will act in a manner similar to a whole one, it is easy to explain
the construction of a heliometer. An ordinary telescope lens is
sawed in half by means of a thin round metal disk revolved rapidly
by machinery, and fed continually with emery and water at its
edge. The cutting effect of emery is sufficient to make such a
disk enter glass much as an ordinary saw penetrates wood. The two
"semi-lenses," as they are called, are then mounted separately in
metal holders. These are attached to one end of the heliometer,
called the "head," in such a way that the two semi-lenses can slide
side by side upon metal guides. This head is then fastened to one
end of a telescope tube mounted in the usual way. The "head" end
of the instrument is turned toward the sky in observing, and at
the eye-end is placed the usual little magnifier we have already
described.
The observer at the eye-end has control of certain rods by means
of which he can push the semi-lenses on their slides in the head
at the other end of the tube. Now, if he moves the semi-lenses so
as to bring them side by side exactly, the whole arrangement will
act like an ordinary telescope. For the semi-lenses will then fit
together just as if the original glass had never been cut. But
if the half-lenses are separated a little on their slides, each
will act by itself. Being slightly separated, each will cover a
different part of the sky. The whole affair acts as if the observer
at the eye-end were looking through two telescopes at once. For
each semi-lens acts independently, just as if it were a complete
glass of only half the size.
Now, suppose there were a couple of stars in the sky, one in the
part covered by the first semi-lens, and one in the part covered
by the second. The observer would, of course, see both stars at
once upon looking into the little magnifier at the eye-end of the
heliometer.
We must remember that these stars will appear in the field of view
simply as two tiny points of light. The astronomer, as we have
said, is provided with a simple system of long rods, by means of
which he can manipulate the semi-lenses while the observation is
being made. If he slides them very slowly one way or the other, the
two star-points in the field of view will be seen to approach each
other. In this way they can at last be brought so near together
that they will form but a single dot of light. Observation with
the heliometer consists in thus bringing two star-images together,
until at last they are superimposed one upon the other, and we see
one image only. Means are provided by which it is then possible to
measure the amount of separation of the two half-lenses. Evidently
the farther asunder on the sky are the two stars under observation,
the greater will be the separation of the semi-lenses necessary
to make a single image of their light. Thus, measurement of the
lenses' separation becomes a means of determining the separation
of the stars themselves upon the sky.
The two slides in the heliometer head are supplied with a pair of
very delicate measures or "scales" usually made of silver. These
can be examined from the eye-end of the instrument by looking
through a long microscope provided for this special purpose. Thus
an extremely precise value is obtained both of the separation of
the sliders and of the distance on the sky between the stars under
examination. Measures made in this way with the heliometer are
counted the most precise of astronomical observations.
Having thus described briefly the kind of observations obtained
with the heliometer, we shall now touch upon their further
utilization. We shall take as an example but one of their many
uses--that one which astronomers consider the most important--the
measurement of stellar distances. (See also p. 94.)
Even the nearest fixed star is almost inconceivably remote from
us. And astronomers are imprisoned on this little earth; we cannot
bridge the profound distance separating us from the stars, so as
to use direct measurement with tape-line or surveyor's chain. We
are forced to have recourse to some indirect method. Suppose a
certain star is suspected, on account of its brightness, or for
some other reason, of being near us in space, and so giving a
favorable opportunity for a determination of distance. A couple of
very faint stars are selected close by. These, on account of their
faintness, the astronomer may regard as quite immeasurably far
away. He then determines with his heliometer the exact position on
the sky of the bright star with respect to the pair of faint ones.
Half a year is then allowed to pass. During that time the earth has
been swinging along in its annual path or orbit around the sun.
Half a year will have sufficed to carry the observer on the earth
to the other side of that path, and he is then 185,000,000 miles
away from his position at the first observation.
Another determination is made of the bright star's position as
referred to the two faint ones. Now, if all these stars were
equally distant, their relative positions at the second observation
would be just the same as at the former one. But if, as is very
probable, the bright star is very much nearer us than are the two
faint ones, we shall obtain a different position from our second
observation. For the change of 185,000,000 miles in the observer's
location will, of course, affect the direction in which we see
the near star, while it will leave the distant ones practically
unchanged. Without entering into technical details, we may say that
from a large number of observations of this kind, we can obtain
the distance of the bright star by a process of calculation. The
only essential is to have an instrument that can make the actual
observations of position accurately enough; and in this respect the
heliometer is still unexcelled.
OCCULTATIONS
Scarcely anyone can have watched the sky without noticing how
different is the behavior of our moon from that of any other object
we can see. Of course, it has this in common with the sun and stars
and planets, that it rises in the eastern horizon, slowly climbs
the dome of the sky, and again goes down and sets in the west.
This motion of the heavenly bodies is known to be an apparent one
merely, and caused by the turning of our own earth upon its axis. A
man standing upon the earth's surface can look up and see above him
one-half the great celestial vault, gemmed with twinkling stars,
and bearing, perhaps, within its ample curve one or two serenely
shining planets and the lustrous moon. But at any given moment the
observer can see nothing of the other half of the heavenly sphere.
It is beneath his feet, and concealed by the solid bulk of the
earth.
The earth, however, is turning on an axis, carrying the observer
with it. And so it is continually presenting him to a new part of
the sky. At any moment he sees but a single half-sphere; yet the
very next instant it is no longer the same; a small portion has
passed out of sight on one side by going down behind the turning
earth, while a corresponding new section has come into view on
the opposite side. It is this coming into view that we call the
rising of a star; and the corresponding disappearance on the other
side is called setting. Thus rising and setting are, of course,
due entirely to a turning of the earth, and not at all to actual
motions of the stars; and for this reason, all objects in the sky,
without exception, must rise and set again. But the moon really has
a motion of its own in addition to this apparent one caused by the
earth's rotation.
Somewhere in the dawn of time early watchers of the stars thought
out those fancied constellations that survive even down to our own
day. They placed the mighty lion, king of beasts, upon the face
of night, and the great hunter, too, armed with club and dagger,
to pursue him. Among these constellations the moon threads her
destined way, night after night, so rapidly that the unaided eye
can see that she is moving. It needs but little power of fancy's
magic to recall from the dim past a picture of some aged astronomer
graving upon his tablets the Records of the Moon. "To-night she is
near the bright star in the eye of the Bull." And again: "To-night
she rides full, and near to the heart of the Virgin."
And why does the moon ride thus through the stars of night? Modern
science has succeeded in disentangling the intricacies of her
motion, until to-day only one or two of the very minutest details
of that motion remain unexplained. But it has been a hard problem.
Someone has well said that lunar theory should be likened to a
lofty cliff upon whose side the intellectual giants among men can
mark off their mental stature, but whose height no one of them may
ever hope to scale.
But for our present purpose it is unnecessary to pursue the subject
of lunar motion into its abstruser details. To understand why the
moon moves rapidly among the stars, it is sufficient to remember
that she is whirling quickly round the earth, so as to complete
her circuit in a little less than a month. We see her at all times
projected upon the distant background of the sky on which are set
the stellar points of light, as though intended for beacons to
mark the course pursued by moon and planets. The stars themselves
have no such motions as the moon; situated at a distance almost
inconceivably great, they may, indeed, be travellers through empty
space, yet their journeys shrink into insignificance as seen from
distant earth. It requires the most delicate instruments of the
astronomer to so magnify the tiny displacements of the stars on the
celestial vault that they may be measured by human eyes.
Let us again recur to our supposed observer watching the moon night
after night, so as to record the stars closely approached by her.
Why should he not some time or other be surprised by an approach so
close as to amount apparently to actual contact? The moon covers
quite a large surface on the sky, and when we remember the nearly
countless numbers of the stars, it would, indeed, be strange if
there were not some behind the moon as well as all around her.
A moment's consideration shows that this must often be the case;
and in fact, if the moon's advancing edge be scrutinized carefully
through a telescope, small stars can be seen frequently to
disappear behind it. This is a most interesting observation. At
first we see the moon and star near each other in the telescope's
field of view. But the distance between them lessens perceptibly,
even quickly, until at last, with a startling suddenness, the star
goes out of sight behind the moon. After a time, ranging from a few
moments to, perhaps, more than an hour, the moon will pass, and we
can see the star suddenly reappear from behind the other edge.
These interesting observations, while not at all uncommon, can
be made only very rarely by naked-eye astronomers. The reason is
simple. The moon's light is so brilliant that it fairly overcomes
the stars whenever they are at all near, except in the case of
very bright ones. Small stars that can be followed quite easily
up to the moon's edge in a good telescope, disappear from a
naked-eye view while the moon is still a long distance away. But
the number of very bright stars is comparatively small, so that
it is quite unusual to find anyone not a professional astronomer
who has actually seen one of these so-called "occultations."
Moreover, most people are not informed in advance of the occurrence
of an opportunity to make such observations, although they can be
predicted quite easily by the aid of astronomical calculations.
Sometimes we have occultations of planets, and these are the most
interesting of all. When the moon passes between us and one of the
larger planets, it is worth while to observe the phenomenon even
without a telescope.
Up to this point we have considered occultations chiefly as being
of interest to the naked-eye astronomer. Yet occultations have a
real scientific value. It is by their means that we can, perhaps,
best measure the moon's size. By noting with a telescope the time
of disappearance and reappearance of known stars, astronomers can
bring the direct measurement of the moon's diameter within the
range of their numerical calculations. Sometimes the moon passes
over a condensed cluster of stars like the Pleiades. The results
obtainable on these occasions are valuable in a very high degree,
and contribute largely to making precise our knowledge of the lunar
diameter.
There is another thing of scientific interest about occultations,
though it has lost some of its importance in recent years. When
such an event has been observed, the agreement of the predicted
time with that actually recorded by the astronomer offers a most
searching test of the correctness of our theory of lunar motion. We
have already called attention to the great inherent difficulty of
this theory. It is easy to see that by noting the exact instant of
disappearance of a star at a place on the earth the latitude and
longitude of which are known, we can both check our calculations
and gather material for improving our theory. The same principle
can be used also in the converse direction. Within the limits of
precision imposed by the state of our knowledge of lunar theory,
we can utilize occultations to help determine the position on
the earth of places whose longitude is unknown. It is a very
interesting bit of history that the first determination of the
longitude of Washington was made by means of occultations, and that
this early determination led to the founding of the United States
Naval Observatory.
On March 28, 1810, Mr. Pitkin, of Connecticut, reported to
the House of Representatives on "laying a foundation for the
establishment of a first meridian for the United States, by which a
further dependence on Great Britain or any other foreign nation for
such meridian may be entirely removed." This report was the result
of a memorial presented by one William Lambert, who had deduced the
longitude of the Capitol from an occultation observed October 20,
1804. Various proceedings were had in Congress and in committee,
until at last, in 1821, Lambert was appointed "to make astronomical
observations by lunar occultations of fixed stars, solar eclipses,
or any approved method adapted to ascertain the longitude of the
Capitol from Greenwich." Lambert's reports were made in 1822
and 1823, but ten years passed before the establishment of a
formal Naval Observatory under Goldsborough, Wilkes, and Gilliss.
But to Lambert belongs the honor of having marked out the first
fundamental official meridian of longitude in the United States.
MOUNTING GREAT TELESCOPES
There are many interesting practical things about an astronomical
observatory with which the public seldom has an opportunity to
become acquainted. Among these, perhaps, the details connected
with setting up a great telescope take first rank. The writer
happened to be present at the Cape of Good Hope Observatory when
the photographic equatorial telescope was being mounted, and the
operation of putting it in position may be taken as typical of
similar processes elsewhere. (See also p. 86.)
[Illustration: Forty-Inch Telescope, Yerkes Observatory, University
of Chicago.]
In the first place, it is necessary to explain what is meant by an
"equatorial" telescope. One of the chief difficulties in making
ordinary observations arises from the rising and setting of the
stars. They are all apparently moving across the face of the sky,
usually climbing up from the eastern horizon, only to go down again
and set in the west. If, therefore, we wish to scrutinize any
given object for a considerable time, we must move the telescope
continuously so as to keep pace with the motion of the heavens. For
this purpose, the tube must be attached to axles, so that it can be
turned easily in any direction. The equatorial mounting is a device
that permits the telescope to be thus aimed at any part of the sky,
and at the same time facilitates greatly the operation of keeping
it pointed correctly after a star has once been brought into the
field of view.
To understand the equatorial mounting it is necessary to remember
that the rising and setting motions of the heavenly bodies are
apparent ones only, and due in reality to the turning of the earth
on its own axis. As the earth goes around, it carries observer,
telescope, and observatory past the stars fixed upon the distant
sky. Consequently, to keep a telescope pointed continuously at a
given star, it is merely necessary to rotate it steadily backward
upon a suitable axis just fast enough to neutralize exactly the
turning of our earth.
By a suitable axis for this purpose we mean one so mounted as
to be exactly parallel to the earth's own axis of rotation. A
little reflection shows how simply such an arrangement will work.
All the heavenly bodies may be regarded, for practical purposes,
as excessively remote in comparison with the dimensions of our
earth. The entire planet shrinks into absolute insignificance
when compared with the distances of the nearest objects brought
under observation by astronomers. It follows that if we have our
telescope attached to such a rotation-axis as we have described,
it will be just the same for purposes of observation as though the
telescope's axis were not only parallel to the earth's axis, but
actually coincident with it. The two axes may be separated by a
distance equal to that between the earth's surface and its centre;
but, as we have said, this distance is insignificant so far as our
present object is concerned.
There is another way to arrive at the same result. We know that
the stars in rising and setting all seem to revolve about the pole
star, which itself seems to remain immovable. Consequently, if we
mount our telescope so that it can turn about an axis pointing at
the pole, we shall be able to neutralize the rotation of the stars
by simply turning the telescope about the axis at the proper speed
and in the right direction. Astronomical considerations teach us
that an axis thus pointing at the pole will be parallel to the
earth's own axis. Thus we arrive at the same fundamental principle
for mounting an astronomical telescope from whichever point of view
we consider the subject.
Every large telescope is provided with such an axis of rotation;
and for the reason stated it is called the "polar axis." The
telescope itself is then called an "equatorial." The advantage
of this method of mounting is very evident. Since we can follow
the stars' motions by turning the telescope about one axis only,
it becomes a very simple matter to accomplish this turning
automatically by means of clock-work.
The "following" of a star being thus provided for by the device
of a polar axis, it is, of course, also necessary to supply some
other motion so as to enable us to aim the tube at any point in the
heavens. For it is obvious that if it were rigidly attached to the
polar axis, we could, indeed, follow any star that happened to be
in the field of view, but we could not change this field of view at
will so as to observe other stars or planets. To accomplish this,
the telescope is attached to the polar axis by means of a pivot.
By turning the telescope around its polar axis, and also on this
pivot, we can find any object in the heavens; and once found, we
can leave to the polar axis and its automatic clock-work the task
of keeping that object before the observer's eye.
In setting up the Cape of Good Hope instrument the astronomers were
obliged to do a large part of the work of adjustment personally.
Far away from European instrument-makers, the parts of the
mounting and telescope had to be "assembled," or put together, by
the astronomers of the Cape Observatory. A heavy pier of brick
and masonry had been prepared in advance. Upon this was placed a
massive iron base, intended to support the superstructure of polar
axis and telescope. This base rested on three points, one of which
could be screwed in and out, so as to tilt the whole affair a
little forward or backward. By means of this screw we effected the
final adjustment of the polar axis to exact parallelism with that
of the earth. Other screws were provided with which the base could
be twisted a little horizontally either to the right or left. Once
set up in a position almost correct, it was easy to perfect the
adjustment by the aid of these screws.
Afterward the tube and lenses were put in place, and the clock
properly attached inside the big cast-iron base. This clock-work
looked more like a piece of heavy machinery than a delicate clock
mechanism. But it had heavy work to do, carrying the massive
telescope with its weighty lenses, and needed to be correspondingly
strong. It had a driving-weight of about 2,000 pounds, and was so
powerful that turning the telescope affected it no more than the
hour-hand of an ordinary clock affects the mechanism within its
case.
The final test of the whole adjustment consisted in noting whether
stars once brought into the telescopic field of view could be
maintained there automatically by means of the clock. This object
having been attained successfully, the instrument stood ready to be
used in the routine business of the observatory.
Before leaving the subject of telescope-mountings, we must mention
the giant instrument set up at the Paris Exposition of 1900. The
project of having a _Grande Lunette_ had been hailed by newspapers
throughout the world and by the general public in their customary
excitable way. It was tremendously over-advertised; exaggerated
notions of the instrument's powers were spread abroad and eagerly
credited; the moon was to be dragged down, as it were, from its
customary place in the sky, so near that we should be able almost
to touch its surface. As to the planets--free license was given to
the journalistic imagination, and there was no effective limitation
to the magnificence of astronomical discovery practically within
our grasp, beyond the necessity for printed space demanded by
sundry wars, pestilences, and other mundane trifles.
[Illustration: Yerkes Observatory, University of Chicago.]
Now, the present writer is very far from advocating a lessening of
the attention devoted to astronomy. Rather would he magnify his
office than diminish it. But let journalistic astronomy be as good
an imitation of sober scientific truth as can be procured at space
rates; let editors encourage the public to study those things
in the science that are ennobling and cultivating to the mind;
let there be an end to the frenzied effort to fabricate a highly
colored account of alleged discoveries of yesterday, capable of
masquerading to-day under heavy head-lines as News.
The manner in which the big telescope came to be built is not
without interest, and shows that enterprise is far from dead, even
in the old countries. A stock company was organized--we should
call it a corporation--under the name _Société de l'Optique_. It
would appear that shares were regularly put on the market, and
that a prospectus, more or less alluring, was widely distributed.
We may say at once that the investing public did not respond
with obtrusive alacrity; but at all events, the promoters'
efforts received sufficient encouragement to enable them to begin
active work. From the very first a vigorous attempt was made to
utilize both the resources of genuine science and the devices of
quasi-charlatanry. It was announced that the public were to be
admitted to look through the big glass (apparently at so much an
eye), and many, doubtless, expected that the man in the street
would be able to make personal acquaintance with the man in the
moon. A telescopic image of the sun was to be projected on a big
screen, and exhibited to a concourse of spectators assembled in
rising tiers of seats within a great amphitheatre. And when clouds
or other circumstances should prevent observing the planets or
scrutinizing the sun, a powerful stereopticon was to be used.
Artificial pictures of the wonders of heaven were to be projected
on the screen, and the public would never be disappointed. It
was arranged that skilled talkers should be present to explain
all marvels: and, in short, financial profit was to be combined
with machinery for advancing scientific discovery. Astronomers
the world over were "circularized," asked to become shareholders,
and, in default of that, to send lantern-slides or photographs of
remarkable celestial objects for exhibition in the magic-lantern
part of the show.