars visible to the naked eye
(of course, he had no telescope) into six classes, according to
their brilliancy. The faintest visible stars were put in the sixth
class, and all the others were assigned somewhat arbitrarily to one
or the other of the brighter classes.
Modern astronomers have devised a more scientific system, which has
been made to conform very nearly to that of Hipparchus, just as
it has come down to us through the ages. We have adopted a certain
arbitrary degree of luminosity as the standard "first-magnitude";
compared with sunlight, this may be represented roughly by a
fraction of which the numerator is 1, and the denominator about
eighty thousand millions. The standard second-magnitude star is one
whose light, compared with a first-magnitude, may be represented
approximately by the fraction ⅖. The third magnitude, in turn, may
be compared with the second by the same fraction ⅖; and so the
classification is extended to magnitudes below those visible to the
unaided eye. Each magnitude compares with the one above it, as the
light of two candles would compare with the light of five.
Rutherfurd did not stop with mere photographs. He realized very
clearly the obvious truth that by making a picture of the sky we
simply change the scene of our operations. Upon the photograph
we can measure that which we might have studied directly in the
heavens; but so long as they remain unmeasured, celestial pictures
have a potential value only. Locked within them may lie hidden
some secret of our universe. But it will not come forth unsought.
Patient effort must precede discovery, in photography, as elsewhere
in science. There is no royal road. Rutherfurd devised an elaborate
measuring-machine in which his photographs could be examined under
the microscope with the most minute exactness. With this machine
he measured a large number of his pictures; and it has been shown
quite recently that the results obtained from them are comparable
in accuracy with those coming from the most highly accredited
methods of direct eye-observation.
And photographs are far superior in ease of manipulation.
Convenient day-observing under the microscope in a comfortable
astronomical laboratory is substituted for all the discomforts
of a midnight vigil under the stars. The work of measurement can
proceed in all weathers, whereas formerly it was limited strictly
to perfectly clear nights. Lastly, the negatives form a permanent
record, to which we can always return to correct errors or
re-examine doubtful points.
Rutherfurd's stellar work extended down to about 1877, and
included especially parallax determinations and the photography of
star-clusters. Each of these subjects is receiving close attention
from later investigators, and, therefore, merits brief mention
here. Stellar parallax is in one sense but another name for stellar
distance. Its measurement has been one of the important problems
of astronomy for centuries, ever since men recognized that the
Copernican theory of our universe requires the determination of
stellar distances for its complete demonstration.
If the earth is swinging around the sun once a year in a mighty
path or orbit, there must be changes of its position in space
comparable in size with the orbit itself. And the stars ought to
shift their apparent places on the sky to correspond with these
changes in the terrestrial observer's position. The phenomenon is
analogous to what occurs when we look out of a room, first through
one window, and then through another. Any object on the opposite
side of the street will be seen in a changed direction, on account
of the observer's having shifted his position from one window to
the other. If the object seemed to be due north when seen from
the first window, it will, perhaps, appear a little east of north
from the other. But this change of direction will be comparatively
small, if the object under observation is very far away, in
comparison with the distance between the two windows.
This is what occurs with the stars. The earth's orbit, vast as
it is, shrinks into almost absolute insignificance when compared
with the profound distances by which we are sundered from even the
nearest fixed stars. Consequently, the shifting of their positions
is also very small--so small as to be near the extreme limit
separating that which is measurable from that which is beyond human
ken.
Photography lends itself most readily to a study of this matter.
Suppose a certain star is suspected of "having a parallax." In
other words, we have reason to believe it near enough to admit of
a successful measurement of distance. Perhaps it is a very bright
star; and, other things being equal, it is probably fair to assume
that brightness signifies nearness. And astronomers have certain
other indications of proximity that guide them in the selection of
proper objects for investigation, though such evidence, of course,
never takes the place of actual measurement.
The star under examination is sure to have near it on the sky a
number of stars so very small that we may safely take them to be
immeasurably far away. The parallax star is among them, but not
of them. We see it projected upon the background of the heavens,
though it may in reality be quite near us, astronomically speaking.
If this is really so, and the star, therefore, subject to the
slight parallactic shifting already mentioned, we can detect it by
noting the suspected star's position among the surrounding small
stars. For these, being immeasurably remote, will remain unchanged,
within the limits of our powers of observation, and thus serve
as points of reference for marking the apparent shifting of the
brighter star we are actually considering.
We have merely to photograph the region at various seasons of the
year. Careful examination of the photographs under the microscope
will then enable us to measure the slightest displacement of the
parallax star. From these measures, by a process of calculation,
astronomers can then obtain the star's distance. It will not
become known in miles; we shall only ascertain how many times the
distance between the earth and sun would have to be laid down like
a measuring-rod, in order to cover the space separating us from the
star: and the subsequent evaluation of this distance "earth to sun"
in miles is another important problem in whose solution photography
promises to be most useful.
The above method of measuring stellar distance is, of course,
subject to whatever slight uncertainty arises from the assumption
that the small stars used for comparison are themselves beyond the
possibility of parallactic shifting. But astronomy possesses no
better method. Moreover, the number of small stars used in this
way is, of course, much larger in photography than it ever can be
in visual work. In the former process, all surrounding stars can
be photographed at once; in the latter each star must be measured
separately, and daylight soon intervenes to impose a limit on
numbers. Usually only two can be used; so that here photography
has a most important advantage. It minimizes the chance of our
parallax being rendered erroneous, by the stars of comparison not
being really infinitely remote. This might happen, perhaps, in the
case of one or two; but with an average result from a large number
we know it to be practically impossible.
Cluster work is not altogether unlike "parallax hunting" in its
preliminary stage of securing the photographic observations. The
object is to obtain an absolutely faithful picture of a star group,
just as it exists in the sky. We have every reason to suppose
that a very large number of stars condensed into one small spot
upon the heavens means something more than chance aggregation.
The Pleiades group (page 10) contains thousands of massive stars,
doubtless held together by the force of their mutual gravitational
attraction. If this be true, there must be complex orbital motion
in the cluster; and, as time goes on, we should actually see the
separate components change their relative positions, as it were,
before our eyes. The details of such motion upon the great scale
of cosmic space offer one of the many problems that make astronomy
the grandest of human sciences.
We have said that time must pass before we can see these things;
there may be centuries of waiting. But one way exists to hurry on
the perfection of our knowledge; we must increase the precision of
observations. Motions that would need the growth of centuries to
become visible to the older astronomical appliances, might yield
in a few decades to more delicate observational processes. Here
photography is most promising. Having once obtained a surpassingly
accurate picture of a star-cluster, we can subject it easily to
precise microscopic measurement. The same operations repeated at a
later date will enable us to compare the two series of measures,
and thus ascertain the motions that may have occurred in the
interval. The Rutherfurd photographs furnish a veritable mine of
information in researches of this kind; for they antedate all other
celestial photographs of precision by at least a quarter-century,
and bring just so much nearer the time when definite knowledge
shall replace information based on reasoning from probabilities.
Rutherfurd's methods showed the advantages of photography as
applied to individual star-clusters. It required only the attention
of some astronomer disposing of large observational facilities,
and accustomed to operations upon a great scale, to apply similar
methods throughout the whole heavens. In the year 1882 a bright
comet was very conspicuous in the southern heavens. It was
extensively observed from the southern hemisphere, and especially
at the British Royal Observatory at the Cape of Good Hope.
Gill, director of that institution, conceived the idea that this
comet might be bright enough to photograph. At that time, comet
photography had been attempted but little, if at all, and it was
by no means sure that the experiment would be successful. Nor was
Gill well acquainted with the work of Rutherfurd; for the best
results of that astronomer had lain dormant many years. He was one
of those men with whom personal modesty amounts to a fault. Loath
to put himself forward in any way, and disliking to rush into
print, Rutherfurd had given but little publicity to his work. This
peculiarity has, doubtless, delayed his just reputation; but he
will lose nothing in the end from a brief postponement. Gill must,
however, be credited with more penetration than would be his due
if Rutherfurd had made it possible for others to know that he had
anticipated many of the newer ideas.
However this may be, the comet was photographed with the help of
a local portrait photographer named Allis. When Gill and Allis
fastened a simple portrait camera belonging to the latter upon the
tube of one of the Cape telescopes, and pointed it at the great
comet, they little thought the experiment would lead to one of the
greatest astronomical works ever attempted by men. Yet this was
destined to occur. The negative they obtained showed an excellent
picture of the comet; but what was more important for the future of
sidereal astronomy, it was also quite thickly dotted with little
black points corresponding to stars. The extraordinary ease with
which the whole heavens could be thus charted photographically
was brought home to Gill as never before. It was this comet
picture that interested him in the application of photography
to star-charting; and without his interest the now famous
astro-photographic catalogue of the heavens would probably never
have been made.
After considerable preliminary correspondence, a congress
of astronomers was finally called to meet at Paris in 1887.
Representatives of the principal observatories and civilized
governments were present. They decided that the end of the
nineteenth century should see the making of a great catalogue
of all the stars in the sky, upon a scale of completeness and
precision surpassing anything previously attempted. It is
impossible to exaggerate the importance of such a work; for upon
our star-catalogues depends ultimately the entire structure of
astronomical science.
The work was far too vast for the powers of any observatory alone.
Therefore, the whole sky, from pole to pole, was divided into
eighteen belts or zones of approximately equal area; and each of
these was assigned to a single observatory to be photographed. A
series of telescopes was specially constructed, so that every
part of the work should be done with the same type of instrument.
As far as possible, an attempt was made to secure uniformity of
methods, and particularly a uniform scale of precision. To cover
the entire sky upon the plan proposed no less than 44,108 negatives
are required; and most of these have now been finished. The further
measurement of the pictures and the drawing up of a vast printed
star-catalogue are also well under way. One of the participating
observatories, that at Potsdam, Germany, has published the first
volume of its part of the catalogue. It is estimated that this
observatory alone will require twenty quarto volumes to contain
merely the final results of its work on the catalogue. Altogether
not less than two million stars will find a place in this, our
latest directory of the heavens.
Such wholesale methods of attacking problems of observational
astronomy are particularly characteristic of photography. The great
catalogue is, perhaps, the best illustration of this tendency; but
of scarcely smaller interest, though less important in reality, is
the photographic method of dealing with minor planets. We have
already said (page 63) that in the space between the orbits of Mars
and Jupiter several hundred small bodies are moving around the sun
in ordinary planetary orbits. These bodies are called asteroids,
or minor planets. The visual method of discovering unknown members
of this group was painfully tedious; but photography has changed
matters completely, and has added immensely to our knowledge of the
asteroids.
Wolf, of Heidelberg, first made use of the new process for
minor-planet discovery. His method is sufficiently ingenious to
deserve brief mention again. A photograph of a suitable region
of the sky was made with an exposure lasting two or three hours.
Throughout all this time the instrument was manipulated so as
to follow the motion of the heavens in the way we have already
explained, so that each star would appear on the negative as a
small, round, black dot.
But if a minor planet happened to be in the region covered by the
plate, its photographic image would be very different. For the
orbital motion of the planet about the sun would make it move a
little among the stars even in the two or three hours during which
the plate was exposed. This motion would be faithfully reproduced
in the picture, so that the planet would appear as a short curved
line rather than a well-defined dot like a star. Thus the presence
of such a line-image infallibly denotes an asteroid.
Subsequent calculations are necessary to ascertain whether the
object is a planet already known or a genuine new discovery. Wolf,
and others using his method in recent years, have made immense
additions to our catalogue of asteroids. Indeed, the matter was
beginning to lose interest on account of the frequency and sameness
of these discoveries, when the astronomical world was startled by
the finding of the Planet of 1898. (Page 58.)
On August 27, 1898, Witt, of Berlin, discovered the small body
that bears the number "433" in the list of minor planets, and has
received the name Eros. Its important peculiarity consists in the
exceptional position of the orbit. While all the other asteroids
are farther from the sun than Mars, and less distant than Jupiter,
Eros can pass within the orbit of the former. At times, therefore,
it will approach our earth more closely than any other permanent
member of the solar system, excepting our own moon. So it is, in
a sense, our nearest neighbor; and this fact alone makes it the
most interesting of all the minor planets. The nineteenth century
was opened by Piazzi's well-known discovery of the first of these
bodies (page 59); it is, therefore, fitting that we should find
the most important one at its close. We are almost certain that it
will be possible to make use of Eros to solve with unprecedented
accuracy the most important problem in all astronomy. This is
the determination of our earth's distance from the sun. When
considering stellar parallax, we have seen how our observations
enable us to measure some of the stars' distances in terms of the
distance "earth to sun" as a unit. It is, indeed, the fundamental
unit for all astronomical measures, and its exact evaluation has
always been considered the basal problem of astronomy. Astronomers
know it as the problem of Solar Parallax.
We shall not here enter into the somewhat intricate details of
this subject, however interesting they may be. The problem offers
difficulties somewhat analogous to those confronting a surveyor
who has to determine the distance of some inaccessible terrestrial
point. To do this, it is necessary first to measure a "base-line,"
as we call it. Then the measurement of angles with a theodolite
will make it possible to deduce the required distance of the
inaccessible point by a process of calculation. To insure accuracy,
however, as every surveyor knows, the base-line must be made long
enough; and this is precisely what is impossible in the case of the
solar parallax.
For we are necessarily limited to marking out our base-line on
the earth; and the entire planet is too small to furnish one of
really sufficient size. The best we can do is to use the distance
between two observatories situated, as near as may be, on opposite
sides of the earth. But even this base is wofully small. However,
the smallness loses some of its harmful effect if we operate upon
a planet that is comparatively near us. We can measure such
a planet's distance more accurately than any other; and this
being known, the solar distance can be computed by the aid of
mathematical considerations based upon Newton's law of gravitation
and observational determinations of the planetary orbital elements.
Photography is by no means limited to investigations in the older
departments of astronomical observation. Its powerful arm has
been stretched out to grasp as well the newer instruments of
spectroscopic study. Here the sensitive plate has been substituted
for the human eye with even greater relative advantage. The
accurate microscopic measurement of difficult lines in stellar
spectra was indeed possible by older methods; but photography
has made it comparatively easy; and, above all, has rendered
practicable series of observations extensive enough in numbers to
furnish statistical information of real value. Only in this way
have we been able to determine whether the stars, in their varied
and unknown orbits, are approaching us or moving farther away. Even
the speed of this approach or recession has become measurable,
and has been evaluated in the case of many individual stars. (See
page 21.)
[Illustration: Solar Corona. Total Eclipse.
Photographed by Campbell, January 22, 1898; Jeur, India.]
The subject of solar physics has become a veritable department of
astronomy in the hands of photographic investigators. Ingenious
spectro-photographic methods have been devised, whereby we have
secured pictures of the sun from which we have learned much that
must have remained forever unknown to older methods.
Especially useful has photography proved itself in the observation
of total solar eclipses. It is only when the sun's bright disk
is completely obscured by the interposed moon that we can see
the faintly luminous structure of the solar corona, that great
appendage of our sun, whose exact nature is still unexplained. Only
during the few minutes of total eclipse in each century can we
look upon it; and keen is the interest of astronomers when those
few minutes occur. But it is found that eye observations made in
hurried excitement have comparatively little value. Half a dozen
persons might make drawings of the corona during the same eclipse,
yet they would differ so much from one another as to leave the
true outline very much in doubt. But with photography we can obtain
a really correct picture whose details can be studied and discussed
subsequently at leisure.
If we were asked to sum up in one word what photography has
accomplished, we should say that observational astronomy has
been revolutionized. There is to-day scarcely an instrument of
precision in which the sensitive plate has not been substituted
for the human eye; scarcely an inquiry possible to the older
method which cannot now be undertaken upon a grander scale.
Novel investigations formerly not even possible are now entirely
practicable by photography; and the end is not yet. Valuable as are
the achievements already consummated, photography is richest in
its promise for the future. Astronomy has been called the "perfect
science"; it is safe to predict that the next generation will
wonder that the knowledge we have to-day should ever have received
so proud a title.
TIME STANDARDS OF THE WORLD