MOUNTAIN OBSERVATORIES
The century that has elapsed since the time of Sir William Herschel,
known as the father of the new or descriptive astronomy, has witnessed
all the advances of the science that have been made possible by adopting
the photographic method of making the record, instead of depending upon
the human eye. Only one eye can be looking at the eyepiece at a time:
the photograph can be studied by a thousand eyes.
At mountain elevations telescopes are now extensively employed, and
there the camera is of especial and additional value, because the
photograph taken on the mountain can be brought down for the expert to
study, at ease and in the comfort of a lower elevation. We shall next
trace the movement that has led the astronomer to seek the summits of
mountains for his observatories, and the photographer to follow him.
Not only did the genius of Newton discover the law of universal
gravitation, and make the first experiments in optics essential to the
invention of the spectroscope, but he was the real originator also of
the modern movement for the occupation of mountain elevations for
astronomical observatories. His keen mind followed a ray of light all
the way from its celestial source to the eye of the observer, and
analyzed the causes of indistinct and imperfect vision.
Endeavoring to improve on the telescope as Galileo and his followers had
left it, he found such inherent difficulties in glass itself that he
abandoned the refracting type of telescope for the reflector, to the
construction of which he devoted many years. But he soon found out, what
every astronomer and optician knew to their keen regret, that a
telescope, no matter how perfectly the skill of the optician's hand may
make it, cannot perform perfectly unless it has an optically perfect
atmosphere to look through.
So Newton conceived the idea of a mountain observatory, on the summit of
which, as he thought, the air would be not only cloudless, but so steady
and equable that the rays of light from the heavenly bodies might reach
the eye undisturbed by atmospheric tremors and quiverings which are
almost always present in the lower strata of the great ocean of air that
surrounds our planet.
This is the way Newton puts the question in his treatise on
_Opticks_--he says: "The Air through which we look upon the Stars, is in
a perpetual Tremor; as may be seen by the tremulous Motion of Shadows
cast from high Towers, and by the twinkling of the fix'd stars.... The
only remedy is a most serene and quiet Air, such as may perhaps be found
on the tops of the highest Mountains above the grosser Clouds."
Newton's suggestion is that the _highest_ mountains may afford the best
conditions for tranquillity; and it is an interesting coincidence that
the summits of the highest mountains, about 30,000 feet in elevation,
are at about the same level where the turbulence of the atmosphere most
likely ceases, according to the indications of recent meteorological
research. These heights are far above any elevations permanently
occupied as yet, but a good beginning has been made and results of great
value have already been reached.
Curiously, investigation of mountain peaks and their suitability for
this purpose was not undertaken till nearly two centuries after Newton,
when Piazzi Smyth in 1856 organized his expedition to the summit of a
mountain of quite moderate elevation, and published his "Teneriffe: an
Astronomer's Experiment." Teneriffe is an accessible peak of about
10,000 feet, on an island of the Canaries off the African coast, where
Smyth fancied that conditions of equability would exist; and on reaching
the summit with his apparatus and spending a few days and nights there,
he was not disappointed. Could he have reached an elevation of 13,000
feet, he would have had fully one-third of all the atmosphere in weight
below him, and that the most turbulent portion of all. Nevertheless, the
gain in steadiness of the atmosphere, providing "better seeing," as the
astronomer's expression is, even at 10,000 feet, was most encouraging,
and led to attempts on other peaks by other astronomers, a few of whom
we shall mention.
Davidson, an observer of the United States Coast Survey, with a broad
experience of many years in mountain observing, investigated the summit
of the Sierra Nevada mountains as early as 1872, at an elevation of
7,200 feet. His especial object was to make an accurate comparison
between elevated stations at different heights. He found the seeing
excellent, especially on the sun; but the excessive snowfall at his
station, 45 feet annually, was a condition very adverse to permanent
occupation.
In the summer of 1872, Young spent several weeks at Sherman, Wyoming, at
an elevation exceeding 8,300 feet. He carried with him the 9.4-inch
telescope of Dartmouth College, where he was then professor, and this
was the first expedition on which a large glass was used by a very
skillful observer at great elevation. He found the number of good days
and nights small, but the sky was exceedingly favorable when clear. Many
7th magnitude stars could be detected with the naked eye. Young's
observations at Sherman were mainly spectroscopic, however, and they
demonstrated the immense advantage of a high-level station, far above
the dust and haze of the lower atmosphere. He pronounced the 9.4-inch
glass at 8,000 feet the full equivalent of a 12-inch at sea level.
Mont Blanc of 15,000 feet elevation was another summit where the veteran
Janssen of Paris maintained a station for many years; but the
continental conditions of atmospheric moisture and circulation were not
favorable on the whole. Janssen was mainly interested in the sun, and
the daylight seeing is rarely benefited, owing to the strong upward
currents of warm air set in motion by the sun itself.
Mountains in the beautiful climate of California were among the earliest
investigated, and when in 1874 the trustees of Mr. James Lick's estate
were charged with equipping an observatory with the most powerful
telescope in existence, they wisely located on the summit of Mount
Hamilton. It is 4,300 feet above sea level, and Burnham and other
astronomers made critical tests of the steadiness of vision there by
observing double stars, which afford perhaps the best means of comparing
the optical quality of the atmosphere of one region with another. The
writer was fortunate in having charge of the observations of the transit
of Venus in 1882 on the mountain, when the Observatory was in process of
construction, and the quality of the photographs obtained on that
occasion demonstrated anew the excellence of the site. Particularly at
night, for about nine months of the year, the seeing is exceptionally
good, especially when fog banks rolling in from the Pacific, cover the
valleys below like a blanket, preventing harmful radiation from the soil
below.
The great telescope mounted in 1888, a 36-inch refractor by Alvan Clark,
has fulfilled every expectation of its projectors, and justified the
selection of the site in every particular. The elevation, although
moderate, is still high enough to secure very marked advantage in
clearness and steadiness of the air, and at the same time not so high
that the health and activities of the observers are appreciably affected
by the thinner air of the summit. This telescope is known the world over
for the monumental contributions to science made by the able astronomers
who have worked with it: among them Barnard who discovered the fifth
satellite of Jupiter in 1892; Burnham, Hussey, and Aitken, who have
discovered and measured thousands of close double stars; Keeler, who
spent many faithful years on the summit; and Campbell, the present
director, whose spectroscopic researches on stellar movements have added
greatly to our knowledge of the structure of the universe. Among the
many lines of research now in progress at the Lick Observatory and in
the D. O. Mills Observatory at Santiago, Chile, are the discoveries of
stars whose velocities in space are not constant, but variable with the
spectral type of the star. Mr. Lick's bequest for the Observatory was
about $700,000. So ably has this scientific trust been administered that
he might well have endowed it with his entire estate, exceeding
$4,000,000.
Another California mountain that was early investigated is Mount
Whitney. Its summit elevation is nearly 15,000 feet, and in 1881 Langley
made its ascent for the purpose of measuring the solar constant. He
found conditions much more favorable than on Mount Etna,
Sicily--elevation about 10,000 feet--which he had visited the year
before. But the height of Mount Whitney was such as to occasion him much
inconvenience from mountain sickness, an ailment which is most
distressing and due partly to lack of oxygen and partly to mere
diminution of mechanical pressure. Mount Whitney was also visited many
years after by Campbell for investigating the spectrum of Mars in
comparison with that of the moon. Langley found on Mount Whitney an
excellent station lower down, at about 12,000 feet elevation; and by
equipping the two stations with like apparatus for measuring the solar
heat, he obtained very important data on the selective absorption of the
atmosphere.
Returning from the transit of Venus in 1882, Copeland of Edinburgh
visited several sites in the Andes of Peru, ascending on the railway
from Mollendo. Vincocaya was one of the highest, something over 14,000
feet elevation. His report was most enthusiastic, not only as to
clearness and transparency of the atmosphere, but also as to its
steadiness, which for planetary and double star observations is almost
as important. Copeland's investigation of this region of the Andes has
led many other astronomers to make critical tests in the same general
region. Climatic conditions are particularly favorable, and the sites
for high-level research are among the best known, the atmosphere being
not only clear a large part of the year, but in certain favored spots
exceedingly steady.
In 1887 the writer ascended the summit of Fujiyama, Japan, 12,400 feet
elevation. The early September conditions as to steadiness of atmosphere
were extraordinarily fine, but the mountain is covered by cloud many
months in each year. There is a saddle on the inside of the crater that
would form an ideal location for a high-level observatory. This
expedition was undertaken at the request of the late Professor
Pickering, director of Harvard College Observatory, which had recently
received a bequest from Uriah A. Boyden, amounting to nearly a quarter
of a million dollars, to "establish and maintain, in conjunction with
others, an astronomical observatory on some mountain peak."
Great elevations were systematically investigated in Colorado and
California, the Chilean desert of Atacama was visited, and a temporary
station established at Chosica, Peru, elevation about 5,000 feet.
Atmospheric conditions becoming unfavorable, a permanent station was
established in 1891 at Arequipa, Peru, elevation 8,000 feet, which has
been maintained as an annex to the Harvard Observatory ever since. The
cloud conditions have been on the whole less favorable than was
expected, but the steadiness of the air has been very satisfactory. In
addition to planetary researches conducted there in the earlier years by
W. H. Pickering, many large programs of stellar research have been
executed, especially relating to the magnitudes and spectra of the
stars. In conjunction with the home observatory in the northern
hemisphere, this afforded a vast advantage in embracing all the stars of
the entire heavens, on a scale not attempted elsewhere. The Bruce
photographic telescope of 24-inch aperture has been employed for many
years at Arequipa, and with it the plates were taken which enabled
Pickering to discover the ninth satellite of Saturn (Phoebe), and the
splendid photographs of southern globular clusters in which Bailey has
found numerous variable stars of very short periods--very faint objects,
but none the less interesting, and of much significance in modern study
of the evolution and structure of the stellar universe. The crowning
research of the observatory is the Henry Draper catalogue of stellar
spectra, now in process of publication, which is of the first order of
importance in statistical studies of stellar distribution with reference
to spectral type, and in studying the relation of parallax and distance,
proper motion, radial velocity and its variation to the spectral
characteristics of the stars.
Perrine of Cordova is now establishing on Sierra Chica about twenty-five
miles southwest of Cordova, a great reflecting telescope comparable in
size with the instruments of the northern hemisphere, for investigation
of the southern nebulae and clusters, and motions of the stars. The
elevation of this new Argentine observatory will be 4,000 feet above sea
level.
Another observatory at mountain elevation and in a highly favorable
climate is the Lowell Observatory, located at about 7,000 feet elevation
at Flagstaff, Arizona. Many localities were visited and the atmosphere
tested especially for steadiness, an optical quality very essential for
research on the planetary surfaces. Mexico was one of these stations,
but local air currents and changes of temperature there were such that
good seeing was far from prevalent, as had been expected. At Flagstaff,
on the other hand, conditions have been pretty uniformly good, and an
enormous amount of work on the planet Mars has been accumulated and
published. The first successful photographs of this planet were taken
there in 1905, and Jupiter, Saturn, the zodiacal light and many other
test objects have been photographed, which demonstrates the excellence
of the site for astronomical research. Within recent years spectrum
research by Slipher, especially on the nebulae, has been added to the
program, and the rotation and radial velocities of many nebulae have been
determined.
On Mount Wilson, near Pasadena, California, at an elevation of nearly
6,000 feet, is the Carnegie Solar Observatory, founded and equipped
under the direction of Professor George E. Hale, as a department of the
Carnegie Institution of Washington, of which Dr. John Campbell Merriam
is President. The climatology of the region was carefully investigated
and tests of the seeing made by Hussey and others. Although equipped
primarily for study of the sun, the program of the observatory has been
widely amplified to include the stars and nebulae. The instrumental
equipment is unique in many respects. To avoid the harmful effect of
unsteadiness of air strata close to the ground a tower 150 feet high was
erected, with a dome surmounting it and covering a coelostat with mirror
for reflecting the sun's rays vertically downward. Underneath the tower
a dry well was excavated to a depth equal to 1/2 the height of the tower
above it. In the subterranean chamber is the spectroheliograph of
exceptional size and power. The sun's original image is nearly 17 inches
in diameter on the plate, and the solar chromosphere and prominences,
together with the photosphere and faculae, are all recorded by
monochromatic light.
Connected with the observatory on Mount Wilson are the laboratories,
offices and instrument shops in Pasadena, 16 miles distant, where the
remarkable apparatus for use on the mountain is constructed. A
reflecting telescope with silver-on-glass mirror 60 inches in diameter
was first built by Ritchey and thoroughly tested by stellar photographs.
Also the northern spiral nebulae were photographed, exhibiting an
extraordinary wealth of detail in apparent star formation. The success
of this instrument paved the way for one similar in design, but with a
mirror 100 inches in diameter, provided by gift of the late John D.
Hooker of Los Angeles. The telescope was completed in 1919.
Notwithstanding its huge size and enormous weight, the mounting is very
successful, as well as the mirror. Mercurial bearings counterbalance the
weight of the polar axis in large part. This great telescope, by far the
largest and most powerful ever constructed, is now employed on a program
of research in which its vast light-gathering power will be utilized to
the full. Under the skillful management of Hale and his enthusiastic and
capable colleagues, the confines of the stellar heavens will be
enormously extended, and secrets of evolution of the universe and of its
structure no doubt revealed.
In all the mountain stations hitherto established, as the Lick
Observatory at 4,000 feet, the Mount Wilson Observatory at 6,000 feet,
the Lowell Observatory at 7,000 feet, the Harvard Observatory at 8,000
feet; and Teneriffe and Etna at 10,000, Fujiyama at 12,000, Pike's Peak
at 14,000, Mont Blanc and Mount Whitney at 15,000, the researches that
have been carried on have fully demonstrated the vast advantage of
increased elevation in localities where climatological conditions as
well as elevation are favorable. Nevertheless, only one-half of the
extreme altitude contemplated by Sir Isaac Newton has yet been attained.
Can the greater heights be reached and permanently occupied?
Geographically and astronomically the most favorably located mountain
for a great observatory is Mount Chimborazo in Ecuador. Its elevation is
22,000 feet, and it was ascended by Edward Whymper in 1880. Situated
very nearly on the earth's equator, almost the entire sidereal heavens
are visible from this single station, and all the planets are favored by
circumzenith conditions when passing the meridian. No other mountain in
the world approaches Chimborazo in this respect. But the summit is
perpetually snow-capped, exceedingly inaccessible, and the defect of
barometric pressure would make life impossible up there in the open.
Only one method of occupation appears to be feasible. The permanent snow
line is at about 16,000 feet, where excellent water power is available.
By tunneling into the mountain at this point, and diagonally upward to
the summit, permanent occupation could be accomplished, at a cost not to
exceed one million dollars.
The rooms of the summit observatory would need to be built as steel
caissons, and supplied with compressed air at sea-level tension. The
practicability of this plan was demonstrated by the writer in
September, 1907, at Cerro de Pasco, Peru. A steel caisson was carried up
to an elevation exceeding 14,000 feet. Patients suffering acutely with
mountain sickness were placed inside this caisson, and on restoring the
atmospheric pressure within it artificially all unfavorable
symptoms--headache, high respiration and accelerated pulse--disappeared.
There was every indication that if persons liable to this uncomfortable
complaint were brought up to this elevation, or indeed any attainable
elevation, under unreduced pressure, the symptoms of mountain sickness
would be unknown. Comfortable occupation of the highest mountain summits
was thereby assured.
The working of astronomical instruments from within air-tight
compartments does not present any insurmountable difficulties, either
mechanical or physical. Since the time these experiments were made, the
Guayaquil-Quito railway has been constructed over a saddle of
Chimborazo, at an elevation of 12,000 feet; and only six miles of
railway would need to be built from this station to the point where the
tunnel would enter the mountain.
Only by the execution of some such plan as this can astronomers hope to
overcome the baleful effects of an ever mobile atmosphere, and secure
the advantages contemplated by Sir Isaac Newton in that tranquillity of
atmosphere, which he conceived as perpetually surrounding the summits of
the highest mountains.
In Russell's theory of the progressive development of the stars, from
the giant class to the dwarf, an element of verification from
observation is lacking, because hitherto no certain method of measuring
the very minute angular diameters of the stars has been successfully
applied. The apparent surface brightness corresponding to each spectral
type is pretty well known, and by dividing it into the total apparent
brightness, we have the angular area subtended by the star, quite
independent of the star's distance. This makes it easy to estimate the
angular diameter of a star, and Betelgeuse is the one which has the
greatest angular diameter of all whose distances we know. Antares is
next in order of angular diameter, 0".043, Aldebaran 0".022, Arcturus
0".020, Pollux 0".013, and Sirius only 0".007.
Can these theoretical estimates be verified by observation? Clearly it
is of the utmost importance and the exceedingly difficult inquiry has
been undertaken with the 100-inch reflector on Mount Wilson, employing
the method of the interferometer developed by Michelson and described
later on, an instrument undoubtedly capable of measuring much smaller
angles than can be measured by any other known method. Unquestionably
the interference of atmospheric waves, or in other words what
astronomers call "poor seeing," will ultimately set the limit to what
can be accomplished. "But even if," says Eddington, "we have to send
special expeditions to the top of one of the highest mountains in the
world, the attack on this far-reaching problem must not be allowed to
languish."