What is Astronomy?
Astronomy is the science that deals with the origin, evolution,
composition, distance, and motion of all bodies and scattered
matter in the universe. It includes astrophysics, which
discusses the physical properties and structure of all cosmic
matter.
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Until the invention of the telescope and
the discovery of the laws of motion and gravity in the
17th century, astronomy was primarily concerned with
noting and predicting the positions of the Sun, Moon,
and planets, initially for calendrical and astrological
purposes and later for navigational applications and
scientific interest. The catalog of objects now studied
is much broader and includes, in order of increasing
distances, the solar system, the stars that make up
the Milky Way Galaxy, and other more distant stellar
objects and galaxies. With the advent of scientific
space probes, the Earth also has come to be studied
as one of the planets, though its more detailed investigation
remains the domain of the geologic sciences. |
Impact of Astronomy
No area of science is
totally self-contained. Discoveries in one field find
applications in others, often unpredictably. There
are various notable examples of this involving astronomical
studies. Newton's laws of motion and gravity emerged
from the analysis of planetary and lunar orbits. The
behaviour of nuclear matter and of some elementary
particles is now better understood as a result of
measurements of neutron stars and the cosmological
helium abundance, respectively. Study of the theory
of synchrotron radiation was greatly stimulated by
its detection in radiation emitted by the Crab Nebula,
and machines are now being used to produce synchrotron
radiation to probe the structure of materials.
Astronomical knowledge also has had impact outside
of science. The earliest calendars were based on astronomical
observations of the cycles of repeated solar and lunar
positions. Also, for centuries, familiarity with the
positions and apparent motions of the stars enabled
sea voyagers to navigate with reasonable accuracy.
Perhaps the single greatest effect that astronomical
studies have had on modern society has been in molding
the attitude of its members. The development of what
is known as the scientific method was strongly influenced
by astronomical observations. The power of science
to provide the basis for accurate predictions of such
phenomena as eclipses and the positions of the planets
and later, so dramatically, of comets has generated
an attitude toward science that remains an important
social force today. |
Telescopic Observations
| Before Galileo's use of telescopes for astronomical
investigations in 1609, all observations were made by
naked eye, with corresponding limits on the faintness
and degree of detail that could be seen. Since that
time, telescopes have become central to astronomy. With
apertures much larger than the pupil of the human eye,
telescopes permit the study of faint and distant objects.
In addition, sufficient radiant energy can be collected
in short time intervals to permit rapid fluctuations
in intensity to be detected. |
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Optical telescopes are either refractors or reflectors that use lenses or mirrors,
respectively, for their main light-collecting elements (objectives).
Refractors are effectively limited to apertures with a diameter
of 102 centimetres (40 inches) or less because of problems
inherent in the use of large lenses. These distort under
their own weight and can be supported only around the perimeter;
an appreciable amount of light is lost due to absorption
in the glass. Large-aperture refractors are extremely long
and require large and expensive domes. The largest modern
telescopes are all reflectors. They are not subject to the
chromatic problems of refractors, can be better supported
mechanically, and can be housed in smaller domes because
of their more compact dimensions.
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The atmosphere does not transmit radiation
of all wavelengths equally well. This restricts ground-based
astronomy to the visible and radio regions of the spectrum,
with some relatively narrow “windows” in
the nearer infrared. Longer infrared wavelengths are
heavily absorbed by atmospheric water vapour and carbon
dioxide. Atmospheric effects can be reduced by careful
site selection and by carrying out observations at high
altitudes. Most major optical observatories are now
located on high mountains, well away from cities and
their reflected lights. Infrared telescopes have been
constructed atop Mauna Kea in Hawaii and in the Canary
Islands where atmospheric humidity is very low. Airborne
telescopes designed mainly for infrared observations,
such as the one aboard the Kuiper Airborne Observatory,
operate at
an altitude of about 12,000 metres (40,000 feet), with
flight durations limited to a few hours. Telescopes
for infrared-, X-, and gamma-ray observations have been
carried
to altitudes of more than 30,000 metres by balloons.
Higher altitudes can be attained during short-duration
rocket flights for ultraviolet observations. Telescopes
for all wavelengths from infrared to gamma rays have
been carried by remote-controlled Earth-orbiting satellites
as well as by some manned space vehicles. The Hubble
Space Telescope, launched in 1990 and fully operational
since late 1993, has provided remarkably detailed images
of not only familar objects like nebulae and galaxies,
but also of objects invisible to terrestrial observatories. |
Angular resolution better than one milliarcsecond has been
achieved at radio wavelengths by using several telescopes
in an array. In such an arrangement, the effective aperture
(D) becomes the greatest distance between component telescopes.
For example, in the Very Large Array (VLA), operated near
Socorro, N.M., by the National Radio Astronomy Observatory,
27 movable radio dishes are set out along tracks that extend
for nearly 21 kilometres. In a technique called very long
baseline interferometry (VLBI), simultaneous observations
are made with radio telescopes thousands of kilometres apart.
The Earth is a moving platform for astronomical observations.
It is important that the specification of precise celestial
coordinates be made in ways that correct for telescope location,
the position of the Earth in its orbit around the Sun, and
the epoch of observation, since the Earth's axis of rotation
moves slowly over the years. Time measurements are now based
on atomic clocks rather than on the Earth's rotation, and
telescopes can be driven continuously to compensate for
the planet's rotation so as to permit tracking of a given
astronomical object.
Use of Radiation Detectors
The atmosphere does not transmit radiation of all wavelengths
equally well. This restricts ground-based astronomy to the
visible and radio regions of the spectrum, with some relatively
narrow “windows” in the nearer infrared. Longer
infrared wavelengths are heavily absorbed by atmospheric
water vapour and carbon dioxide. Atmospheric effects can
be reduced by careful site selection and by carrying out
observations at high altitudes. Most major optical observatories
are now located on high mountains, well away from cities
and their reflected lights. Infrared telescopes have been
constructed atop Mauna Kea in Hawaii and in the Canary Islands
where atmospheric humidity is very low. Airborne telescopes
designed mainly for infrared observations, such as the one
aboard the Kuiper Airborne Observatory, operate at an altitude
of about 12,000 metres (40,000 feet), with flight durations
limited to a few hours. Telescopes for infrared-, X-, and
gamma-ray observations have been carried to altitudes of
more than 30,000 metres by balloons. Higher altitudes can
be attained during short-duration rocket flights for ultraviolet
observations. Telescopes for all wavelengths from infrared
to gamma rays have been carried by remote-controlled Earth-orbiting
satellites as well as by some manned space vehicles. The
Hubble Space Telescope, launched in 1990 and fully operational
since late 1993, has provided remarkably detailed images
of not only familar objects like nebulae and galaxies, but
also of objects invisible to terrestrial observatories.
Angular resolution better than one milliarcsecond has been
achieved at radio wavelengths by using several telescopes
in an array. In such an arrangement, the effective aperture
(D) becomes the greatest distance between component telescopes.
For example, in the Very Large Array (VLA), operated near
Socorro, N.M., by the National Radio Astronomy Observatory,
27 movable radio dishes are set out along tracks that extend
for nearly 21 kilometres. In a technique called very long
baseline interferometry (VLBI), simultaneous observations
are made with radio telescopes thousands of kilometres apart.
The Earth is a moving platform for astronomical observations.
It is important that the specification of precise celestial
coordinates be made in ways that correct for telescope location,
the position of the Earth in its orbit around the Sun, and
the epoch of observation, since the Earth's axis of rotation
moves slowly over the years. Time measurements are now based
on atomic clocks rather than on the Earth's rotation, and
telescopes can be driven continuously to compensate for
the planet's rotation so as to permit tracking of a given
astronomical object.
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