Agreeably to the announcement of the annual report of Harvard
College Observatory for 1877, as to photometry, a beginning was made
by constructing a photometer suitable to be attached to the great
telescope. Other photometers have been devised at different times for
use independently. One of the earliest was applied during the year
beginning Oct. 12, 1877, in measuring the light of all known satellites
excepting the two inner ones of Uranus, which are too faint to be
discerned, even by the great telescope. The first prolonged observation
entered upon was of the eclipses of Jupiter’s satellites.
As there are four satellites and as the plane of their orbits is nearly
the same as that of the planet itself, eclipses are frequent. The
plan proposed the observation of all these eclipses visible during a
revolution of Jupiter around the sun, a period of about 12 years. The
work was begun June 23, 1878, and has been regularly pursued. The final
result will be of the highest value in that, among its utilities it
will permit a new and independent computation to be made of the earth’s
distance from the sun, which distance is a prime factor in theoretical
astronomy.
Computations hitherto made, based upon data derived from these
eclipses, are not authoritative, because of disagreements among
different observers using different telescopes, and because of defects
in the method of observation.
The director’s report for 1878 says: “Errors of this kind are much
lessened by photometric observations of the satellites as they
gradually enter or emerge from the shadow of Jupiter, using the planet
itself or another satellite as a standard. Each comparison thus
obtained gives an independent determination of the time of the eclipse,
free from the errors due to the condition of the air or the power of
the telescope employed and less likely to be affected by personal
equation than the observation of a disappearance or a reappearance.
By the ordinary method an observation during twilight can have little
value, while good photometric observations may be made as well then as
at any other time. It is even possible to make them before sunset.”
In 1879 a work of magnitude was begun—the photometric observation
of all stars down to those of the sixth magnitude visible in this
latitude. For greater facility, and particularly to avoid loss of time
in identifying stars of small magnitude, it was decided to make a new
departure in method and in construction of an instrument. The new
instrument was called the meridian photometer, and stars were observed
by it only when near the meridian. The position of any star being well
known, the time of its appearance in the field of the telescope could
be foreseen.
Each that was desired for a particular night had, therefore, only to
be waited for, not sought for. The original instrument consisted of a
fixed horizontal telescope pointed west and having two objectives.
The light of the pole star, which was taken for the standard or unit of
measurement, was reflected by a prism into one object glass, and that
of the star to be measured into the other. The light of the brighter
star was then reduced to exactly that of the fainter by the turning of
a screw having a register attached. The indication of the register gave
the measure, which was confirmed by repeated observations. Telescopes
mounted in the ordinary way continued to be used in other branches of
photometric work.
[Illustration: HARVARD OBSERVING STATION AT WILLOWS, CAL., JANUARY 1,
1889.]
The photometric survey of the sixth magnitude and brighter stars was
completed Aug. 25, 1881. In 1882 a new and more powerful meridian
instrument was constructed and a photometric survey of a list of about
21,000 stars, from the sixth to the ninth magnitude, was entered
upon. This work was finished Sept. 29, 1888, and soon afterwards the
instrument, with others, was sent to Peru in charge of Mr. S. I. Bailey
of the observatory corps, where, May 11, 1889, a corresponding survey
of the stars, from the first to the ninth magnitude, inclusive,
between 30° south and the southern pole, was begun. Thus the facts
relating to all the stars in the sky of these classifications will be
embodied in the final result.
The record, which will comprise several volumes, one or more of which
have already been published, will have an identity throughout as
respects the method, the instrument, and the unit of measurement. It
will be authoritative as a text book or series of text books, and will
enhance the value for reference, and comparison of various records of
the light of stars, both those of modern and ancient date.
The successful working of the two meridian photometers led to the
construction of one still more powerful, having an aperture of 12
inches. The first was of 1½ inch aperture, and the second of four
inches.
The three differ somewhat in mechanism, but are the same in principle.
The 12-inch is called by distinction the “horizontal telescope.” It
will be available in case a photometric survey of stars of fainter
magnitudes shall be undertaken, but its use is not limited to
photometry.
In 1879, a photometer was devised for measuring the light of nebulæ,
thus applying to these objects and to stars the same unit and scale. In
1881, photometric observations of certain bright parts of the moon,
were made for the Selenographical Society of England, the particular
parts being selected by that society. It thus was shown that the
lunar scale of light in common use may be closely expressed in terms
of stellar magnitude, each degree of the lunar scale answering to
six-tenths of a magnitude. Photometry has been very extensively applied
at Harvard in study of variable stars.
A history of any department of practical astronomy, written from the
point of view of a mechanician, could hardly fail to be of interest.
Among the curious experiences at Harvard in the line of photometry is
one which illustrates this point, and, at the same time, indicates the
refinements in observation which are resorted to, and demonstrates one
of the utilities of the photometric method.
[Illustration: HARVARD OBSERVING STATION IN PERU.]
In 1877 announcement was made of the discovery at Washington of two
satellites of the planet Mars. The Harvard telescope being applied
they were after a little effort descried as two faint points of light,
showing no visible disks. To ascertain the diameter of each satellite
might therefore seem impossible: but it was done, approximately, by the
photometric method.
The mechanical problem was to reduce the light of the planet as seen in
the telescope to an equality with the light of one of the satellites
as thus seen. Five or six different mechanics were employed to drill
in a piece of metal a hole, making a true circle, and small enough to
produce the equality sought for by sufficiently diminishing the light
of the planet. It may be remarked that one of those who succeeded best
had already, for his own purposes, managed to drill a hole, lengthwise,
through a fine cambric needle, making a steel tube of it.
What he made for Prof. Pickering was a hole in a steel plate, the
diameter of which was one eighteen hundredth (1-1800) of an inch. It
was so nearly circular that the various diameters, including errors of
measurement, only differed one one hundred thousandth (1-100,000) of an
inch.
Other mechanical devices were resorted to for corroboration, and the
results reached were that the diameter of one of the satellites is
about six miles, and that of the other about seven miles. They are the
smallest known in the solar system.
The availability of the spectroscope in astronomy had early been
appreciated by the profession. In experiments in this line it had been
found that a classification of the nebulæ might be made upon the basis
of their spectra. In 1880 the study was carried a stage further at
Harvard in ascertaining by the spectroscope that certain faint objects,
which, by direct vision, had been judged to be stars, are in fact
nebulæ. In 1881, it was found that the spectroscope is serviceable in
the discovery of variable stars. Thus incited, a new instrument was
imported from London, but it did not prove satisfactory.
Nothing of importance appears to have been done in this department
thereafter until 1886, when the proposition of Mrs. Draper opened the
way to investigation of spectra by aid of photography. For this the
11-inch photographic telescope, which had been used by Dr. Draper,
was loaned by Mrs. Draper, who also met the expense of a new mounting
and a special observatory building. A beginning was made with an
eight-inch instrument, known as the Bache telescope. It is of the
pattern described as the “doublet,” and offers the advantage of a
large field of view. With it the spectra of about 10,500 stars of the
sixth magnitude and brighter, between the pole and 25° south, were
photographed before the close of the year 1888.
The instrument was then sent to Peru, where a like survey of the
Southern sky is in progress. Spectroscopic observations of the brighter
stars have been continued at Cambridge with the 11-inch Draper
telescope and of fainter stars with an 8-inch doublet similar to the
Bache instrument. In this work it was found that by giving a certain
chemical stain to the photographic plate the yellow and green portions
of the spectrum of even the fainter stars can profitably be studied.
Furthermore, what seems incredible at first thought, it appears to be
demonstrated that the components of binary stars whose juxtaposition
does not permit them to be separated in any telescope, may, by
spectroscopic photography, be shown to be in revolution about each
other. Two or more such objects have been found in which the changes
regularly succeeding in the lines of the spectrum not only prove that
the components are in motion, but permit the period of revolution to be
determined.
Prior to 1883 photography is mentioned in the annual reports of the
present director only as incidental to other work. In that year a
systematic investigation was undertaken, having among other objects
in view, the construction of a photographic map of the whole heavens.
An early application of photography in this investigation was in the
direction of determining the color of stars, measuring their brightness
by an independent method, picturing their spectra, exhibiting the
effect of atmospheric absorption of light in a series of plates
covering the period of a year, and ascertaining by images of stars
trailed upon the plate, the clearness and steadiness of the atmosphere.
In 1887 the Boyden fund being available, the first step was taken in
the important enterprise of giving a continental expansion to the work
of the observatory. The aim of the testator in making his bequest
could well be furthered in conducting observations simultaneously
in photometry, spectroscopy and photography. In following up the
project, the Draper memorial funds appear also to have been available
to a considerable extent in the two latter methods of observation.
Experimental stations were established in Colorado in the summer of
1887 on mountain peaks of 14,000, 11,000 and 6000 feet in height,
respectively, and the meteorological conditions, including the
transparency and steadiness of the upper atmosphere, were duly tested.
This investigation was continued at the expense of the Boyden fund
during the following winter by local observers whose stations were at
considerable height.
In 1889 the movement was further extended by establishing an
observatory on a peak about 6500 feet high in Peru, 25 or 30 miles
distant from the sea coast and the city of Lima. Local official
sanction was given to naming the peak, “Monte Harvard.” About the
same time other observers of the Harvard corps set up an experimental
observatory on Mt. Wilson, 6000 feet high, in Southern California. The
station is about 30 miles from the sea coast and somewhat less from the
city of Los Angeles.
The experimental purpose is the same as in Colorado, and looks to
the ultimate establishment of a permanent observatory as a branch of
the Harvard institution at some favorable point where the superior
atmospheric conditions of the Pacific mountain regions can be had. In
the special direction of picturing celestial objects at Mt. Wilson
remarkable photographic results are already possessed at Cambridge in
plates showing lunar surfaces, Saturn’s rings, Jupiter’s belts and the
most brilliant of the nebulæ. That among them which is of the greatest
scientific interest, as being a novelty, is the picture on a negative
plate of the great spiral nebula of Orion. It is a Harvard discovery by
the photographic method, and is quite other than that heretofore known
as the great nebula in Orion. That is an object having a span of about
half a degree. The new great nebula has a span of nearly 17 degrees;
its outline includes all the stars of the constellation, and it is too
faint an object to be discerned by the naked eye.
It is one of the principal advantages of the photographic method in
astronomical work that the sensitive plate will denote objects which
the eye reinforced by a telescope of any power cannot detect. The
great nebula thus discovered is within reach of the telescope, but its
dimensions are so much larger than the field of the telescope, and its
outline so faint, that its true character would not thus originally be
apprehended.
Photography at Cambridge has already produced several series of
plates, each plate covering a section of the northern sky, the whole
of which when perfected and collated will be a self-recorded, and so,
indisputable atlas, showing the position of all stars down to those of
the 11th magnitude. It will be an atlas in sheets of glass, and frailer
in some respects than if composed of sheets of paper. But for study
of the science the glass is better than any product of the engraver’s
art, and better than any sun picture printed by the plate itself.
Indeed, it is one of the triumphs of the photographic method that a
perfect photographic negative discloses more to the student than does
a telescopic view of that area of the sky of which the photograph is a
copy. Astronomical research is now constantly made at the observatory
in this manner, and with results equal to or better than those reached
by former methods.
Celestial objects are thus originally discovered and the positions
of familiar objects remeasured or otherwise compared, and this work
might be continued throughout the whole 24 hours were it so desired,
regardless of the glare of the sun by day or of impenetrable clouds by
night.
The work in progress in Peru will give other series of plates offering
equal facilities for the study at Cambridge of that part of the sky
which is beyond our southern horizon. Some of the results which these
extensive investigations of the light, the spectra and the positions
of the stars will yield will anticipate the doings of other great
observatories of the world. But there is no necessary limit at stars of
the magnitudes named; there will remain other worlds to conquer.
A special encouragement to new enterprises at Harvard is in the
munificent gift of $50,000, made within the year past by Miss Catherine
W. Bruce of New York for the construction of a telescope of 24 inches
aperture, to be used in photography. A contract for this instrument has
been made. It is intended that its first use shall be to photograph
maps of the fainter stars, and it is hoped that those as faint as the
16th magnitude can thus be represented. The basis of this sanguine
forecast is the fact that with an eight-inch telescope of the pattern
of the proposed 24-inch, and an exposure of the plate for one hour,
twice as many stars are photographed as are visible with a telescope
of 15 inches aperture. Prof. Pickering received the honorary degree of
A.M. from Harvard in 1880, and that of LL.D. from the University of
California in 1886, and from the University of Michigan in 1887. Like
his predecessor, Prof. G. P. Bond, he has been honored by the Royal
Astronomical Society in the bestowal of its gold medal.
The several investigations of chief importance which are now in
progress at Harvard College Observatory have already been mentioned
as part of the record of the half-century past. They also go into the
record with which the second half-century now begins. As such they
may be briefly recapitulated, viz.: The survey, for the purposes of
the great European standard catalogue known as the “Astronomische
Gessellschaft,” of the zone between 9° 50′ south and 14° 10′ south; the
photometric, spectroscopic, and photographic special surveys making in
south latitude to complete like surveys hitherto made at Cambridge,
extending to about 30° south; the systematic work in photography,
which includes much classifiable as spectroscopy, carried on both at
Cambridge and in Peru as the Draper Memorial work: other systematic
work of like importance done under the special restrictions of the
Boyden fund; and what perhaps may be called the orbital observations of
eclipses of Jupiter’s satellites.
That planet has now nearly completed its circuit around the sun, and
the last of its satellite eclipses to be observed will occur on Dec.
17 ensuing. During the period of 12 years about 450 of these eclipses
have been observed and recorded. Perhaps as many others for which
preparations were made at the observatory, passed unseen, because of
interposing clouds. Except to an expert these figures give no hint
of the magnitude of the work. All that need here be said is that in
its completed form it will be one of the great achievements of the
observatory.
The enumeration of these unfinished works and those completed, which
has now been made, will have fulfilled its purpose if it shall have
impressed upon the mind of the general reader the fact, with which it
is presumable everybody is somewhat familiar, that a great oak has
grown from the little acorn planted on Harvard College campus 50 years
ago.
Transcriber’s Notes:
- Text enclosed by underscores is in italics (_italics_).
- Blank pages have been removed.
- Silently corrected typographical errors.