Among the astronomers of antiquity, two great men stand out with
unchallenged pre-eminence. Hipparchus and Ptolemy entertained the same
large organic designs; they worked on similar methods; and, as the
outcome, their performances fitted so accurately together that between
them they re-made celestial science. Hipparchus fixed the chief data of
astronomy--the lengths of the tropical and sidereal years, of the
various months, and of the synodic periods of the five planets;
determined the obliquity of the ecliptic and of the moon's path, the
place of the sun's apogee, the eccentricity of his orbit, and the moon's
horizontal parallax; all with approximate accuracy. His loans from
Chaldaean experts appear, indeed, to have been numerous; but were
doubtless independently verified. His supreme merit, however, consisted
in the establishment of astronomy on a sound geometrical basis. His
acquaintance with trigonometry, a branch of science initiated by him,
together with his invention of the planisphere, enabled him to solve a
number of elementary problems; and he was thus led to bestow especial
attention upon the position of the equinox, as being the common point of
origin for measures both in right ascension and longitude. Its steady
retrogression among the stars became manifest to him in 130 B.C., on
comparing his own observations with those made by Timocharis a century
and a half earlier; and he estimated at not less than 36" (the true
value being 50") the annual amount of "precession."
The choice made by Hipparchus of the geocentric theory of the universe
decided the future of Greek astronomy. He further elaborated it by the
introduction of "eccentrics," which accounted for the changes in orbital
velocity of the sun and moon by a displacement of the earth, to a
corresponding extent, from the centre of the circles they were assumed
to describe. This gave the elliptic inequality known as the "equation of
the centre," and no other was at that time obvious. He attempted no
detailed discussion of planetary theory; but his catalogue of 1080
stars, divided into six classes of brightness, or "magnitudes," is one
of the finest monuments of antique astronomy. It is substantially
embodied in Ptolemy's _Almagest_ (see PTOLEMY).
Ptolemy.
An interval of 250 years elapsed before the constructive labours of
Hipparchus obtained completion at Alexandria. His observations were
largely, and somewhat arbitrarily, employed by Ptolemy. Professor
Newcomb, who has compiled an instructive table of the equinoxes
severally observed by Hipparchus and Ptolemy, with their errors deduced
from Leverrier's solar tables, finds palpable evidence that the
discrepancies between the two series were artificially reconciled on the
basis of a year 6^m too long, adopted by Ptolemy on trust from his
predecessor. He nevertheless holds the process to have been one that
implied no fraudulent intention.
Arab astronomers.
The Ptolemaic system was, in a geometrical sense, defensible; it
harmonized fairly well with appearances, and physical reasonings had not
then been extended to the heavens. To the ignorant it was recommended by
its conformity to crude common sense; to the learned, by the wealth of
ingenuity expended in bringing it to perfection. The _Almagest_ was the
consummation of Greek astronomy. Ptolemy had no successor; he found only
commentators, among the more noteworthy of whom were Theon of Alexandria
(fl. A.D. 400) and his daughter Hypatia (370-415). With the capture of
Alexandria by Omar in 641, the last glimmer of its scientific light
became extinct, to be rekindled, a century and a half later, on the
banks of the Tigris. The first Arabic translation of the _Almagest_ was
made by order of Harun al-Rashid about the year 800; others followed,
and the Caliph al-Mamun built in 829 a grand observatory at Bagdad. Here
Albumazar (805-885) watched the skies and cast horoscopes; here Tobit
ben Korra (836-901) developed his long unquestioned, yet misleading
theory of the "trepidation" of the equinoxes; Abd-ar-rahman al-Suf
(903-986) revised at first hand the catalogue of Ptolemy;[13] and
Abulwefa (939-998), like al-Sufi, a native of Persia, made continuous
planetary observations, but did not (as alleged by L. Sedillot)
anticipate Tycho Brahe's discovery of the moon's variation. Ibn Junis
(c. 950-1008), although the scene of his activity was in Egypt, falls
into line with the astronomers of Bagdad. He compiled the Hakimite
Tables of the planets, and observed at Cairo, in 977 and 978, two solar
eclipses which, as being the first recorded with scientific
accuracy,[14] were made available in fixing the amount of lunar
acceleration. Nasir ud-din (1201-1274) drew up the Ilkhanic Tables, and
determined the constant of precession at 51". He directed an observatory
established by Hulagu Khan (d. 1265) at Maraga in Persia, and equipped
with a mural quadrant of 12 ft. radius, besides altitude and azimuth
instruments. Ulugh Beg (1394-1449), a grandson of Tamerlane, was the
illustrious personification of Tatar astronomy. He founded about 1420 a
splendid observatory at Samarkand, in which he re-determined nearly all
Ptolemy's stars, while the Tables published by him held the primacy for
two centuries.[15]
Moorish Astronomy.
European Astronomy.
Purbach.
Walther.
Arab astronomy, transported by the Moors to Spain, flourished
temporarily at Cordova and Toledo. From the latter city the Toletan
Tables, drawn up by Arzachel in 1080, took their name; and there also
the Alfonsine Tables, published in 1252, were prepared under the
authority of Alphonso X. of Castile. Their appearance signalized the
dawn of European science, and was nearly coincident with that of the
_Sphaera Mundi_, a text-book of spherical astronomy, written by a
Yorkshireman, John Holywood, known as Sacro Bosco (d. 1256). It had an
immense vogue, perpetuated by the printing-press in fifty-nine editions.
In Germany, during the 15th century, a brilliant attempt was made to
patch up the flaws in Ptolemaic doctrine. George Purbach (1423-1461)
introduced into Europe the method of determining time by altitudes
employed by Ibn Junis. He lectured with applause at Vienna from 1450;
was joined there in 1452 by Regiomontanus (q.v.); and was on the point
of starting for Rome to inspect a manuscript of the _Almagest_ when he
died suddenly at the age of thirty-eight. His teachings bore fruit in
the work of Regiomontanus, and of Bernhard Walther of Nuremberg
(1430-1504), who fitted up an observatory with clocks driven by weights,
and developed many improvements in practical astronomy.
Copernicus.
Meantime, a radical reform was being prepared in Italy. Under the
searchlights of the new learning, the dictatorship of Ptolemy appeared
no more inevitable than that of Aristotle; advanced thinkers like
Domenico Maria Novara (1454-1504) promulgated _sub rosa_ what were
called Pythagorean opinions; and they were eagerly and fully
appropriated by Nicolaus Copernicus during his student-years (1496-1505)
at Bologna and Padua. He laid the groundwork of his heliocentric theory
between 1506 and 1512, and brought it to completion in _De
Revolutionibus Orbium Coelestium_ (1543). The colossal task of remaking
astronomy on an inverted design was, in this treatise, virtually
accomplished. Its reasonings were solidly founded on the principle of
the relativity of motion. A continuous shifting of the standpoint was in
large measure substituted for the displacements of the objects viewed,
which thus acquired a regularity and consistency heretofore lacking to
them. In the new system, the sphere of the fixed stars no longer
revolved diurnally, the earth rotating instead on an axis directed
towards the celestial pole. The sun too remained stationary, while the
planets, including our own globe, circulated round him. By this means,
the planetary "retrogradations" were explained as simple perspective
effects due to the combination of the earth's revolutions with those of
her sister orbs. The retention, however, by Copernicus of the antique
postulate of uniform circular motion impaired the perfection of his
plan, since it involved a partial survival of the epicyclical machinery.
Nor was it feasible, on this showing, to place the sun at the true
centre of any of the planetary orbits; so that his ruling position in
the midst of them was illusory. The reformed scheme was then by no means
perfect. Its simplicity was only comparative; many outstanding anomalies
compromised its harmonious working. Moreover, the absence of sensible
parallaxes in the stellar heavens seemed inconsistent with its validity;
and a mobile earth outraged deep-rooted prepossessions. Under these
disadvantageous circumstances, it is scarcely surprising that the
heliocentric theory, while admired as a daring speculation, won its way
slowly to acceptance as a truth.
Observatory of Cassel.
The _Tabulae Prutenicae_, calculated on Copernican principles by Erasmus
Reinhold (1511-1553), appeared in 1551. Although they represented
celestial movements far better than the Alfonsine Tables, large
discrepancies were still apparent, and the desirability of testing the
novel hypothesis upon which they were based by more refined observations
prompted a reform of methods, undertaken almost simultaneously by the
landgrave William IV. of Hesse-Cassel (1532-1592), and by Tycho Brahe.
The landgrave built at Cassel in 1561 the first observatory with a
revolving dome, and worked for some years at a star-catalogue finally
left incomplete. Christoph Rothmann and Joost Burgi (1552-1632) became
his assistants in 1577 and 1579 respectively; and through the skill of
Burgi, time-determinations were made available for measuring right
ascensions. At Cassel, too, the altitude and azimuth instrument is
believed to have made its first appearance in Europe.[16]
Tycho Brahe.
Tycho's labours were both more strenuous and more effective. He
perfected the art of pre-telescopic observation. His instruments were on
a scale and of a type unknown since the days of Nasir ud-din. At
Augsburg, in 1569, he ordered the construction of a 19-ft. quadrant, and
of a celestial globe 5 ft. in diameter; he substituted equatorial for
zodiacal armillae, thus definitively establishing the system of
measurements in right ascension and declination; and improved the
graduation of circular arcs by adopting the method of "transversals." By
these means, employed with consummate skill, he attained an
unprecedented degree of accuracy, and as an incidental though valuable
result, demonstrated the unreality of the supposed trepidation of the
equinoxes.
Kepler.
No more congruous arrangement could have been devised than the
inheritance by Johann Kepler of the wealth of materials amassed by Tycho
Brahe. The younger man's genius supplied what was wanting to his
predecessor. Tycho's endowments were of the practical order; yet he had
never designed his observations to be an end in themselves. He thought
of them as means towards the end of ascertaining the true form of the
universe. His range of ideas was, however, restricted; and the attempt
embodied in his ground-plan of the solar system to revive the ephemeral
theory of Heraclides failed to influence the development of thought.
Kepler, on the contrary, was endowed with unlimited powers of
speculation, but had no mechanical faculty. He found in Tycho's ample
legacy of first-class data precisely what enabled him to try, by the
touchstone of fact, the successive hypotheses that he imagined; and his
untiring patience in comparing and calculating the observations at his
disposal was rewarded by a series of unique discoveries. He long adhered
to the traditional belief that all celestial revolutions must be
performed equably in circles; but a laborious computation of seven
recorded oppositions of Mars at last persuaded him that the planet
travelled in an ellipse, one focus of which was occupied by the sun.
Pursuing the inquiry, he found that its velocity was uniform with
respect to no single point within the orbit, but that the areas
described, in equal times, by a line drawn from the sun to the planet
were strictly equal. These two principles he extended, by direct proof,
to the motion of the earth; and, by analogy, to that of the other
planets. They were published in 1609 in _De Motibus Stellae Martis_. The
announcement of the third of "Kepler's Laws" was made ten years later,
in _De Harmonice Mundi_. It states that the squares of the periods of
circulation round the sun of the several planets are in the same ratio
as the cubes of their mean distances. This numerical proportion, as
being a necessary consequence of the law of gravitation, must prevail in
every system under its sway. It does in fact prevail among the
satellite-families of our acquaintance, and presumably in stellar
combinations as well. Kepler's ineradicable belief in the existence of
some such congruity was derived from the Pythagorean idea of an
underlying harmony in nature; but his arduous efforts for its
realization took a devious and fantastic course which seemed to give
little promise of their surprising ultimate success. The outcome of his
discoveries was, not only to perfect the geometrical plan of the solar
system, but to enhance very materially the predicting power of
astronomy. The Rudolphine Tables (Ulm, 1627), computed by him from
elliptic elements, retained authority for a century, and have in
principle never been superseded. He was deterred from research into the
orbital relations of comets, by his conviction of their perishable
nature. He supposed their tails to result from the action of solar rays,
which, in traversing their mass, bore off with them some of their
subtler particles to form trains directed away from the sun. And through
the process of waste thus set on foot, they finally dissolved into the
aether, and expired "like spinning insects." (_De Cometis; Opera_, ed.
Frisch, t. vii. p. 110.) This remarkable anticipation of the modern
theory of light-pressure was suggested to him by his observations of the
great comets of 1618.
The formal astronomy of the ancients left Kepler unsatisfied. He aimed
at finding out the cause as well as the mode of the planetary
revolutions; and his demonstration that the planes in which they are
described all pass through the sun was an important preliminary to a
physical explanation of them. But his efforts to supply such an
explanation were rendered futile by his imperfect apprehension of what
motion is in itself. He had, it is true, a distinct conception of a
force analogous to that of gravity, by which cognate bodies tended
towards union. Misled, however, into identifying it with magnetism, he
imagined circulation in the solar system to be maintained through the
material compulsion of fibrous emanations from the sun, carried round by
his axial rotation. Ignorance regarding the inertia of matter drove him
to this expedient. The persistence of movement seemed to him to imply
the persistence of a moving power. He did not recognize that motion and
rest are equally natural, in the sense of requiring force for their
alteration. Yet his rationale of the tides in _De Motibus Stellae_ is
not only memorable as an astonishing forecast of the principle of
reciprocal attraction in the proportion of mass, but for its bold
extension to the earth of the lunar sphere of influence.
Galileo Galilei, Kepler's most eminent contemporary, took a foremost
part in dissipating the obscurity that still hung over the very
foundations of mechanical science. He had, indeed, precursors and
co-operators. Michel Varo of Geneva wrote correctly in 1584 on the
composition of forces; Simon Stevin of Bruges (1548-1620) independently
demonstrated the principle; and G.B. Benedetti expounded in his
_Speculationum Liber_ (Turin, 1585) perfectly clear ideas as to the
nature of accelerated motion, some years in advance of Galileo's
dramatic experiments at Pisa. Yet they were never assimilated by Kepler;
while, on the other hand, the laws of planetary circulation he had
enounced were strangely ignored by Galileo. The two lines of inquiry
remained for some time apart. Had they at once been made to coalesce,
the true nature of the force controlling celestial movements should have
been quickly recognized. As it was, the importance of Kepler's
generalizations was not fully appreciated until Sir Isaac Newton made
them the corner-stone of his new cosmic edifice.
Galileo.
Galileo's contributions to astronomy were of a different quality from
Kepler's. They were easily intelligible to the general public: in a
sense, they were obvious, since they could be verified by every
possessor of one of the Dutch perspective-instruments, just then in
course of wide and rapid distribution. And similar results to his were
in fact independently obtained in various parts of Europe by Christopher
Scheiner at Ingolstadt, by Johann Fabricius at Osteel in Friesland, and
by Thomas Harriot at Syon House, Isleworth. Galileo was nevertheless by
far the ablest and most versatile of these early telescopic observers.
His gifts of exposition were on a par with his gifts of discernment.
What he saw, he rendered conspicuous to the world. His sagacity was
indeed sometimes at fault. He maintained with full conviction to the end
of his life a grossly erroneous hypothesis of the tides, early adopted
from Andrea Caesalpino; the "triplicate" appearance of Saturn always
remained an enigma to him; and in regarding comets as atmospheric
emanations he lagged far behind Tycho Brahe. Yet he unquestionably ranks
as the true founder of descriptive astronomy; while his splendid
presentment of the laws of projectiles in his dialogue of the "New
Sciences" (Leiden, 1638) lent potent aid to the solid establishment of
celestial mechanics.
Gravitational Astronomy.
Bacon.
Descartes.
Newton.
Euler, Clairault, D'Alembert.
The accumulation of facts does not in itself constitute science.
Empirical knowledge scarcely deserves the name. _Vere scire est per
causas scire._ Francis Bacon's prescient dream, however, of a living
astronomy by which the physical laws governing terrestrial relations
should be extended the highest heavens, had long to wait for
realization. Kepler divined its possibility; but his thoughts, derailed
(so to speak) by the false analogy of magnetism, brought him no farther
than to the rough draft of the scheme of vortices expounded in detail by
Rene Descartes in his _Principia Philosophiae_ (1644). And this was a
Descartes _cul-de-sac._ The only practicable road struck aside from it.
The true foundations of a mechanical theory of the heavens were laid by
Kepler's discoveries, and by Galileo's dynamical demonstrations; its
construction was facilitated by the development of mathematical methods.
The invention of logarithms, the rise of analytical geometry, and the
evolution of B. Cavalieri's "indivisibles" into the infinitesimal
calculus, all accomplished during the 17th century, immeasurably widened
the scope of exact astronomy. Gradually, too, the nature of the problem
awaiting solution came to be apprehended. Jeremiah Horrocks had some
intuition, previously to 1639, that the motion of the moon was
controlled by the earth's gravity, and disturbed by the action of the
sun. Ismael Bouillaud (1605-1694) stated in 1645 the fact of planetary
circulation under the sway of a sun-force decreasing as the inverse
square of the distance; and the inevitableness of this same "duplicate
ratio" was separately perceived by Robert Hooke, Edmund Halley and Sir
Christopher Wren before Newton's discovery had yet been made public. He
was the only man of his generation who both recognized the law, and had
power to demonstrate its validity. And this was only a beginning. His
complete achievement had a twofold aspect. It consisted, first, in the
identification, by strict numerical comparisons, of terrestrial gravity
with the mutual attraction of the heavenly bodies; secondly, in the
following out of its mechanical consequences throughout the solar
system. Gravitation was thus shown to be the sole influence governing
the movements of planets and satellites; the figure of the rotating
earth was successfully explained by its action on the minuter particles
of matter; tides and the procession of the equinoxes proved amenable to
reasonings based on the same principle; and it satisfactorily accounted
as well for some of the chief lunar and planetary inequalities. Newton's
investigations, however, were very far from being exhaustive. Colossal
though his powers were, they had limits; and his work could not but
remain unterminated, since it was by its nature interminable. Nor was it
possible to provide it with what could properly be called a sequel. The
synthetic method employed by him was too unwieldy for common use. Yet no
other was just then at hand. Mathematical analysis needed half a century
of cultivation before it was fully available for the arduous tasks
reserved for it. They were accordingly taken up anew by a band of
continental inquirers, primarily by three men of untiring energy and
vivid genius, Leonhard Euler, Alexis Clairault, and Jean le Rond
d'Alembert. The first of the outstanding gravitational problems with
which they grappled was the unaccountably rapid advance of the lunar
perigee. But the apparent anomaly disappeared under Euler's powerful
treatment in 1749, and his result was shortly afterwards still further
assured by Clairault. The subject of planetary perturbations was next
attacked. Euler devised in 1753 a new method, that of the "variation of
parameters," for their investigation, and applied it to unravel some of
the earth's irregularities in a memoir crowned by the French Academy in
1756; while in 1757, Clairault estimated the masses of the moon and
Venus by their respective disturbing effects upon terrestrial movements.
But the most striking incident in the history of the verification of
Newton's law was the return of Halley's comet to perihelion, on the 12th
of March 1759, in approximate accordance with Clairault's calculation of
the delays due to the action of Jupiter and Saturn. Visual proof was
thus, it might be said, afforded of the harmonious working of a single
principle to the uttermost boundaries of the sun's dominion.
Lagrange.
These successes paved the way for the higher triumphs of Joseph Louis
Lagrange and of Pierre Simon Laplace. The subject of the lunar
librations was treated by Lagrange with great originality in an essay
crowned by the Paris Academy of Sciences in 1764; and he filled up the
lacunae in his theory of them in a memoir communicated to the Berlin
Academy in 1780. He again won the prize of the Paris Academy in 1766
with an analytical discussion of the movements of Jupiter's satellites
(_Miscellanea_, Turin Acad. t. iv.); and in the same year expanded
Euler's adumbrated method of the variation of parameters into a highly
effective engine of perturbational research. It was especially adapted
to the tracing out of "secular inequalities," or those depending upon
changes in the orbital elements of the bodies affected by them, and
hence progressing indefinitely with time; and by its means, accordingly,
the mechanical stability of the solar system was splendidly demonstrated
through the successive efforts of Lagrange and Laplace. The proper share
of each in bringing about this memorable result is not easy to
apportion, since they freely imparted and profited by one another's
advances and improvements; it need only be said that the fundamental
proposition of the invariability of the planetary major axes laid down
with restrictions by Laplace in 1773, was finally established by
Lagrange in 1776; while Laplace in 1784 proved the subsistence of such a
relation between the eccentricities of the planetary orbits on the one
hand, and their inclinations on the other, that an increase of either
element could, in any single case, proceed only to a very small extent.
The system was thus shown, apart from unknown agencies of subversion, to
be constructed for indefinite permanence. The prize of the Berlin
Academy was, in 1780, adjudged to Lagrange for a treatise on the
perturbations of comets, and he contributed to the Berlin Memoirs,
1781-1784, a set of five elaborate papers, embodying and unifying his
perfected methods and their results.
Laplace.
The crowning trophies of gravitational astronomy in the 18th century
were Laplace's explanations of the "great inequality" of Jupiter and
Saturn in 1784, and of the "secular acceleration" of the moon in 1787.
Both irregularities had been noted, a century earlier, by Edmund Halley;
both had, since that time, vainly exercised the ingenuity of the ablest
mathematicians; both now almost simultaneously yielded their secret to
the same fortunate inquirer. Johann Heinrich Lambert pointed out in 1773
that the motion of Saturn, from being retarded, had become accelerated.
A periodic character was thus indicated for the disturbance; and Laplace
assigned its true cause in the near approach to commensurability in the
periods of the two planets, the cycle of disturbance completing itself
in about 900 (more accurately 929-1/2) years. The lunar acceleration,
too, obtains ultimate compensation, though only after a vastly
protracted term of years. The discovery, just one hundred years after
the publication of Newton's _Principia_, of its dependence upon the
slowly varying eccentricity of the earth's orbit signalized the removal
of the last conspicuous obstacle to admitting the unqualified validity
of the law of gravitation. Laplace's calculations, it is true, were
inexact. An error, corrected by J.C. Adams in 1853, nearly doubled the
value of the acceleration deducible from them; and served to conceal a
discrepancy with observation which has since given occasion to much
profound research (see MOON).
The _Mecanique celeste_, in which Laplace welded into a whole the items
of knowledge accumulated by the labours of a century, has been termed
the "Almagest of the 18th century" (Fourier). But imposing and complete
though the monument appeared, it did not long hold possession of the
field. Further developments ensued. The "method of least squares," by
which the most probable result can be educed from a body of
observational data, was published by Adrien Marie Legendre in 1806, by
Carl Friedrich Gauss in his _Theoria Motus_ (1809), which described also
a mode of calculating the orbit of a planet from three complete
observations, afterwards turned to important account for the recapture
of Ceres, the first discovered asteroid (see PLANETS, MINOR). Researches
into rotational movement were facilitated by S.D. Poisson's application
to them in 1809 of Lagrange's theory of the variation of constants;
Philippe de Pontecoulant successfully used in 1829, for the prediction
of the impending return of Halley's comet, a system of "mechanical
quadratures" published by Lagrange in the Berlin Memoirs for 1778; and
in his _Theorie analytique du systeme du monde_ (1846) he modified and
refined general theories of the lunar and planetary revolutions. P.A.
Hansen in 1829 (_Astr. Nach._ Nos. 166-168, 179) left the beaten track
by choosing time as the sole variable, the orbital elements remaining
constant. A.L. Cauchy published in 1842-1845 a method similarly
conceived, though otherwise developed; and the scope of analysis in
determining the movements of the heavenly bodies has since been
perseveringly widened by the labours of Urbain J.J. Leverrier, J.C.
Adams, S. Newcomb, G.W. Hill, E.W. Brown, H. Gylden, Charles Delaunay,
F. Tisserand, H. Poincare and others too numerous to mention. Nor were
these abstract investigations unaccompanied by concrete results. Sir
George Airy detected in 1831 an inequality, periodic in 240 years,
between Venus and the earth. Leverrier undertook in 1839, and concluded
in 1876, the formidable task of revising all the planetary theories and
constructing from them improved tables. Not less comprehensive has been
the work carried out by Professor Newcomb of raising to a higher grade
of perfection, and reducing to a uniform standard, all the theories and
constants of the solar system. His inquiries afford the assurance of a
nearly exact conformity among its members to strict gravitational law,
only the moon and Mercury showing some slight, but so far unexplained,
anomalies of movement. The discovery of Neptune in 1846 by Adams and
Leverrier marked the first solution of the "inverse problem" of
perturbations. That is to say, ascertained or ascertainable effects were
made the starting-point instead of the goal of research.
Descriptive and practical astronomy.
Bayer.
Gassendi.
Horrocks.
Huygens.
Gascoigne.
Hevelius.
Observational astronomy, meanwhile, was advancing to some extent
independently. The descriptive branch found its principle of development
in the growing powers of the telescope, and had little to do with
mathematical theory; which, on the contrary, was closely allied, by
relations of mutual helpfulness, with practical astronomy, or
"astrometry." Meanwhile, the elementary requirement of making visual
acquaintance with the stellar heavens was met, as regards the unknown
southern skies, when Johann Bayer published at Nuremberg in 1603 a
celestial atlas depicting twelve new constellations formed from the rude
observations of navigators across the line. In the same work, the
current mode of star-nomenclature by the letters of the Greek alphabet
made its appearance. On the 7th of November 1631 Pierre Gassendi watched
at Paris the passage of Mercury across the sun. This was the first
planetary transit observed. The next was that of Venus on the 24th of
November (O.S.) 1639, of which Jeremiah Horrocks and William Crabtree
were the sole spectators. The improvement of telescopes was prosecuted
by Christiaan Huygens from 1655, and promptly led to his discoveries of
the sixth Saturnian moon, of the true shape of the Saturnian appendages,
and of the multiple character of the "trapezium" of stars in the Orion
nebula. William Gascoigne's invention of the filar micrometer and of the
adaptation of telescopes to graduated instruments remained submerged for
a quarter of a century in consequence of his untimely death at Marston
Moor (1644). The latter combination had also been ineffectually proposed
in 1634 by Jean Baptiste Morin (1583-1656); and both devices were
recontrived at Paris about 1667, the micrometer by Adrien Auzout (d.
1691), telescopic sights (so-called) by Jean Picard (1620-1682), who
simultaneously introduced the astronomical use of pendulum-clocks,
constructed by Huygens eleven years previously. These improvements were
ignored or rejected by Johann Hevelius of Danzig, the author of the last
important star-catalogue based solely upon naked-eye determinations.
He, nevertheless, used telescopes to good purpose in his studies of
lunar topography, and his designations for the chief mountain-chains and
"seas" of the moon have never been superseded. He, moreover, threw out
the suggestion (in his _Cometographia_, 1668) that comets move round the
sun in orbits of a parabolic form.
The Paris observatory.