Romer.
The establishment, in 1671 and 1676 respectively, of the French and
English national observatories at once typified and stimulated progress.
The Paris institution, it is true, lacked unity of direction. No
authoritative chief was assigned to it until 1771. G.D. Cassini, his son
and his grandson were only _primi inter pares_. Claude Perrault's
stately edifice was equally accessible to all the more eminent members
of the Academy of Sciences; and researches were, more or less
independently, carried on there by (among others) Philippe de la Hire
(1640-1718), G.F. Maraldi (1665-1729), and his nephew, J.D. Maraldi,
Jean Picard, Huygens, Olaus Romer and Nicolas de Lacaille. Some of the
best instruments then extant were mounted at the Paris observatory. G.D.
Cassini brought from Rome a 17-ft. telescope by G. Campani, with which
he discovered in 1671 Iapetus, the ninth in distance of Saturn's family
of satellites; Rhea was detected in 1672 with a glass by the same maker
of 34-ft. focus; the duplicity of the ring showed in 1675; and, in 1684,
two additional satellites were disclosed by a Campani telescope of 100
ft. Cassini, moreover, set up an altazimuth in 1678, and employed from
about 1682 a "parallactic machine," provided with clockwork to enable it
to follow the diurnal motion. Both inventions have been ascribed to
Olaus Romer, who used but did not claim them, and must have become
familiar with their principles during the nine years (1672-1681) spent
by him at the Paris observatory. Romer, on the other hand, deserves full
credit for originating the transit-circle and the prime vertical
instrument; and he earned undying fame by his discovery of the finite
velocity of light, made at Paris in 1675 by comparing his observations
of the eclipses of Jupiter's satellites at the conjunctions and
oppositions of the planet.
Flamsteed.
The organization of the Greenwich observatory differed widely from that
adopted at Paris. There a fundamental scheme of practical amelioration
was initiated by John Flamsteed, the first astronomer royal, and has
never since been lost sight of. Its purpose is the attainment of so
complete a power of prediction that the places of the sun, moon and
planets may be assigned without noticeable error for an indefinite
future time. Sidereal inquiries, as such, made no part of the original
programme in which the stars figured merely as points of reference. But
these points are not stationary. They have an apparent precessional
movement, the exact amount of which can be arrived at only by prolonged
and toilsome enquiries. They have besides "proper motions," detected in
1718 by E. Halley in a few cases, and since found to prevail
universally. Further, James Bradley discovered in 1728 the annual
shifting of the stars due to the aberration of light (see ABERRATION),
and in 1748, the complicating effects upon precession of the "nutation"
of the earth's axis. Hence, the preparation of a catalogue recording the
"mean" positions of a number of stars for a given epoch involves
considerable preliminary labour; nor do those positions long continue to
satisfy observation. They need, after a time, to be corrected, not only
systematically for precession, but also empirically for proper motion.
Before the stars can safely be employed as route-marks in the sky, their
movements must accordingly be tabulated, and research into the method of
such movements inevitably follows. We perceive then that the fundamental
problems of sidereal science are closely linked up with the elementary
and indispensable procedures of celestial measurement.
The history of the Greenwich observatory is one of strenuous efforts for
refinement, stimulated by the growing stringency of theoretical
necessities. Improved practice, again, reacted upon theory by bringing
to notice residual errors, demanding the correction of formulae, or
intimating neglected disturbances. Each increase of mechanical skill
claims a corresponding gain in the subtlety of analysis; and vice versa.
And this kind of interaction has gone on ever since Flamsteed
reluctantly furnished the "places of the moon," which enabled Newton to
lay the foundations of lunar theory.
Halley.
Bradley.
Bliss.
Maskelyne.
Pond.
Airy.
Edmund Halley, the second astronomer royal, devoted most of his official
attention to the moon. But his plan of attack was not happily chosen; he
carried it out with deficient instrumental means; and his administration
(1720-1742) remained comparatively barren. That of his successor, though
shorter, was vastly more productive. James Bradley chose the most
appropriate tasks, and executed them supremely well, with the
indispensable aid of John Bird (1700-1776), who constructed for him an
8-ft. quadrant of unsurpassed quality. Bradley's store of observations
has accordingly proved invaluable. Those of 3222 stars, reduced by F.W.
Bessel in 1818, and again with masterly insight by Dr A. Auwers in 1882,
form the true basis of exact astronomy, and of our knowledge of proper
motions. Those relating to the moon and planets, corrected by Sir George
Airy, 1840-1846, form part of the standard materials for discussing
theories of movement in the solar system. The fourth astronomer royal,
Nathaniel Bliss, provided in two years a sequel of some value to
Bradley's performance. Nevil Maskelyne, who succeeded him in 1764, set
on foot, in 1767, the publication of the _Nautical Almanac_, and about
the same time had an achromatic telescope fitted to the Greenwich mural
quadrant. The invention, perfected by John Dollond in 1757, was long
debarred from becoming effective by difficulties in the manufacture of
glass, aggravated in England by a heavy excise duty levied until 1845.
More immediately efficacious was the innovation made by John Pond
(astronomer royal, 1811-1836) of substituting entire circles for
quadrants. He further introduced, in 1821, the method of duplicate
observations by direct vision and by reflection, and by these means
obtained results of very high precision. During Sir George Airy's long
term of office (1836-1881) exact astronomy and the traditional purposes
of the royal observatory were promoted with increased vigour, while the
scope of research was at the same time memorably widened. Magnetic,
meteorological, and spectroscopic departments were added to the
establishment; electricity was employed, through the medium of the
chronograph, for the registration of transits; and photography was
resorted to for the daily automatic record of the sun's condition.
Wargentin.
Lacaille.
Tobias Mayer.
Lalande.
Meanwhile, advances were being made in various parts of the continent of
Europe. Peter Wargentin (1717-1783), secretary to the Swedish Academy of
Sciences, made a special study of the Jovian system. James Bradley had
described to the Royal Society on the 2nd of July 1719 the curious
cyclical relations of the three inner satellites; and their period of
437 days was independently discovered by Wargentin, who based upon it in
1746 a set of tables, superseded only by those of J.B.J. Delambre in
1792. Among the fruits of the strenuous career of Nicolas Louis de
Lacaille were tables of the sun, in which terms depending upon planetary
perturbations were, for the first time, introduced (1758); an extended
acquaintance with the southern heavens; and a determination of the
moon's parallax from observations made at opposite extremities of an arc
of the meridian 85 deg. in length. Tobias Mayer of Gottingen (1723-1762)
originated the mode of adjusting transit-instruments still in vogue;
drew up a catalogue of nearly a thousand zodiacal stars (published
posthumously in 1775); and deduced the proper motions of eighty stars
from a comparison of their places as given by Olaus Romer in 1706 with
those obtained by himself in 1756. He executed besides a chart and forty
drawings of the moon (published at Gottingen in 1881), and calculated
lunar tables from a skilful development of Euler's theory, for which a
reward of L3000 was in 1765 paid to his widow by the British government.
They were published by the Board of Longitude, together with his solar
tables, in 1770. The material interests of navigation were in these
works primarily regarded; but the imaginative side of knowledge had
also potent representatives during the latter half of the 18th century.
In France, especially, the versatile activity of J.J. Lalande
popularized the acquisitions of astronomy, and enforced its demands; and
he had a German counterpart in J.E. Bode.
Distance of the sun.
Between the time of Aristarchus and the opposition of Mars in 1672, no
serious attempt was made to solve the problem of the sun's distance. In
that year, however, Jean Richer at Cayenne and G.D. Cassini at Paris
made combined observations of the planet, which yielded a parallax for
the sun of 9.5", corresponding to a mean radius for the terrestrial
orbit of 87,000,000 m. This result, though widely inaccurate, came much
nearer to the truth than any previously obtained; and it instructively
illustrated the feasibility of concerted astronomical operations at
distant parts of the earth. The way was thus prepared for availing to
the full of the opportunities for a celestial survey offered by the
transits of Venus in 1761 and 1769. They had been signalized by E.
Halley in 1716; they were later insisted upon by Lalande; an enthusiasm
for co-operation was evoked, and the globe, from Siberia to Otaheite,
was studded with observing parties. The outcome, nevertheless,
disappointed expectation. The instants of contact between the limbs of
the sun and planet defied precise determination. Optical complications
fatally impeded sharpness of vision, and the phenomena took place in a
debateable borderland of uncertainty. J.F. Encke, it is true, derived
from them in 1822-1824 what seemed an authentic parallax of 8.57",
implying a distance of 95,370,000 m.; but the confidence it inspired was
finally overthrown in 1854 by P.A. Hansen's announcement of its
incompatibility with lunar theory. An appeal then lay to the 19th
century pair of transits in 1874 and 1882; but no peremptory decision
ensued; observations were marred by the same optical evils as before.
Their upshot, however, had lost its essential importance; for a fresh
series of investigations based on a variety of principles had already
been started. Leverrier, in 1858, calculated a value of 8.95" for the
solar parallax (equivalent to a distance of 91,000,000 m.) from the
"parallactic inequality" of the moon; Professor Newcomb, using other
forms of the gravitational method, derived in 1895 a parallax of 8.76".
Again, since the constant of aberration defines the ratio between the
velocity of light and the earth's orbital speed, the span of the
terrestrial circuit, in other words, the distance of the sun, is
immediately deducible from known values of the first two quantities. The
rate of light-transmission was accordingly made the subject of an
elaborate set of experiments by Professor Newcomb in 1880-1882; and the
result, taken in connexion with the aberration-constant as determined at
Pulkowa, yielded a solar parallax of 8.79", or a distance (in round
numbers) of 93,000,000 m. But the direct or geometrical mode of attack
has still the preference over any of the indirect plans. Sir David Gill
derived a highly satisfactory value of 8.78" for the long-sought
constant from the opposition of Mars in 1877, and from combined
heliometer observations at five observatories in 1888-1889 of the minor
planets Iris, Victoria and Sappho, the apparently definitive value of
8.80" (equivalent distance, 92,874,000 m.). But an unlooked-for fresh
opportunity was afforded by the discovery in 1898 of the singularly
circumstanced minor planet Eros, which occasionally approaches the earth
more nearly than any other heavenly body except the moon. The opposition
of November 1900, though only moderately favourable, could not be
neglected; an international photographic campaign was organized at Paris
with the aid of 58 observatories; and the voluminous collected data
imply, so far as they have been discussed, a parallax for the sun a
little greater than 8.8". (See also PARALLAX.)
Reflecting telescopes.
William Herschel.
Sir John Herschel.
Lord Rosse.
The first specimen of a reflecting telescope was constructed by Isaac
Newton in 1668. It was of what is still called "Newtonian" design, and
had a speculum 2 in. in diameter. Through the skill of John Hadley
(1682-1743) and James Short of Edinburgh (1710-1768) the instrument
unfolded, in the ensuing century, some of its capabilities, which the
labours of William Herschel enormously enhanced. Between 1774 and 1789
he built scores of specula of continually augmented size, up to a
diameter of 4 ft., the optical excellence of which approved itself by a
crowd of discoveries. Uranus (q.v.) was recognized by its disk on the
13th of March 1781; two of its satellites, Oberon and Titania, disclosed
themselves on the 11th of January 1787; while with the giant 48-in.
mirror, used on the "front-view" plan, Mimas and Enceladus, the
innermost Saturnian moons, were brought to view on the 28th of August
and the 17th of September 1789. These were incidental trophies;
Herschel's main object was the exploration of the sidereal heavens. The
task, though novel and formidable, was executed with almost incredible
success. Charles Messier (1730-1817) had catalogued in 1781 103 nebulae;
Herschel discovered 2500, laid down the lines of their classification,
divined the laws of their distribution, and assigned their place in a
scheme of development. The proof supplied by him in 1802 that coupled
stars mutually circulate threw open a boundless field of research; and
he originated experimental inquiries into the construction of the
heavens by systematically collecting and sifting stellar statistics. He,
moreover, definitively established, in 1783, the fact and general
direction of the sun's movement in space, and thus introduced an element
of order into the maze of stellar proper motions. Sir John Herschel
continued in the northern, and extended to the southern hemisphere, his
father's work. The third earl of Rosse mounted, at Parsonstown in 1845,
a speculum 6 ft. in diameter, which afforded the first indications of
the spiral structure shown in recent photographs to be the most
prevalent characteristic of nebulae. Down to near the close of the 19th
century, both the use and the improvement of reflectors were left mainly
in British hands; but the gift of the "Crossley" instrument in 1895, to
the Lick observatory, and its splendid subsequent performances in
nebular photography, brought similar tools of research into extensive
use among American astronomers; and they are now, for many of the
various purposes of astrophysics, strongly preferred to refractors.
Giuseppe Piazzi.
Max Wolf.
Acquaintance with the asteroidal family began as the 19th century
opened. On the 1st of January 1801 Giuseppe Piazzi (1746-1826)
discovered Ceres, at Palermo, while engaged in collecting materials for
his star-catalogues. A prolonged succession of similar events followed.
But in the mode of detecting these swarming bodies, a typical change was
made on the 22nd of December 1891, when Dr Max Wolf of Heidelberg
photographically captured No. 323. Repetitions of the feat are now
counted by the score.
Lassell.
Bond.
Hall.
Barnard.
Perrine.
W.H. Pickering.
Practical astronomy was only secondarily concerned with the addition of
Neptune, on the 23rd of September 1846, to the company of known planets;
but William Lassell's discovery of its satellite, on the 10th of October
following, was a consequence of the perfect figure and high polish of
his 2-ft. speculum. With the same instrument, he further detected, on
the 19th of September 1848, Hyperion, the seventh of Saturn's
attendants, and, on the 24th of October 1851, Ariel and Umbriel, the
interior moons of Uranus. Simultaneously with Lassell, on the opposite
shore of the Atlantic, W.C. Bond identified Hyperion; and he perceived,
on the 15th of November 1850, Saturn's dusky ring, independently
observed, a fortnight later, by W.R. Dawes, at Wateringbury in Kent.
With the Washington 26-in. refractor, on the 11th of August 1877,
Professor Asaph Hall descried the moons of Mars, Deimos and Phobos; and
a minute light-speck, noticed by Professor E.E. Barnard in the close
neighbourhood of Jupiter on the 9th of September 1892, proved
representative of a small inner satellite, invisible with less perfect
and powerful instruments than the Lick 36-in. achromatic. The Jovian
system has been reinforced by three remote and extremely faint members,
two photographed by Professor C.D. Perrine with the Crossley reflector
in 1904-1905, and the third at Greenwich in 1908; and a pair of
Saturnian moons, designated Phoebe and Themis, were tracked out by
Professor W.H. Pickering, in 1898 and 1905 respectively, amid the
thicket of stars imprinted on negatives taken at Arequipa with the Bruce
24-in. doublet lens. This raises to 26 the number of discovered
satellites in the solar system.
Comets.
Meteors.
Cometary science has ramified in unexpected ways during the last hundred
years. The establishment of a class of "short-period" comets by the
computations of J.F. Encke in 1819, and of Wilhelm von Biela in 1826,
led to the theory of their "capture" by the great planets, for which a
solid mathematical basis was provided by H. Newton, F. Tisserand and O.
Callandreau. An argument for the aboriginal connexion of comets with the
solar system, founded by R.C. Carrington in 1860 upon their
participation in its translatory movement, was more fully developed by
L. Fabry in 1893; and the close orbital relationships of cometary
groups, accentuated by the pursuit of each other along nearly the same
track by the comets of 1843, 1880 and 1882, singularly illustrated the
probable vicissitudes of their careers. The most remarkable event,
however, in the recent history of cometary astronomy was its
assimilation to that of meteors, which took unquestionable cosmical rank
as a consequence of the Leonid tempest of November 1833. The affinity of
the two classes of objects became known in 1866 through G.V.
Schiaparelli's announcement that the orbit of the bright comet of 1862
agreed strictly with the elliptic ring formed by the circulating Perseid
meteors; and three other cases of close coincidence were soon afterwards
brought to light. Tebbutt's comet in 1881 was the first to be
satisfactorily photographed. The study of such objects is now carried on
mainly through the agency of the sensitive plate. The photographic
registration of meteor-trails, too, has been lately attempted with
partial success. The full realization of the method will doubtless
provide adequate data for the detailed investigation of meteoric paths.
Sidereal astronomy.
Star catalogues.
The progress of science during the 19th century had no more distinctive
feature than the rapid growth of sidereal astronomy (see STAR). Its
scope, wide as the universe, can be compassed no otherwise than by
statistical means, and the collection of materials for this purpose
involves most arduous preliminary labour. The multitudinous enrolment of
stars was the first requisite. Only one "catalogue of precision"--Nevil
Maskelyne's of 36 fundamental stars--was available in 1800. J.J.
Lalande, however, published in 1801, in his _Histoire celeste_, the
approximate places of 47,390 from a re-observation of which the great
Paris catalogue (1887-1892) has been compiled. A valuable catalogue of
about 7600 stars was issued by Giuseppe Piazzi in 1814; Stephen
Groombridge determined 4239 at Blackheath in 1806-1816; while through
the joint and successive work of F.W. Bessel and W.A. Argelander, exact
acquaintance was made with 90,000, a more general acquaintance with the
324,000 stars recorded in the _Bonn Durchmusterung_ (1859-1862). The
southern hemisphere was subsequently reviewed on a similar duplicate
plan by E. Schonfeld (1828-1891) at Bonn, by B.A. Gould and J.M. Thome
at Cordoba. Moreover, the imposing catalogue set on foot in 1865 at
thirteen observatories by the German astronomical society has recently
been completed; and adjuncts to it have, from time to time, been
provided in the publications of the royal observatories at Greenwich and
the Cape of Good Hope, and of national, imperial and private
establishments in the United States and on the continent of Europe. But
in the execution of these protracted undertakings, the human eye has
been, to a large and increasing extent, superseded by the camera.
Photographic star-charting was begun by Sir David Gill in 1885, and the
third and concluding volume of the _Cape Photographic Durchmusterung_
appeared in 1900. It gives the co-ordinates of above 450,000 stars,
measured by Professor J.C. Kapteyn at Groningen on plates taken by C.
Ray Woods at the Cape observatory. And this comprehensive work was
merely preparatory to the International Catalogue and Chart, the
production of which was initiated by the resolutions of the Paris
Photographic Congress of 1887. Eighteen observatories scattered north
and south of the equator divided the sky among them; and the outcome of
their combined operations aimed at the production of a catalogue of at
least 2,000,000 strictly determined stars, together with a colossal map
in 22,000 sheets, showing stars to the fourteenth magnitude, in numbers
difficult to estimate. (Sea PHOTOGRAPHY, CELESTIAL.)
Photometric catalogues.
The arrangement of the stars in space can be usefully discussed only in
connexion with their apparent light-power, or "magnitude." Photometric
catalogues, accordingly, form an indispensable part of stellar
statistics; and their construction has been zealously prosecuted. The
_Harvard Photometry_ of 4260 lucid stars was issued by Professor E.C.
Pickering in 1884, the _Uranometria Nova Oxoniensis_, giving the
relative lustre of 2784 stars, by C. Pritchard in 1885. The instrument
used at Harvard was a "meridian photometer," constructed on the
principle of polarization; while the "method of extinctions," by means
of a wedge of neutral-tinted glass, served for the Oxford
determinations. At Potsdam, some 17,000 stars have been measured by
C.H.G. Muller and P.F.F. Kempf with a polarizing photometer; but by far
the most comprehensive work of the kind is the Harvard _Photometric
Durchmusterung_ (1901-1903), embracing all stars to 7.5 magnitude, and
extended to the southern pole by measurements executed at Arequipa. The
embarrassing subject of photographic photometry has also been attacked
by Professor Pickering. The need is urgent of fixing a scale, and
defining standards of actinic brightness; but it has not yet been
successfully met.
Double stars.
The investigation of double stars was carried on from 1819 to 1850 with
singular persistence and ability at Dorpat and Pulkowa by F.G.W. Struve,
and by his son and successor, O.W. Struve. The high excellence of the
data collected by them was a combined result of their skill, and of the
vast improvement in refracting telescopes due to the genius of Joseph
Fraunhofer (1787-1826). Among the inheritors of his renown were Alvan
Clark and Alvan G. Clark of Cambridgeport, Massachusetts; and the superb
definition of their great achromatics rendered practicable the division
of what might have been deemed impossibly close star-pairs. These
facilities were remarkably illustrated by Professor S.W. Burnham's
record of discovery, which roused fresh enthusiasm for this line of
inquiry by compelling recognition of the extraordinary profusion
throughout the heavens of compound objects. Discoveries with the
spectroscope have ratified and extended this conclusion.
Stellar parallax.
Only spurious star-parallaxes had claimed the attention of astronomers
until F.W. Bessel announced, in December 1838, the perspective yearly
shifting of 61 Cygni in an ellipse with a mean radius of about one-third
of a second. Thomas Henderson (1798-1844) had indeed measured the larger
displacements of [alpha] Centauri at the Cape in 1832-1833, but delayed
until 1839 to publish his result. Out of several hundred stars since
then examined, seventy or eighty have yielded fairly accurate, though
very small parallaxes. But this amount of knowledge, however valuable in
itself, is utterly inadequate to the needs of sidereal research; and
various attempts have accordingly been made, chiefly by Professors J.C.
Kapteyn and Simon Newcomb, to estimate, through the analysis of their
proper motions, the "mean parallax" of stars assorted by magnitude. And
the data thus arrived at are reassuringly self-consistent. A wide
photographic survey, by which parallaxes might be secured wholesale, has
further been recommended by Kapteyn; but is unlikely to be undertaken in
the immediate future.
Proper motions.
The exhaustive ascertainment of stellar parallaxes, combined with the
visible facts of stellar distribution, would enable us to build a
perfect plan of the universe in three dimensions. Its perfection would,
nevertheless, be undermined by the mobility of all its constituent
parts. Their configuration at a given instant supplies no information as
to their configuration hereafter unless the mode and laws of their
movements have been determined. Hence, one of the leading inducements
to the construction of exact and comprehensive catalogues has been to
elicit, by comparisons of those for widely separated epochs, the proper
motions of the stars enumerated in them. Little was known on the subject
at the beginning of the 19th century. William Herschel founded his
determination in 1783 of the sun's route in space upon the movements of
thirteen stars; and he took into account those of only six in his second
solution of the problem in 1805. But in 1837 Argelander employed 390
proper motions as materials for the treatment of the same subject; and
L. Struve had at his disposal, in 1887, no less than 2800. From the
re-observation of Lalande's stars, after the lapse of not far from a
century, J. Bossert was enabled to deduce 2675 proper motions, published
at Paris in four successive memoirs, 1887-1902; and the sum-total of
those ascertained probably now exceeds 6000. Yet this number, although
it represents a portentous expenditure of labour, is insignificant
compared with the multitude of the stellar throng; nor had any general
tendency been discerned to regulate what seemed casual flittings until
Professor Kapteyn, in 1904, adverted to the prevalence among all the
brighter stars of opposite stream-flows towards two "vertices" situated
in the Milky Way (see STAR). The assured general fact as regards the
direction of stellar movements was that they included a common
parallactic element due to the sun's translation. And it is by the
consideration of this partial accordance in motion that the advance
through space of the solar system has been ascertained.
Astrophysics.
Spectrum analysis.
The apex of the sun's way was fixed by Professor Newcomb in 1898 at a
point about 4 deg. S. of the brilliant star Vega; but was shifted nearly
7 deg. to the S.W. by J.C. Kapteyn's inquiry in 1901; so that the range
of uncertainty as to its position continues unsatisfactorily wide. The
speed with which our system progresses is, on the other hand, fairly
well known. It cannot differ much from 12-1/2 m. a second, the rate
assigned to it by Professor W.W. Campbell in 1902. He employed in his
discussion the radial velocities of 280 stars, spectroscopically
determined; and the upshot signally exemplified the community of
interests between the rising science of astrophysics and the ancient
science of astrometry. Their characteristic purposes are, nevertheless,
entirely different. The positions of the heavenly bodies in space, and
the changes of those positions with time, constitute the primary subject
of investigation by the elder school; while the new astronomy concerns
itself chiefly with the individual peculiarities of suns and planets,
with their chemistry, physical habitudes and modes of luminosity. Its
distinctive method is spectrum analysis, the invention and development
of which in the 19th century have fundamentally altered the purpose and
prospects of celestial inquiries.
Wollaston.
Fraunhofer.
Kirchhoff.
Chemistry of the sun.
A beam of sunlight admitted into a darkened room through a narrow
aperture, and there dispersed into a vario-tinted band by the
interposition of a prism, is not absolutely continuous. Dr W.H.
Wollaston made the experiment in 1802, and perceived the spaces of
colour to be interrupted by seven obscure gaps, which took the shape of
lines owing to his use of rectangular slit. He thus caught a preliminary
glimpse of the "Fraunhofer lines," so called because Joseph Fraunhofer
brought them into prominent notice by the diligence and insight of his
labours upon them in 1814-1815. He mapped 324, chose out nine, which he
designated by the letters of the alphabet, to be standards of
measurement for the rest, and ascertained the coincidence in position
between the double yellow ray derived from the flame of burning sodium
and the pair of dark lines named by him "D" in the solar spectrum. There
ensued forty-five years of groping for a law which should clear up the
enigma of the solar reversals. Partial anticipations abounded. The vital
heart of the matter was barely missed by W.A. Miller in 1845, by L.
Foucault in 1849, by A.J. Angstrom in 1853, by Balfour Stewart in 1858;
while Sir George Stokes held the solution of the problem in the hollow
of his hand from 1852 onward. But it was the synthetic genius of Gustav
Kirchhoff which first gave unity to the scattered phenomena, and finally
reconciled what was elicited in the laboratory with what was observed in
the sun. On the 15th of December 1859 he communicated to the Berlin
Academy of Sciences the principle which bears his name. Its purport is
that glowing vapours similarly circumstanced absorb the identical
radiations which they emit. That is to say, they stop out just those
sections of white light transmitted through them which form their own
special luminous badges. Moreover, if the white light come from a source
at a higher temperature than theirs, the sections, or lines, absorbed by
them show dark against a continuous background. And this is precisely
the case with the sun. Kirchhoff's principle, accordingly, not only
afforded a simple explanation of the Fraunhofer lines, but availed to
found a far-reaching science of celestial chemistry. Thousands of the
dark lines in the solar spectrum agree absolutely in wave-length with
the bright rays artificially obtained from known substances, and
appertaining to them individually. These substances must then exist near
the sun. They are in fact suspended in a state of vapour between our
eyes and the photosphere, the dazzling prismatic radiance of which they,
to a minute extent, intercept, thus writing their signatures on the
coloured scroll of dispersed sunshine. By persistent research,
powerfully aided by the photographic camera and by the concave gratings
invented by H.A. Rowland (1848-1901) in 1882, about forty terrestrial
elements have been identified in the sun. Among them, iron, sodium,
magnesium, calcium and hydrogen are conspicuous; but it would be rash to
assert that any of the seventy forms of matter provisionally enumerated
in text-books are wholly absent from his composition.
Solar eclipses.
Solar physics has profited enormously by the abolition of glare during
total eclipses. That of the 8th of July 1842 was the first to be
efficiently observed; and the luminous appendages to the sun disclosed
by it were such as to excite startled attention. Their investigation has
since been diligently prosecuted. The corona was photographed at
Konigsberg during the totality of the 28th of July 1851; similar records
of the red prominences, successively obtained by Father Angelo Secchi
and Warren de la Rue, as the shadow-track crossed Spain on the 18th of
July 1860, finally demonstrated their solar status. The Indian eclipse
of the 18th of August 1868 supplied knowledge of their spectrum, found
to include the yellow ray of an exotic gas named by Sir Norman Lockyer
"helium." It further suggested, to Lockyer and P. Janssen separately,
the spectroscopic method of observing these objects in daylight. Under
cover of an eclipse visible in North America on the 7th of August 1869,
the bright green line of the corona was discerned; and Professor C.A.
Young caught the "flash spectrum" of the reversing layer, at the moment
of second contact, at Xerez de la Frontera in Spain, on the 22nd of
December 1870. This significant but evanescent phenomenon, which
represents the direct emissions of a low-lying solar envelope, was
photographed by William Shackleton on the occasion of an eclipse in
Novaya Zemlya on the 9th of August 1896; and it has since been
abundantly registered by exposures made during the obscurations of 1898,
1900, 1901 and 1905. A singular and unlooked-for result of eclipse-work
has been to include the corona within the scope of solar periodicity.
Heinrich Schwabe established, in 1851, the cyclical variation, in eleven
years, of spot-frequency; terrestrial magnetic disturbances manifestly
obeyed the same law; and the peculiar winged aspect of the corona
disclosed by the eclipse of the 29th of July 1878, at an epoch of
minimum sun-spots, intimated to A.C. Ranyard a theory of coronal types,
changing concurrently with the fluctuations of spot-activity. This was
amply verified at subsequent eclipses.
Prominence photography.
The photography of prominences was, after some preliminary trials by
C.A. Young and others, fully realized in 1891 by Professor George E.
Hale at Chicago, and independently by Henri Deslandres at Paris. The
pictures were taken, in both cases, with only one quality of light; the
violet ray of calcium, the remaining superfluous beams being eliminated
by the agency of a double slit. The last-named expedient had been
described by Janssen in 1867. Hale devised on the same principle the
"spectroheliograph," an instrument by which the sun's disk can be
photographed in calcium-light by imparting a rapid movement to its image
relatively to the sensitive plate; and the method has proved in many
ways fruitful.
Stellar spectroscopy.
The likeness of the sun to the stars has been shown by the spectroscope
to be profound and inherent. Yet the general agreement of solar and
stellar chemistry does not exclude important diversities of detail.
Fraunhofer was the pioneer in this branch. He observed, in 1823, dark
lines in stellar spectra which Kirchhoff's discovery supplied the means
of interpreting. The task, attempted by G.B. Donati in 1860, was
effectively taken in hand, two years later, by Angelo Secchi, William
Huggins and Lewis M. Rutherfurd. There ensued a general classification
of the stars by Secchi into four leading types, distinguished by
diversities of spectral pattern; and the recognition by Huggins of a
considerable number of terrestrial elements as present in stellar
atmospheres. Nebular chemistry was initiated by the same investigator
when, on the 29th of August 1864, he observed the bright-line spectrum
of a planetary nebula in Draco. About seventy analogous objects,
including that in the Sword of Orion, were found by him to give light of
the same quality; and thus after seventy-three years, verification was
brought to William Herschel's hypothesis of a "shining fluid" diffused
through space, the possible raw material of stars. In 1874, Dr H.C.
Vogel published a modification of Secchi's scheme of stellar
diversities, and gave it organic meaning by connecting spectral
differences with advance in "age." And in 1895, he set apart, as in the
earliest stage of growth, a new class of "helium stars," supposed to
develop successively into Sirian, solar, Antarian, or alternatively into
carbon stars.
Spectra of comets.
On the 5th of August 1864, G.B. Donati analysed the light of a small
comet into three bright bands. Sir William Huggins repeated the
experiment on Winnecke's comet in 1868, obtained the same bands, and
traced them to their origin from glowing carbon-vapour. A photograph of
the spectrum of Tebbutt's comet, taken by him on the 24th of June 1881,
showed radiations of shorter wave-lengths but identical source, and in
addition, a percentage of reflected solar light marked as such by the
presence of some well-known Fraunhofer lines. Further experience has
generalized these earlier results. The rule that comets yield
carbon-spectra has scarcely any exceptions. The usual bands were,
however, temporarily effaced in the two brilliant apparitions of 1882 by
vivid rays of sodium and iron, emitted during the excitement of
perihelion-passage.
Progress in spectrography.
The adoption, by Sir William Huggins in 1876, of gelatine or dry plates
in celestial photography was a change of decisive import. For it made
long exposures possible; and only with long exposures could autographic
impressions be secured of such faint objects as nebulae, telescopic
comets, and the immense majority of stars, or of the dim ranges of
stellar and nebular spectra. The first conspicuous triumph of the new
"spectrographic" art thus established was the record by Huggins in 1879
of the dispersed light of several "white" or Sirian stars, in which the
chief traits of absorption were the rhythmical series of hydrogen-lines,
then memorably discovered. Again by Sir William Huggins, the spectrum of
the Orion nebula was photographed on the 7th of March 1882; and the
method has gradually become nearly exclusive in the study of nebular
emanations. The "Draper Catalogue" of 10,351 stellar spectra was
published by Professor E.C. Pickering in 1890. The materials for it were
rapidly accumulated by the use of an objective prism, that is, of a
prism placed in front of, instead of behind the object-lens, by which
means the spectra of all the stars in the field, to the number often of
many score, imprinted themselves simultaneously on the sensitive plate.
The progress of this survey was marked by a number of important
discoveries of "new" and variable stars and of spectroscopic binaries,
mainly through the acumen of Mrs Williamina Paton Fleming of Harvard
College in scrutinizing the negatives forming the data for the great
catalogue.
Doppler's principle.
The principle that the refrangibility of light is altered by end-on
motion was enunciated by Christian Doppler of Prague in 1842. The pitch
of a steam-whistle quite obviously rises and falls as the engine to
which it is attached approaches and recedes from a stationary auditor;
and light-pulses are modified like sound-waves by velocity in the line
of sight. They are crowded together and therefore rendered shorter and
more frequent by the advance of their source, but drawn apart and
lengthened by its recession. These effects vary with the rate of motion,
which they consequently serve to measure; and they are produced
indifferently by movements of the spectator or of the light-source. But
Doppler's idea that they might be detected by colour-change was entirely
illusory. It would apply only if the spectrum had no infra-red and
ultraviolet extensions. These, however, since they share the general
lengthening or shortening of wave-length through motion, are thereby
shifted, to a certain definite extent, into visibility, and so produce
accurate chromatic compensation. Integrated light, accordingly, tells
nothing about velocity; but analysed light does, when it includes bright
or dark rays the normal positions of which are known. The distinction
was pointed out by Hippolyte Fizeau in 1848. By comparison with their
analogues in the laboratory it can be determined whether, in which
direction, and how much, lines of recognized origin are displaced in the
spectra of the heavenly bodies. This subtle mode of research was made
available by Sir William Huggins in 1868. He employed it, with an
outcome of striking promise, to measure the radial speed of some of the
brighter stars. In the following year, Sir Norman Lockyer was enabled to
prove, by its means, the extraordinary vehemence of chromospheric
disturbances, the bright prominence-rays in his spectroscope betraying,
through their opposite shiftings, movements and counter-movements up to
120 m. a second; while its validity and refinement were, in 1871,
vouched for by H.C. Vogel's observations on the 9th of June 1871, of
differences due to the sun's rotation in the refrangibility of
Fraunhofer lines derived respectively from the east and west limbs.
Stellar line-of-sight work, however, made no satisfactory progress
until, in 1888, Vogel changed the _venue_ from the eye to the camera. A
high degree of precision in measurement thus became attainable, and has
since been fully attained. Not only the grosser facts concerning radial
velocity, but variations in it so small as a mile, or less, per second,
have been recorded and interpreted in terms of deep meaning. For the
investigation of the general scheme of sidereal structure, the
multiplication of results of the kind is indispensable. But as yet, the
recessional or approaching movements of only a few hundred stars have
been registered; and this store of information is scanty indeed compared
with the needs of research. How the stars really move in space, and how
the sun travels among them, can be ascertained only with the aid of
materials collected by the spectrograph, which has now fortunately been
brought to comply with the arduous conditions of exactitude requisite
for collaboration with the transit instrument and its allies, the clock
and chronograph. And here, to their great mutual advantage, the old and
the new astronomies meet and join forces.
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de l'astr. au moyen age; Hist. de l'astr. moderne; Hist, de l'astr. au
XVIII^e siecle_; J.S. Bailly, _Histoire de l'astronomie_ (5 vols.,
1775-1787); J.F. Weidler, _Historia Astronomiae_ (1741); J.H. Madler,
_Geschichte der Himmelskunde_ (1873); R. Wolf, _Geschichte der
Astronomie_ (1876); _Handbuch der Astronomie_ (1890-1892); W. Whewell,
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during the 19th Century_ (4th ed., 1903); A. Berry, _Hist. of
Astronomy_ (1898); J.K. Schaubach, _Geschichte der griechischen
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Observatory_, vol. xix. (1896). (A. M. C.)