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ASTRONOMY,
science dealing with all the celestial bodies in the universe,
including the planets and their satellites, comets and meteors,
the stars and interstellar matter, the star systems known as galaxies,
and clusters of galaxies. Modern astronomy is divided into several
branches, namely, astrometry, the observational study of the positions
and motions of these bodies; celestial mechanics the mathematical
study of their motions as explained by the theory of gravitation;
astrophysics, the study of their chemical composition and physical
condition from spectrum analysis and the laws of physics; and cosmology,
the study of the universe as a whole.
Curiosity of ancient peoples concerning day and night and
the sun, moon, and stars led eventually to the observation that
the heavenly bodies appear to move in a regular manner that is useful
in defining time and direction on the earth. Astronomy grew out
of problems originating with the first civilizations, that is, the
need to establish with precision the proper times for planting and harvesting
crops and for religious celebrations and to find bearings and latitudes
on long trading journeys or voyages.
To ancient peoples the sky exhibited many regularities of
behavior. The bright sun, which divided daytime from nighttime,
rose every morning from one direction, the east, moved steadily
across the sky during the day, and set in a nearly opposite direction,
the west. At night more than 1000 visible stars followed a similar course,
appearing to rotate in permanent groupings, called constellations,
around a fixed point in the sky, which is known as the north celestial
pole.
In the North Temperate Zone, people noticed that daytime and
nighttime were unequal in length. On long days the sun rose north
of east and climbed high in the sky at noon; on days with long nights
the sun rose south of east and did not climb so high at noon. Observation
of the stars that appear in the west after sunset or in the east
before sunrise showed that the relative position of the sun among
the stars changes gradually. The Egyptians may have been the first
to discover that the sun moves completely around the sphere of the
fixed stars in approximately 365 days and nights.
Further study showed that the sky also holds the moon and
five bright planets. These bodies, together with the sun, move around
the star sphere within a narrow belt called the zodiac. The moon
traverses the zodiac quickly, overtaking the sun about once every
29.5 days, the period known as the synodic month. Star watchers
in ancient times attempted to arrange the days and either the months
or the years into a consistent time system, or calendar. Inasmuch
as neither an entire month nor an entire year contains an exactly
integral number of days, the calendar makers assigned to successive
months or years different numbers of days, having a long-range average
that would approximate the true value. Thus, the modern calendar
provides for 97 leap years in every 400-year period, so that the
average number of days per year is 365.2425, very close to the astronomically
determined number, which is 365.24220.
The sun and moon always traverse the zodiac from west to east.
However, the five bright planets—Mercury, Venus, Earth,
Mars, and Jupiter, which also have a generally eastward motion against
the background of the stars—move westward, or retrograde,
for varying durations during each synodic period. Thus, the planets
appear to pursue an eastward course erratically, with periodic loops
in their paths. In ancient times, people imagined that celestial
events, especially the planetary motions, were connected with their
own fortunes. This belief, called astrology, encouraged the development
of mathematical schemes for predicting the planetary motions and
thus furthered the progress of astronomy during ancient times.
Interesting constellation maps and useful calendars were developed
by several ancient peoples, notably the Egyptians, the Mayans, and
the Chinese, but the Babylonians accomplished even greater achievements.
To perfect their calendar, they studied the motions of the sun and
moon. It was their custom to designate as the beginning of each
month the day after the new moon, when the lunar crescent first
appears after sunset. Originally this day was determined by observations, but
later the Babylonians wanted to calculate it in advance. About 400 bc they
realized that the apparent motions of the sun and moon from west
to east around the zodiac do not have a constant speed. These bodies appear
to move with increasing speed for half of each revolution to a definite
maximum and then to decrease in speed to the former minimum. The
Babylonians attempted to represent this cycle arithmetically by
giving the moon, for example, a fixed speed for its motion during
half its cycle and a different fixed speed for the other half. Later
they refined the mathematical method by representing the speed of
the moon as a factor that increases linearly from the minimum to
maximum during half of its revolution and then decreases to the minimum
at the end of the cycle. With these calculations of the lunar and
solar motions, Babylonian stargazers could predict the time of the
new moon and the day on which the new month would begin. As a by-product,
they knew the daily positions of moon and sun for every day during
the month.
In a similar manner the planetary positions were calculated,
with both their eastward and retrograde motions represented. Archaeologists
have unearthed hundreds of cuneiform tablets that show these calculations.
A few of these tablets, which originated in the cities of Babylon
and Uruk, on the Euphrates River, bear the name of Naburiannu (fl.
about 491 bc) or Kidinnu (fl. 383–379 bc),
astrologers who may have invented the systems of calculation.
The ancient Greeks made important theoretical contributions
to astronomy. The Odyssey of Homer refers to such
constellations as the Great Bear, Orion, and the Pleiades and describes
how the stars may serve as a guide in navigation. The poem Works
and Days by Hesiod informs the farmer which constellations
rise before dawn at different seasons of the year to indicate the
proper times for plowing, sowing, and harvesting.
Scientific contributions are associated with the names of
the Greek philosophers Thales of Miletus and Pythagoras of Sámos,
but none of their own writings survives. The legend that Thales
predicted a total solar eclipse on May 28, 585 bc, is probably
apocryphal. About 450 bc the Greeks began a study of planetary motions.
Philolaus (fl. about 500–475 bc), a follower of
Pythagoras, believed that the earth, sun, moon, and planets moved
around a central fire hidden from view by an interposed counterearth.
According to his theory, the revolution of the earth around the
fire every 24 hours accounted for the daily motions of the sun and
stars. About 370 bc the astronomer Eudoxus of Cnidus explained
observed motions by the supposition that a huge sphere bearing the
stars on its inner surface moved around the earth at its center
in a daily rotation. In addition, to account for solar, lunar, and
planetary motions, he assumed that inside the star sphere were many interconnected
transparent spheres that revolved in various ways.
Perhaps the most original ancient observer of the heavens
was Aristarchus of Sámos, a Greek. He believed that motions
in the sky could be explained by the hypothesis that the earth turns
on its axis once every 24 hours and, along with the other planets,
revolves around the sun. This theory was rejected by most Greek
philosophers, who regarded the heavy earth as a motionless globe
around which the light, incorporeal bodies revolve. This theory,
the geocentric system, remained virtually unchallenged for about
2000 years.
In the 2d century ad, the Greeks combined celestial
theories with carefully planned observations. The astronomer Ptolemy
expanded on the findings of the 2d-century bc astronomer
Hipparchus, who had compiled the first catalog of stars and determined
the positions of about 1000 bright stars. This star chart was used
as a background for measuring the motions of the planets. Abandoning
the spheres of Eudoxus for a more flexible system of circles, they
postulated a series of eccentric circles with the earth near a common center
to represent the general eastward motions at varying speeds of the
sun, moon, and planets around the zodiac. To explain the periodic
variations in the speed of the sun and moon and the retrogressions
of the planets, they postulated that each of these bodies revolved
uniformly around a second circle, called an epicycle, the center
of which was situated on the first. By proper choice of the diameters
and speeds for the two circular motions ascribed to each body, its
observed motion usually could be represented.. In some cases a third
circle was required. This technique was described by Ptolemy in
his great work the Almagest. Hypatia (c. 370–415),
a follower of Plato, wrote commentaries on mathematical and astronomical
topics and is regarded today as the first female astronomer.
Greek astronomy was transmitted eastward to the Syrians, the Hindus,
and the Arabs. The Arabian astronomers compiled new star catalogs
in the 9th and 10th centuries and subsequently developed tables
of planetary motion. Although the Arabs were good observers, they
made few useful contributions to astronomical theories. In the 13th
century, Arabic translations of Ptolemy’s Almagest filtered
into western Europe, stimulating interest in astronomy. Initially,
Europeans were content to make tables of planetary motions, based
on Ptolemy’s system, or to write short popular digests
of his theory. Later the German philosopher and mathematician Nicholas
of Cusa and the Italian artist and scientist Leonardo da Vinci questioned
the basic assumptions of the centrality and immobility of the earth.
The history of astronomy took a dramatic turn in the 16th century
as a result of the contributions of the Polish astronomer Nicolaus
Copernicus. He was educated in Italy and was a canon of the Roman
Catholic church. He spent most of his life pursuing astronomy, however, and
he compiled a new star catalog from personal observations. Copernicus
is most famous for his great work On the Revolutions of
the Celestial Spheres (1543; trans. 1952), in which he
analyzed critically the Ptolemaic theory of an earth-centered universe
and showed that the planetary motions can be explained by assuming
a central position for the sun rather than for the earth.
Little attention was paid to the Copernican, or heliocentric,
system until Galileo discovered evidence to support it. Long a secret
admirer of Copernicus’s work, Galileo saw his chance to
test the Copernican theory of a moving earth when the telescope
was invented in the Netherlands. He made (1609) a small refracting telescope,
turned it skyward, and discovered the phases of Venus, indicating
that this planet revolves around the sun; he also discovered four
moons revolving around Jupiter, as well as the rings of Saturn.
Convinced that some bodies, at least, do not circle the earth, he
began to speak and write in favor of the Copernican system. His
attempts to publicize the Copernican system caused him to be tried
by the ecclesiastical authorities. Although he was forced to repudiate
his beliefs and writings, the powerful theory could not be suppressed.
From the scientific viewpoint, the Copernican theory was only a
rearrangement of the planetary orbits, as conceived by Ptolemy.
The ancient Greek theory of planets moving around circles at fixed
speeds was retained in the Copernican system. From 1576 to 1596
the Danish astronomer Tycho Brahe observed the sun, moon, and planets
at his island observatory near Copenhagen and later in Germany.
Based on the data compiled by Brahe, his German assistant, Johannes
Kepler, formulated the laws of planetary motion, stating that the
planets revolve around the sun, not in circular orbits with uniform
motion but in elliptical orbits at varying speeds, and that their
relative distances from the sun can be determined from the observed
periods of revolution.
The British physicist Sir Isaac Newton advanced a simple principle
to explain Kepler’s laws of planetary motion. By mathematical
reasoning, he argued that an attractive force exists between the
sun and each of the planets. This force, which depends on the masses
of the sun and planets and on the distances between them, provides
the basis for the physical interpretation of Kepler’s laws.
Newton’s mathematical discovery is called the law of universal
gravitation.
After Newton’s time, astronomy branched out in several
directions. With his law of gravitation, the old problem of planetary
motion was studied anew as celestial mechanics. Improved telescopes
permitted the scanning of planetary surfaces, the discovery of many
faint stars, and the measurement of stellar distances . In the 19th
century a new instrument, the spectroscope, yielded information
about the chemical composition and motions of heavenly bodies.
During the 20th century, increasingly larger reflecting telescopes
have been built, including one with a mirror 236 in. (6 m) in diameter.
Studies with these instruments have revealed the structure of huge
distant assemblages of stars, called galaxies, and of clusters of
galaxies. In the second half of the 20th century, developments in
physics led to new classes of astronomical instruments, some of
which have been placed on earth-orbiting satellite observatories.
These instruments are sensitive to a wide variety of radiation wavelengths,
including the gamma-ray, X-ray, ultraviolet, infrared, and radio
regions of the electromagnetic spectrum. Astronomers now study not
only planets, stars, and galaxies but also plasmas (hot, ionized
gases) surrounding double stars, interstellar regions that are the
birthplaces of new stars, cold dust grains that are invisible in
the optical regions, energetic nuclei of galaxies that may contain
black holes, and photons originating from the big bang that may
yield information about the early history of the universe. .
Newton’s law of gravitation postulated an attractive
force between the sun and each of the planets in order to explain
Kepler’s laws of elliptical motion. It also implied, however, that
much smaller forces must exist between the planets themselves and
between the sun and other bodies such as comets. The interplanetary
gravitational forces cause the orbits of the planets to deviate
from simple elliptical motion. Most of these irregularities, predicted
on the basis of Newton’s theory, could be observed only
with the telescope.
Observation of planet positions was improved as a result of
the development of more accurate astronomical instruments and photographic
techniques. Correspondingly, mathematical calculations enable present-day
astronomers to predict planetary positions years in advance, with
an accuracy approximating that of the observed positions. Electronic
computing machines are now extensively utilized for such calculations.
With the use of the telescope many new members of the solar
system were discovered, including the planet Uranus in 1781 by the
British astronomer Sir William Herschel; the planet Neptune in 1846
by the German astronomer Johann Gottfried Galle (1812–1910)
based on calculations made in 1841 by the British astronomer John
Couch Adams (1819–92) and in 1846 by the French astronomer
Urbain Jean Joseph Leverrier; and Pluto in 1930 by the American
astronomer Clyde William Tombaugh (1906–97). The number of
known natural satellites is increasing as unmanned probes fly by
the outer planets. The earth has 1 natural moon; Mars, 2; Jupiter,
16; Saturn, more than 20; Uranus, 15; Neptune, 8; and Pluto, 1.
These numbers may continue to increase as astronomers get better
views of the planets. More than 1600 asteroids have been followed
as they move around the sun, mostly between the orbits of Mars and
Jupiter. Several hundred separate comets are cataloged. Countless
smaller bodies exist as stony and metallic meteoroids.
The chemical analysis and physical study of inaccessible heavenly
bodies were made possible by the invention in 1814 of the diffraction-grating
spectroscope by the German physicist Joseph von Fraunhofer and his
subsequent discovery of the spectral lines in starlight. The German
scientists Gustav Robert Kirchhoff and Robert Wilhelm Bunsen developed
the prism spectroscope in its modern form and discovered that every chemical
element exhibits a unique set or sets of spectral lines. Analyses
of planetary and stellar spectra have demonstrated that heavenly
bodies are composed of the same chemical elements known on earth. Spectroscopic
studies also provide clues about such conditions as the surface
temperatures, surface gravities, and motions of the heavenly bodies.
Instrument-bearing satellites, which drew close to Mercury,
Venus, Mars, Jupiter, Saturn, and Uranus in the 1970s and ’80s
to gather chemical and physical data from these bodies, discovered
rings about Jupiter and new moons of that planet, Saturn, and Uranus,
and supplied information that cast doubt on the possible presence
of life on other planets in the solar system; a decade later, however,
information gathered by space exploration programs that included
the Galileo mission to Jupiter and the Hubble Space
Telescope indicated otherwise. Signs of early life found in a Martian
rock prompted two new missions to Mars in the late 1990s.
Before the invention of the telescope the stars were regarded
as merely a convenient backdrop for scanning the wanderings of the
sun, moon, and planets. For the modern astronomer equipped with
telescope and spectroscope, the study of the stars is a challenging
aspect of astronomy.
Basic to the study of a star is the knowledge of its distance
from the earth, which is found by measuring the position of the
star in the sky at intervals six months apart, when the earth is
on opposite sides of its orbit. As the earth swings around the sun,
the star appears to shift back and forth in the sky. This annual
shift, called parallax, can be used to determine the distance from
the earth of a star near the sun. The greater the distance, the
smaller is the parallax of the star. The nearest star to the earth,
at a distance of 4.3 light years (or about 40 trillion km/25
trillion mi), is Alpha Centauri C, also known as Proxima Centauri,
one of the three stars in the Alpha Centauri system. Alpha Centauri
is about 260,000 times farther from the earth than is the sun. The
first star distances were measured independently by three astronomers
in 1838.
All stars are hot, gaseous bodies like the sun, but differ
from it and from one another in various ways. The most important
physical data about a star are its intrinsic brightness, size, mass,
and chemical composition. Although all fixed stars appear much fainter
than the sun because of their great distances from the earth, some of
them are intrinsically brighter. Star masses can be determined directly
for the sun and for pairs of stars, such as eclipsing binaries,
that exhibit a mutual revolution, similar to the motions of the
planets around the sun. Assuming that these revolutions are due
to gravitation, astronomers apply the law of gravitation to determine the
stellar masses mathematically. Of the 50 nearest stars for which
information is fairly complete, 10 percent are brighter, larger,
and more massive than the sun. Spectroscopic studies show that the
majority of the stars are composed largely of hydrogen.
The source of the vast energy radiated by the sun was long
a mystery. The sun produces 3.86 x 1026 W (5.18 x 1023 hp).
Geological evidence shows that life has existed on earth for some
billion years, indicating that solar energy must have been expended
at about its present rate for hundreds of millions of years. In
1938 the American physicist Hans Bethe advanced the theory that
solar energy is produced by the nuclear fusion of hydrogen atoms
into helium. His discovery helped pave the way for the development
of a nuclear-fusion hydrogen bomb approximately 15 years later.
Stars at least 1.4 times more massive than the sun pass through
their entire life cycle much faster than the sun. Optical telescopes
have revealed the principal steps in this cycle. First the star
begins to condense from inside but generally near one edge of a
dense molecular cloud, or “cocoon.” This condensation
initiates a period of contracting and internal heating followed
by a long period as a main-sequence star. Near the end of its lifetime,
the star expands to a red giant state, contracts back to the main
sequence, and degenerates to a white dwarf.
In the 1960s the British radio astronomer Jocelyn Bell (1943– )
discovered rapidly varying signals coming from starlike objects.
Studies by the British radio astronomer Antony Hewish showed these
to be pulsating sources, named pulsars, that consist of matter even
more condensed than white dwarfs. A pulsar is apparently the last
stage in the life cycle before final extinction as a black hole,
which is matter so dense that nothing, not even radiation, can escape
from it. In 1974 the existence of a black hole in the constellation Cygnus
was suggested by detection of X radiation from gas accelerated to
nearly the speed of light as it fell into the black hole. Since
that time other possibilities have been proposed, including the
existence of tremendous black holes located at the center of intensely
radiating galaxies. In 1994, astronomers found convincing evidence
of the existence of a black hole at the center of a galaxy in the
constellation Virgo.
In the late 18th century, Sir William Herschel constructed
the largest reflecting telescopes of his day and used them to explore
the heavens. The first serious student of the universe, he discovered
not only the planet Uranus but also a number of satellites and many
double stars, in addition to myriad star clusters and nebulae. His
counts of stars in different regions of the heavens convinced Herschel that
the sun is one of a vast cloud of stars arranged like the grains
of abrasive in a grindstone. According to his analogy, a person
on a small planet near the sun deep inside the grindstone can look
toward its edges and see a belt of faint, distant stars, which is
called the Milky Way, or the earth’s galaxy, stretching
completely around the sky; looking above or below, the person can
see relatively few nearby stars.
Modern investigations confirm that the Milky Way is a galaxy
of stars that are all gravitationally bound and rotating about a
distant center. Of primary importance in studying the structure
of the Milky Way is a knowledge of star distances. The parallax
method of determining these distances can be applied only to a few thousand
of the nearest stars. A special class of stars exists, the Cepheid
variables, which vary in brightness in periods that depend on their
intrinsic intensities. Comparison of the observed brightnesses with
the known intrinsic brightness of these stars provides a means of
determining their distances. Following the discovery of the relation
between period and luminosity by the U.S. astronomer Henrietta Swan
Leavitt, U.S. astronomer Harlow Shapley used the Cepheid variables,
scattered throughout the Milky Way, to measure its size. A ray of light,
moving at a speed of about 300,000 km/sec (about 186,000
mi/sec), would require 400,000 years to traverse the Milky
Way from edge to edge of its extended halo (described below). The
visible spiral is somewhat less than half as wide. Altogether, the
Milky Way consists of about a million million stars rotating about
a common center. The sun, located about 30,000 light-years from
the center of the Milky Way, travels at a speed of about 210 km/sec
(about 130 mi/sec) and completes an entire revolution approximately
every 200 million years.
The Milky Way includes great quantities of dust and gas particles
scattered between the stars. This interstellar matter intercepts
the visible light emitted by distant stars so that observers on
earth cannot view in detail distant parts of the Milky Way. A new
branch of astronomy was initiated when the American electronic engineer
Karl G. Jansky discovered in 1932 that radio waves are emitted in
the Milky Way. Later study traced this radiation partly to interstellar
matter and partly to discrete sources, formerly called radio stars.
Radio waves emitted by distant parts of the Milky Way can penetrate
interstellar matter, which is opaque to visible light, and thus
enable astronomers to observe regions hidden to optical instruments.
Such observations have revealed the Milky Way to be a spiral galaxy
with a flattened bulge of old stars, an outer disk of hot young stars
that make up the spiral arms, and a large, extended halo of faint
stars.. From observations of the outer disk by radio telescope in
1986, astronomers saw, for the first time in history, the birth
of a star, in the constellation Ophiuchus, or the Serpent Bearer,
500 light-years away.
The nucleus of the Milky Way was until recently a mysterious
region, obscured from view by dark clouds of interstellar dust.
Astronomers began getting their first detailed picture of the region
in 1983, when the Infrared Astronomical Satellite (IRAS) was launched.
Freed from the obscuring effects of the earth’s atmosphere,
sensors board IRAS recorded in unprecedented detail the positions
and shapes of the myriad sources of infrared energy that occupy
the heart of the Milky Way galaxy. Among these was discovered one massive
object, not a star and too compact to be a star cluster, that may
yet prove to be a black hole.
Despite its vast size the Milky Way is only one of many great
star systems, called galaxies, that populate the known universe.
Studies conducted by the American astronomer Edwin Hubble resolved
in 1924 the question as to the nature of the spiral nebulae, showing
them to be individual galaxies like the Milky Way but located at
very great distances. Some galaxies have a spiral form, like the
Milky Way, while others are spheroidal, without the spiral arms,
or of irregular shape. The largest optical telescope, the 387-in.
(9.82-m) Keck Telescope at Mauna Kea Observatory in Hawaii, has
revealed galaxies as far away as several billion light-years.
Spectrum analysis of the light from exterior galaxies shows
that the stars making up these systems are composed of the same
chemical elements known on earth. Somewhat unexpectedly, it also
demonstrates that the galaxies are all moving away from the Milky
Way: the more distant a galaxy, the faster its recession. This is currently
taken as evidence that the universe is expanding, and that it originated
from an extremely hot, dense state of matter by an explosion called
the big bang. The possible conditions that could have initiated
the explosion are treated in a cosmological theory of the early
1980s known as the Inflationary Theory. Big bang radiation has been
cooling ever since; its present temperature is about 3 K above absolute
zero (about –454° F). Radiation of this temperature,
coming from all directions, was discovered in 1965 by the American physicists
Arno Penzias (1933– )
and Robert W. Wilson (1936– ),
and is currently the best indicator of the early history of the
universe. Albert Einstein’s relativistic theory of gravitation
also supports the big bang theory.
Quasars, discovered in the 1950s with the use of radio telescopes,
are believed by most astronomers to be the energetic nuclei of very
distant galaxies. For reasons not yet known, they have brightened
so much that they mask the light from their underlying galaxies.
Often they occur in extremely distant clusters of galaxies. The
spectral lines of quasars display very large red shifts, which would
indicate that these objects are traveling away from earth’s
galaxy at speeds in the range of 80 percent of the speed of light..
Their apparent great speed also means that they are among the most
distant of cosmological objects. A quasar 12 billion light-years distant
was discovered in 1991 by astronomers using the 200-in. (5.08-m)
reflector at Palomar Observatory.