In 1672, Newton had described his production
of a solar spectrum. About 1814, Joseph von Fraunhofer discovered that
the band of colors was crossed by numerous dark lines. By the middle of
the nineteenth century it had become accepted that each chemical substance,
when rendered incandescent, gave a characteristic spectrum of bright lines
affording a test of its presence. In 1848, Léon Foucault observed
that a flame containing sodium would absorb the yellow light emitted by
a strong electric arc placed behind it. Numerous and exact coincidences
of this kind were established for a number of elements by Gustav Kirchhoff
(1824-1887) of Heidelberg, from 1859 onward. He conceived of the sun as
consisting of an incandescent core surrounded by a cooler atmosphere of
elements that selectively absorb light of the same wavelengths as those
they emit. Analysis of the sun's atmosphere thus became possible, and the
spectroscopic technique was extended by Sir William Huggins (1824-1910)
to investigate the chemical composition of the brighter stars.
The classification of stars according to their spectra, begun by Huggins, was elaborated by Pietro Angelo Secchi (1818-1878), Hermann Carl Vogel (1841-1907), and, on an immense scale, by the Harvard astronomers, under Edward Charles Pickering (1846-1919). Their labors bore fruit in the Henry Draper Catalogue (1918-1924), the first large-scale compilation of stars by spectral types. Making use of a physical principle formulated in 1842 by Christian Johann Doppler, Huggins sought to deduce the speeds at which stars approach or recede from the observer by comparing their spectra with that of light from a stationary source, the speeds being indicated by relative displacements of the spectral lines. The earliest reliable estimates of such radial velocities were those obtained by Vogel in 1892. See also Spectra.)
In 1843, Heinrich Schwabe of Dessau reported
that there was a periodic variation in the frequency of occurrence of sunspots,
the cycle having its culmination in approximately eleven years. Concomitant
fluctuations were shortly afterward established in certain terrestrial
magnetic phenomena. Beginning in 1866, Sir Norman Lockyer (1836-1920) employed
the spectroscope to analyze light emanating from sunspots, from the flamelike
solar prominences, and from other selected portions of the sun. With the
results of these investigations he formulated theories of the solar constitution.
The spectroheliograph, devised by George E. Hale about 1890 (and independently
developed by Henri Deslandres at about the same time) made it possible
to photograph the sun in light of a selected wavelength, thereby revealing
the solar distribution of the corresponding element and the structure of
sunspots and prominences.
Since the middle of the 1800's, efforts have been made to determine the rate at which the sun radiates heat and light and to deduce its surface temperature. Calorimetric experiments were carried out by Sir John Herschel (1792-1871), Claude Servais Mathias Pouillet (1791-1868), and Charles Greeley Abbot (1872-1973). The purpose was to estimate the "solar constant" -- the rate of incidence of solar energy upon a unit surface area at the earth's mean distance from the sun -- and so to find the rate of solar emission. The establishment by Josef Stefan in 1879 of his classic law con necting radiation and temperature, enabled the "effective temperature" of an ideal solar surface to be fixed at about 6000°C. Selective atmospheric absorption of solar radiation was investigated by Samuel Pierpont Langley (1834-1906) of the Allegheny Observatory beginning in 1880. He devised his spectrobolometer for determining how the solar energy is distributed over the region of the spectrum in which the sun chiefly radiates, thereby arriving at an independent estimate of the sun's effective temperature.
In 1848, Julius Robert Mayer suggested the impact of meteors upon the solar surface as the source of the sun's heat and light. Hermann von Helmholtz, in 1854, invoked the sun's contraction under gravity; while in 1938-1939 Hans Bethe and Carl von Weizsäcker traced the solar radiation to thermonuclear processes occurring in the sun's interior.
The Sun's Distance
The earliest reasonably accurate estimate
of the sun's distance from the earth (an important astronomical unit) was
obtained in 1672 by Giovanni Domenico Cassini and Jean Richer. They made
concerted observations of Mars from Paris, France, and Cayenne, French
Guiana, respectively, and measured the planet's parallax -- the difference
in direction of a heavenly body as viewed from two separated points --
between their two stations. Modified procedures, involving concerted observations
of a transit of Venus across the sun's disc, were devised by Halley and
by Joseph Nicolas Delisle (1688-1768); they were tested, with disappointing
results, at the transits of 1761, 1769, 1874, and 1882. Elaborate determinations
of the solar distance, based upon extensive observations of the minor planet
Eros when it was near its point of opposition, were organized in 1900-1901
and 1930-1931. On the latter occasion the operations were directed and
the results were formulated by Sir Harold Spencer Jones (1890-1960). (See
Classification of the moon's surface features
and their interpretation as the work of natural forces was begun by J.
H. Schroeter toward the close of the eighteenth century. The charting of
its visible hemisphere was greatly assisted, from about 1890, by the use
of photography. The reduction of the moon's complicated motion to a numerical
theory yielding serviceable tables continued to occupy mathematicians;
since 1923 the lunar tables of Ernest William Brown (1866-1938) have held
the field. The mechanical relations of the earth and the moon, with their
bearing upon the possible origin of the moon, were investigated by Sir
George Howard Darwin (1845-1912), and more recently by Sir Harold Jeffreys
(1891-1989). (See also Moon.)
The progress of modern planetary astronomy
may be briefly summarized, considering the planets in order of their increasing
distance from the sun. Wherever possible, their surface markings have been
scrutinized and their periods of rotation estimated. Spectroscopic studies
have thrown light upon the composition of planetary atmospheres.
The planet Mercury was charted by Giovanni Virginio Schiaparelli (1835-1910), who, in 1881, estimated that planet's period of rotation as equal to that of its revolution. In 1889 he discovered the pattern of markings in or beneath the dazzlingly bright atmosphere which totally obscures the surface of Venus. Spectroscopic evidence indicates much carbon dioxide, no free oxygen or water vapor, and surface temperatures upward of 800°F. (425°C.). Schiaparelli also made an elaborate study of the surface of Mars at the planet's opposition of 1877. He discovered the so-called canals, now known to be an optical illusion. Also in 1877 Asaph Hall discovered two tiny satellites of Mars.
On Jan. 1, 1801, Giuseppe Piazzi (1746-1826) discovered Ceres, the earliest-known member of the class of minor planets (or asteroids) which generally circulate between the orbits of Mars and Jupiter. Other objects of this class have since been discovered, especially following the use of photography for this purpose in 1891 by M. F. J. Wolf. Orbits of over 1,200 such bodies are known.
The constitution of Saturn's rings was investigated mathematically by James Clerk Maxwell (1831-1879). He showed, in 1857, that they must consist of a large number of individual satellites, a view which James Edward Keeler of the Allegheny Observatory confirmed spectroscopically in 1895. The three planets beyond Saturn --Uranus, Neptune, and Pluto -- have been discovered within the last two centuries. Sir William Herschel (1738-1822) discovered Uranus on Mar. 13, 1781, identifying it as a planet by its disklike appearance. Observations of Uranus over a span of years revealed unexplained disturbances of its orbit which could be attributed to the attraction of an unknown planet more remote from the sun. Two mathematicians, John Couch Adams (1819-1892) and Urbain Jean Joseph Leverrier (1811-1877), the one British and the other French, independently computed the position of the hypothetical planet from these perturbations of Uranus. In 1846, acting on information supplied to him by Leverrier, Johann Gottfried Galle, of Berlin, detected Neptune close to the predicted position.
Additional unexplained disturbances in the motion of Uranus prompted Lowell, in 1915, to compute the elements of a hypothetical trans-Neptunian (beyond the orbit of Neptune) planet. In his will Lowell provided funds for a search that ended in 1930, when Clyde W. Tombaugh of the Lowell Observatory discovered a planet which was subsequently named Pluto. Its mass, however, appeared inadequate to produce the perturbations which had prompted the search, leading to speculation of the existence of still another, more distant trans-Neptunian planet.
Meteors and Comets
The fact that meteors are cosmic bodies was
established in 1794 by Ernst Florens Friedrich Chladni. During the nineteenth
century the existence was demonstrated of meteor swarms revolving round
the sun, and Schiaparelli proved the close connection between meteors and
comets. Nineteenth-century studies on comets concentrated on their origin
(whether "captured" by major planets, ejected by "celestial volcanoes,"
or due to some other cause), the nature of their "tails," and the mechanism
by which these tails are generally directed away from the sun. It was supposed
that the particles of the tail were repelled by a solar force conceived
of as electrical by Heinrich Wilhelm Olbers (1758-1840) and as a radiation
pressure by Svante Arrhenius (1859-1927). During the past century comets
have been studied spectroscopically; recent speculations have tended to
regard them as conglomerates of ice and dust. (See also Comet;
In his systematic survey of the heavens at
the end of the eighteenth century, William Herschel established the existence
of binary stars -- close pairs revolving gravitationally around a common
center of mass. From an analysis of the known proper motions of a limited
number of stars, Herschel concluded that the sun, conceived of as a star
with planets, was moving toward a point in the constellation of Hercules.
Herschel's broad findings were confirmed, with some reinterpretation, by
later investigations based upon more extensive and accurate determinations
of proper motions. The use of the spectroscope for determining the speeds
of approach or recession of stars enabled the speed as well as the direction
of the sun's motion to be estimated; the first large-scale application
of this technique to the problem was by William Wallace Campbell
(1862-1938) of the Lick Observatory in the early years of the present century.
The distance of a star from the solar system was first published by Friedrich W. Bessel in 1838. His computation was based on measurements of the star's annual parallax. Later, such measurements were improved through the use of photographic techniques. In 1844, Friedrich Wilhelm August Argelander inaugurated the serious study of the numerous variable stars.
During the 19th century the Alexandrian classification of stars into magnitudes according to their brightness was placed upon a scientific foundation. Photometric techniques were devised for estimating apparent magnitudes.
In that century stars were generally believed to cool from white heat to extinction with an accompanying spectral progression. The suggestion that a star might originate as a red-hot body, warm up to a maximum temperature, and then cool to extinction was made by Sir Joseph Norman Lockyer (1836-1920) on the basis of his "meteoritic hypothesis" of the constitution of celestial objects (1888). The idea of an ascending and a descending curve of stellar temperature was supported by the recognition by Henry Norris Russell in 1913 of two classes of red stars of very different luminosities. These were the giants and dwarfs independently distinguished by Ejnar Hertzsprung (1873-1967).
From 1916 onward, theories of the internal constitution of the stars were brought into line with modern ideas in physics chiefly through the researches of Sir Arthur Stanley Eddington (1882-1944), Sir James Jeans (1877-1946), and E. A. Milne (1896-1950). (See also Star.)
The Milky Way, or Galaxy, was interpreted
by Thomas Wright in 1750 and William Herschel in 1784 as an optical phenomenon
due to the stars being arranged in a shallow layer with the sun near its
median plane. Herschel's conclusions have been modified, but broadly upheld,
by subsequent applications of statistical techniques to the problem, notably
by Jacobus Cornelius Kapteyn (1851-1922), whose announcement, in 1904,
of the phenomenon of "star streaming" afforded a foundation for speculations
on the rotation of the Galaxy. Meanwhile, Harlow Shapley (1885-1972) drew
conclusions as to the configuration and dimensions of the galactic system
from a study of the distribution in space of the associated globular star
Herschel conceived of certain types of nebulae as remote stellar aggregations comparable to our galactic system; contrasted nebular forms were shown by Huggins to possess the bright-line spectra indicative of a gaseous constitution. The first half of the 20th century saw the establishment, largely through the researches of Edwin P. Hubble (1889-1953), of the existence of innumerable external galaxies comparable to our own. They appear to form an evolutionary sequence and to be scattered with a broad uniformity to the limits of observable space. Their spectra exhibit displacements which, interpreted according to Doppler's principle, would indicate speeds of recession from the observer increasing in proportion to the estimated distance of the nebulae. This phenomenon has been brought into relation with the relativist hypothesis of the "expanding universe"; it is of crucial significance in both the "evolutionary" and the "steady-state" conceptions of the universe.
The years following World War II saw two
developments of revolutionary importance to astronomy. Radio waves reaching
the earth from space were detected and analyzed and their cosmic sources,
where possible, located by radio telescopes designed for the purpose. The
successful orbiting of instrument-carrying artificial satellites has surmounted
the barrier which the earth's atmosphere opposes to astronomical observations,
made possible photography of the moon's formerly concealed hemisphere,
and afforded the prospect of exploratory approaches to more distant members
of the solar system.