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*** Introduction
branch of science that studies the motions and natures of celestial
bodies, such as planets , stars , and galaxies ; more generally, the
study of matter and energy in the universe at large.
Ancient
Astronomy
Astronomy is the oldest of the physical sciences. In many early civilizations
the regularity of celestial motions was recognized, and attempts were
made to keep records and predict future events. The first practical
function of astronomy was to provide a basis for the calendar , the
units of month and year being determined by astronomical observations.
Later, astronomy served in navigation and timekeeping. The Chinese
had a working calendar as early as the 13th cent. BC About 350 BC,
Shih Shen prepared the earliest known star catalog, containing 800
entries. Ancient Chinese astronomy is best known today for its observations
of comets and supernovas . The Babylonians, Assyrians, and Egyptians
were also active in astronomy. The earliest astronomers were priests,
and no attempt was made to separate astronomy from astrology . In
fact, an early motivation for the detailed study of planetary positions
was the preparation of horoscopes.
Greek Innovations
The highest development of astronomy in the ancient world came with
the Greeks in the period from 600 BC to AD 400. The methods employed
by the Greek astronomers were quite distinct from those of earlier
civilizations, such as the Babylonian. The Babylonian approach was
numerological and best suited for studying the complex lunar motions
that were of overwhelming interest to the Mesopotamian peoples. The
Greek approach, on the contrary, was geometric and schematic, best
suited for complete cosmological models. Thales, an Ionian philosopher
of the 6th cent. BC, is credited with introducing geometrical ideas
into astronomy. Pythagoras, about a hundred years later, imagined
the universe as a series of concentric spheres in which each of the
seven ˇ°wanderersˇ± (the sun, the moon, and the five known planets)
were embedded. Euxodus developed the idea of rotating spheres by introducing
extra spheres for each of the planets to account for the observed
complexities of their motions. This was the beginning of the Greek
aim of providing a theory that would account for all observed phenomena.
Aristotle (384-322 BC) summarized much of the Greek work before him
and remained an absolute authority until late in the Middle Ages.
Although his belief that the earth does not move retarded astronomical
progress, he gave the correct explanation of lunar eclipses and a
sound argument for the spherical shape of the earth.
The
Alexandrian School and the Ptolemaic System
The apex of Greek astronomy was reached in the Hellenistic period
by the Alexandrian school. Aristarchus (c.310-c.230 BC) determined
the sizes and distances of the moon and sun relative to the earth
and advocated a heliocentric (sun-centered) cosmology. Although there
were errors in his assumptions, his approach was truly scientific;
his work was the first serious attempt to make a scale model of the
universe. The first accurate measurement of the actual (as opposed
to relative) size of the earth was made by Eratosthenes (284-192 BC).
His method was based on the angular difference in the sun's position
at the high noon of the summer solstice in two cities whose distance
apart was known.
The greatest astronomer of antiquity was Hipparchus (190-120 BC).
He developed trigonometry and used it to determine astronomical distances
from the observed angular positions of celestial bodies. He recognized
that astronomy requires accurate and systematic observations extended
over long time periods. He therefore made great use of old observations,
comparing them to his own. Many of his observations, particularly
of the planets, were intended for future astronomers. He devised a
geocentric system of cycles and epicycles (a compounding of circular
motions) to account for the movements of the sun and moon. Ptolemy
(AD 85-165) applied the scheme of epicycles to the planets as well.
The resulting Ptolemaic system was a geometrical representation of
the solar system that predicted the motions of the planets with considerable
accuracy. Among his other achievements was an accurate measurement
of the distance to the moon by a parallax technique. His 13-volume
treatise, the Almagest, summarized much of ancient astronomical knowledge
and, in many translations, was the definitive authority for the next
14 centuries.
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*** Development of Modern Astronomy
The Copernican Revolution
After the fall of Rome, European astronomy was largely dormant, but
significant work was carried out by the Muslims and the Hindus. It
was by way of Arabic translations that Greek astronomy reached medieval
Europe. One of the great landmarks of the revival of learning in Europe
was the publication (1543) by Nicolaus Copernicus (1473-1543) of his
De revolutionibus orbium coelestium ( On the Revolutions of the Celestial
Spheres ). According to the Copernican system , the earth rotates
on its axis and, with all the other planets, revolves around the sun.
The assertion that the earth is not the center of the universe was
to have profound philosophical and religious consequences. Copernicus's
principal claim for his new system was that it made calculations easier.
He retained the uniform circular motion of the Ptolemaic system, but
by placing the sun at the center, he was able to reduce the number
of epicycles. Copernicus also determined the sidereal periods (time
for one revolution around the sun) of the planets and their distance
from the sun relative to the sun-earth distance.
Brahe
and Kepler
The great astronomer Tycho Brahe (1546-1601) was principally an observer;
a conservative in matters of theory, he rejected the notion that the
earth moves. Under the patronage of King Frederick II, Tycho established
Uraniborg, a superb observatory on the Danish island of Hveen. Over
a period of 20 years (1576-97), he and his assistants compiled the
most accurate and complete astronomical observations to that time.
At his death his records passed to Johannes Kepler (1571-1630), who
had been his last assistant. Kepler spent nearly a decade trying to
fit Tycho's observations, particularly of Mars, into an improved system
of heliocentric circular motion. At last, he conceived the idea that
the orbit of Mars was an ellipse with the sun at one focus. This led
him to the three laws of planetary motion that bear his name.
Galileo's Telescope
Galileo Galilei (1564-1642) made fundamental discoveries in both astronomy
and physics; he is perhaps best described as the founder of modern
science. Galileo was the first to make astronomical use of the telescope
. His discoveries of the four largest moons of Jupiter and the phases
of Venus were persuasive evidence for the Copernican cosmology. His
discoveries of craters on the moon and blemishes on the sun ( sunspots
) discredited the ancient belief in the perfection of the heavens.
These findings were announced in The Sidereal Messenger, a small book
published in 1610. Galileo's Dialogue on the Two Chief Systems of
the World (1632) was an eloquent argument for the Copernican system
over the Ptolemaic. However, Galileo was called before the Inquisition
and forced to renounce publicly all doctrines considered contrary
to Scripture.
Astrophysical Discoveries
Isaac Newton (1642-1727), possibly the greatest scientific genius
of all time, succeeded in uniting the sciences of astronomy and physics
. His laws of motion and theory of universal gravitation provided
a physical, dynamic basis for the merely descriptive laws of Kepler.
Until well into the 19th cent., all progress in astronomy was essentially
an extension of Newton's work. Edmond Halley 's prediction that the
comet of 1682 would return in 1758 was refined by A. C. Clairault,
who included the perturbing effects of Jupiter and Saturn on the orbit
to calculate the nearly exact date of the return of the comet. In
1781, William Herschel accidentally discovered a new planet, eventually
named Uranus. Discrepancies between the observed and theoretical orbits
of Uranus indicated the existence of a still more distant planet that
was affecting Uranus's motion. J. C. Adams and U. J. J. Leverrier
independently calculated the position where the new planet, Neptune,
was actually discovered (1846). Similar calculations for a large ˇ°Planet
Xˇ± led in 1930 to the discovery of a small planet with the most distant
orbit, Pluto.
By the early 19th cent., the science of celestial mechanics had reached
a highly developed state at the hands of Leonhard Euler, J. L. Lagrange,
P. S. Laplace, and others. Powerful new mathematical techniques allowed
solution of most of the remaining problems in classical gravitational
theory as applied to the solar system. In 1801, Giuseppe Piazzi discovered
Ceres, the first of many asteroids . When Ceres was lost to view,
C. F. Gauss applied the advanced gravitational techniques to compute
the position where the asteroid was subsequently rediscovered. In
1838, F. W. Bessel made the first measurement of the distance to a
star; using the method of parallax with the earth's orbit as a baseline,
he determined the distance of the star 61 Cygni to be 60 trillion
mi (about 10 light-years ), a figure later shown to be 40% too large.
Modern Techniques, Discoveries, and Theories
Astronomy was revolutionized in the second half of the 19th cent.
by the introduction of techniques based on photography and spectroscopy.
Interest shifted from determining the positions and distances of stars
to studying their physical composition. The dark lines in the solar
spectrum that had been observed by W. H. Wollaston and Joseph von
Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff
on the basis of classical physics, although a complete explanation
came only with the quantum theory. Between 1911 and 1913, Ejnar Hertzsprung
and H. N. Russell studied the relation between the colors and luminosities
of typical stars. With the construction of ever more powerful telescopes,
the boundaries of the known universe constantly increased. E. P. Hubble's
study of the distant galaxies led him to conclude that the universe
is expanding. Using Cepheid variables as distance indicators, Harlow
Shapley determined the size and shape of our galaxy, the Milky Way
. During World War II Walter Baade defined two ˇ°populationsˇ± of stars,
and suggested that an examination of these different types might trace
the spiral shape of our own galaxy. In 1951 a Yerkes Observatory group
led by William W. Morgan detected evidence of two spiral arms in the
Milky Way galaxy.
Various rival theories of the origin and overall structure of the
universe, e.g., the big bang and steady state theories, have been
formulated. Albert Einstein's theory of relativity plays a central
role in all modern cosmological theories. In 1963, the moon passed
in front of the radio source 3C-273, allowing Cyril Hazard to calculate
the exact position of the source. With this information, Maarten Schmidt
photographed the object's spectrum using the 200-in. (5-m) reflector
on Palomar Mt., then the world's largest telescope. He interpreted
the result as coming from an object, now known as a quasar , at an
extreme distance and receding from us at a substantial fraction of
the speed of light. In 1967 Antony Hewish and Jocelyn Bell Burnell
discovered a radio source a few hundred light years away featuring
regular pulses at intervals of about 1 second with an accuracy of
repetition of one-millionth of a second. This was the first discovered
pulsar , a rapidly spinning neutron star emitting lighthouse-type
beams of energy, the end result of the death of a star in a supernova
explosion.
The discovery by Karl Jansky in 1931 that radio signals were emitted
by celestial bodies initiated the science of radio astronomy . Most
recently, the frontiers of astronomy have been expanded by space exploration
. Perturbations and interference from the earth's atmosphere make
space-based observations necessary for infrared , ultraviolet , gamma-ray
, and X-ray astronomy . The Surveyor and Apollo spacecraft of the
late 1960s and early 1970s helped launch the new field of astrogeology.
A series of interplanetary probes, such as Mariner 2 (1962) and 5
(1967) to Venus, Mariner 4 (1965) and 6 (1969) to Mars, and Voyager
1 (1979) and 2 (1979), provided a wealth of data about Jupiter, Saturn,
Uranus, and Neptune; more recently, the Magellan probe to Venus (1990)
and the Galileo probe to Jupiter (1995) have continued this line of
research. The Hubble Space Telescope , launched in 1990, has made
possible visual observations of a quality far exceeding those of earthbound
instruments.
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*** Observatory
Introduction
scientific facility especially equipped to detect and record
naturally occurring scientific phenomena. Although geological and
meteorological observatories exist, the term is generally applied
to astronomical observatories.
The Astronomical Observatory
The function of the astronomical observatory is centered
around the telescope . In addition to visual and photographic observations
of astronomical bodies and phenomena, perhaps the most valuable use
of the telescope is in connection with the spectroscopic study of
starlight. The total light from a star is separated into its various
wavelengths, and the intensity of each is measured. The temperature
and chemical composition of stars can be obtained by this method,
as well as information about stellar motion and magnetic fields. Using
computers, astronomers can measure the spectra digitally recorded
by spectrographs and photometers. Observatories specializing in solar
astronomy usually have coronographs and spectroheliographs. Atmospheric
limitations on telescopic observations include weather conditions,
air turbulence, air glow, pollution, and any source of extraneous
illumination. To minimize such conditions optical observatories are
generally located at high altitudes in sparsely populated areas.
Development of the Astronomical Observatory
Early civilizations, such as those of Babylon, China, and
Egypt, recognized the regular and periodic nature of heavenly motions
and established primitive observatories to maintain astronomical records.
The main purposes of these early observatories were to regulate the
calendar and predict the changes of season. Because it was believed
that unusual occurrences, such as comets and eclipses, foretold future
events on earth, the early observatories also served a religious function,
and most of the ancient astronomers were priests. Later observatories
were established to compile accurate star charts and an annual ephemeris
that would be of use to navigators in determining longitude at sea.
For some 600 years, beginning in the 13th cent., Roman Catholic churches
included solar observatories to measure the movements of the sun and
so determine the correct date for Easter.
The instruments in use before the invention of the telescope include
the sextant , quadrant, astrolabe , and armillary sphere. These are
all calibrated sighting devices for determining the angular positions
of stars and planets. The armillary sphere was the most sophisticated
of these instruments. It was composed of a number of rings corresponding
to great circles on the celestial sphere and was used to determine
both the right ascension and the declination of a star. The last great
observatory of the pretelescopic era was built by Tycho Brahe at Uranienborg,
on the island of Ven, Denmark.
The invention of the telescope in the early 17th cent. revolutionized
observational astronomy in two ways. First, the positions and motions
of celestial bodies could be measured much more accurately with telescopes
than with the earlier instruments. Such data provided a source of
precise time signals. Second, the telescope could be used to analyze
the physical nature of celestial bodies themselves. Until the 19th
cent., telescopic images were inspected visually by highly trained
observers who made drawings of what they saw. The development of dry-plate
photography, which permitted long exposure times, however, offered
a much more sensitive method of recording images. In the late 20th
cent., electronic digital detectors utilizing charge-coupled devices
(CCDs) superseded the use of film; a CCD can detect the arrival of
a single photon of light. A recent development is the extension of
astronomical observations to wavelengths outside the visible spectrum.
Most important has been the development of radio astronomy , the study
of radio waves emitted by celestial bodies.
Because the atmosphere interferes with astronomical observations from
the ground, the ideal location for an observatory is beyond the earth's
atmosphere. The past two decades have seen increasing emphasis on
space-based observatories. Several artificial satellites have been
equipped with telescopes for infrared, visible, ultraviolet, and X-ray
observations. The International Ultraviolet Explorer (IUE) satellite,
launched in 1978, is an 18-in. (0.45-meter) space telescope for ultraviolet
studies. Launched in 1983, the Infrared Astronomy Satellite (IRAS)
discovered some 246,000 infrared sources, as well as several stars
around which planetary systems appear to be forming. Skylab was a
manned orbiting space observatory. The largest space-based observatory
is the Hubble Space Telescope , launched in 1990. Other observatories
include the Compton Gamma-Ray Observatory, launched in 1991, and the
Chandra X-ray Observatory, launched in 1999. ROSAT [ RO entgen SAT
ellite], a joint German-U.S.-British project launched in 1990, studies
both X-ray and ultraviolet wavelengths never before imaged from space.
It has detected a new class of bright stars that shine only in the
ultraviolet part of the spectrum. The Cosmic Background Explorer (1989-93)
studied microwave background radiation that no star or other known
object could emitit is believed to have come from the creation of
the universe.
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*** observatory, orbiting
research satellite designed to study solar radiation, electromagnetic
radiation from distant stars, the earth's atmosphere , or the like.
Because the atmosphere and other aspects of the earth's environment
interfere with astronomical observations from the ground, especially
in the ultraviolet and infrared portions of the spectrum, the decades
since the 1960s have seen increasing emphasis on space-based observatories.
The U.S. Orbiting Solar Observatory (OSO) program, comprising eight
satellites launched between 1962 and 1971, was one of the earliest
series of orbiting observatories; it studied the sun's atmosphere
and the sunspot cycle. Also beginning in 1962 and extending through
1979 were the launches of the six satellites in Great Britain's Ariel
program, which concentrated on solar ultraviolet and X radiation.
The Orbiting Geophysical Observatory (OGO) program consisted of six
satellites, launched between 1964 and 1969, that provided data on
the earth's atmosphere, ionosphere , and magnetosphere and on the
solar wind . The Orbiting Astronomical Observatory (OAO) program comprised
four satellites, launched between 1966 and 1972, that studied astronomical
phenomena at ultraviolet and X-ray wavelengths inaccessible to earthbound
equipment.
In the following years, a large number of satellites were launched
to study solar and galactic radio waves, X rays, gamma rays, and ultraviolet
rays. In addition to the United States a number of countries participated,
among them the Netherlands with ANS-1 (1974-76), which studied soft
and hard X radiation; India with Aryabhata (1975), which returned
atmospheric data for only four days before being silenced by a power
failure; Japan with Hakucho (1979-85) and Tenma (1981-84), both of
which studied X radiation; and the European Space Agency (ESA) with
Exosat (1983-86), an X-ray observatory. This period also saw the first
cooperative efforts, such as the International Ultraviolet Explorer
(IUE), a joint effort of the United States, ESA, and Great Britain
(1978-96), which returned data on ultraviolet radiation for 18 years.
ROSAT [ Ro entgen Sat ellite] (1990-99), a joint German-U.S.-British
project, studied both X-ray and ultraviolet wavelengths never before
imaged from space. It detected a new class of bright stars that shine
only in the ultraviolet part of the spectrum and X-ray emissions from
comets. The Cosmic Background Explorer (1989-93) studied microwave
background radiation that no star or other known object could emitit
is believed to have come from the creation of the universe. The Infrared
Space Observatory (ISO; 1995-98), launched by ESA, found water in
the Orion nebula and in the atmospheres of the giant planets and Titan,
found fluoride molecules in interstellar space, and studied the ˇ°coolˇ±
galaxies first seen by the Infrared Astronomy Satellite (IRAS) in
1983. Another European-built orbiting observatory, the Solar and Heliospheric
Observatory (SOHO), was launched by NASA in 1995. After reaching a
position about 950,000 mi (1.5 million km) from the earth, where the
gravitational attraction of the earth and the sun are in balance (called
a Lagrangian point), SOHO initiated a program of solar physics studies,
such as the solar wind and solar plumes.
To fully explore the cosmos it is necessary to collect and analyze
radiation emitted by phenomena throughout the entire electromagnetic
spectrum. Toward that end, NASA proposed the concept of great observatories,
a series of four orbiting observatories designed to conduct astronomical
studies over many different wavelengths. An important aspect of the
program was to overlap the operations phases of the missions to enable
astronomers to make concurrent observations of an object at different
spectral wavelengths. The first member of the program and the largest
orbiting observatory is the Hubble Space Telescope (HST), which was
deployed by a space shuttle in 1990 and repaired in orbit in 1993.
Subsequent servicing missions in 1997 and 1999 (with another planned
for 2002) added capabilities to the HST, which observes the universe
at ultraviolet, visual, and near-infrared wavelengths. The second
great observatory, the Compton Gamma-Ray Observatory, was launched
and deployed by a shuttle in 1991; it continues to collect data on
gamma-ray bursts, which are some of the most violent physical processes
in the universe. The third great observatory, the Chandra X-ray Observatory,
formerly called the Advanced X-ray Astrophysics Facility, was deployed
from a shuttle and boosted into a high earth orbit in 1999; it focuses
on such objects as black holes, quasars, and high-temperature gases
throughout the X-ray portion of the electromagnetic spectrum. The
Space Infrared Telescope Facility (SIRTF) represents the fourth and
final element in the great observatory program; to be launched 2002,
SIRTF will fill an important gap in wavelength coverage not available
from earthbound telescopes.
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*** space probe
space vehicle carrying sophisticated instrumentation but
no crew, designed to explore various aspects of the solar system.
Unlike an artificial satellite , which is placed in more or less permanent
orbit around the earth, a space probe is launched with enough energy
to escape the gravitational field of the earth and navigate among
the planets. Radio-transmitted commands and on-board computers provide
the means for midcourse corrections in the space probe's trajectory;
some advanced craft have executed complex maneuvers on command from
earth when many millions of miles away in space. Radio contact between
the control station on earth and the space probe also provides a channel
for transmitting data recorded by on-board instruments back to earth.
Instruments carried by space probes include radiometers, magnetometers,
and television cameras sensitive to infrared, visible, and ultraviolet
light; there also may be special detectors for micrometeors, cosmic
rays, gamma rays, and solar wind. A probe may be directed to orbit
a planet, to soft-land instrument packages on a planetary surface,
or to fly by as close as a few thousand miles from one or more planets.
The particulars of trajectory and instrumentation of each space probe
are tailored around the mission's scientific and technological objectives;
the data provided by a single space probe may require months or even
years of analysis. Much has been learned from probes about the origins,
composition, and structure of various bodies in the solar system.
Scientists trying to understand the earth's weather by constructing
theoretical models of global weather systems make use of the knowledge
that is gained concerning the atmospheres and meteorology of the planets.
Because conditions on other planets are simpler than on earth, scientists
can check each of their hypotheses separately in isolation from complicating
factors. The earliest space probes in the U.S. space program were
the Mariner series, which investigated Mars, Venus, and Mercury, and
the Pioneer series, which explored the outer planets. Pioneer 10 was
the first human-made object to entirely escape the solar system. Several
Viking space probes voyaged to Mars in the late 1970s, mapping the
planet and searching for life. The Voyager probes, launched in 1977,
returned spectacular photos and data from brushes by Jupiter, Saturn,
Uranus, Neptune, and their moons. The Magellan spacecraft succeeded
in orbiting Venus in 1990, returning a radar map of the planet's hidden
surface. The Japanese probes Sakigake and Suisei and the European
Space Agency's (ESA) probe Giotto both rendezvoused with Halley's
comet in 1986, and Giotto also came within 125 mi (200 km) of the
nucleus of the comet Grigg-Skjellerup in 1992. The U.S. probe Ulysses
returned data about the poles of the sun in 1994, and the ESA Solar
and Heliospheric Observatory ( SOHO ) was orbited in 1995. Launched
in 1989, the Galileo spacecraft followed a circuitous route that returned
data about Venus (1990), the moon (1992), and the asteroids 951 Gaspra
(1991) and 243 Ida (1993) before it reached Jupiter in 1995 and sent
a small probe into the Jovian atmosphere to study its composition.
Over the next two years it orbited Jupiter 11 times, returning data
about the planet's atmosphere and also about Jupiter's largest moons,
Io, Ganymede, Europa, and Callisto; its mission was extended to include
further studies of Europa, the volcanic moon Io, and the thunderstorms
of Jupiter. The joint U.S.-ESA probe Cassini, launched in 1997, will
explore Saturn and some of its moons. Upon its arrival in 2004, Cassini
will send a probe (called Huygens ) into the atmosphere of Saturn's
largest moon, Titan. Over three years, Cassini will conduct detailed
studies of Saturn's atmosphere, rings, and magnetosphere; conduct
close-up studies of Saturn's satellites Iapetus, Dione, and Enceladus;
and characterize Titan's atmosphere and surface. The Mars Pathfinder
and Mars Global Surveyor, both of which arrived at the red planet
in 1997, were highly successful, the former in analyzing the Martian
surface and the latter in mapping it. Both the Mars Climate Orbiter
and Mars Polar Lander, however, were lost upon their arrival at Mars
in 1999, setting NASA's Mars exploration program back by at least
two years. The NEAR (for Near Earth Asteroid Rendezvous) -Shoemaker
probe returned data about the asteroid Mathilde as it flew by in 1997
and the asteroid Eros as it orbited it in 1999 and 2000 and then landed
on its surface in 2001, returning unparalleled data about a minor
planet.
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