| Astronomy (Greek:
αστρονομία = άστρον + νόμος, literally, "law of the stars") is the science involving the observation and explanation of
events occurring beyond the Earth and its atmosphere. It studies the origins, evolution, and physical and chemical properties of objects that may be
observed in the sky (and are beyond the atmosphere), as well as the connected processes and phenomena.
Astronomy is one of the few sciences where amateurs can still
play an active role, especially in the discovery and monitoring of transient phenomena. Astronomy is not to be confused with astrology, which
assumes that people's destiny and human affairs in general are correlated to the apparent positions of astronomical objects in
the sky -- although the two fields share a common origin, they are quite different; astronomers embrace the scientific method, while astrologers do not.
Divisions of astronomy
In ancient Greece and other early civilizations, astronomy consisted
largely of astrometry, measuring positions of stars and planets in the sky.
Later, the work of Kepler and Newton paved the way for celestial
mechanics, mathematically predicting the motions of celestial bodies interacting under gravity, and solar system objects in particular. Much of the effort in these two areas, once
done largely by hand, is highly automated nowadays, to the extent that they are rarely considered as independent disciplines
anymore. Motions and positions of objects are now more easily determined, and modern astronomy concerns itself much more with
trying to observe and understand the actual physical nature of celestial objects—what makes them "tick".
Ever since the twentieth century the field of professional astronomy has tended to split into observational astronomy and theoretical astrophysics. Although most astronomers
incorporate elements of both into their research, because of the different skills involved, most professional astronomers tend to
specialize in one or the other. Observational astronomy is concerned mostly with acquiring data, which involves building and
maintaining instruments and processing the resulting information; this branch is at times referred to as "astrometry" or simply
as "astronomy". Theoretical astrophysics is concerned mainly with figuring out the observational implications of different
models, and involves working with computer or analytic models.
The fields of study can also be categorized in other ways. Categorization by the region of space under study (e.g., Galactic
astronomy, Planetary Sciences); by subject, such as star formation or cosmology; or by the method used for obtaining
information.
By subject or problem addressed
- Astrometry: the study of the position of objects in the sky and their
changes of position. Defines the system of coordinates used and the kinematics
of objects in our galaxy.
- Cosmology: the study of the origin of the universe and its evolution. The
study of cosmology is theoretical astrophysics at its largest scale.
- Galactic astronomy: the study of the structure and
components of our galaxy and of other galaxies.
- Stellar evolution: the study of the evolution of stars from
their formation to their end as a stellar remnant.
- Star formation: the study of the condition and processes that led
to the formation of stars in the interior of gas clouds, and the process of formation itself.
- Astrobiology: the study of the advent and evolution of biological
systems in the universe.
Also, there are other disciplines that may be considered part of astronomy:
See list of astronomical topics for a more
exhaustive list of astronomy-related pages.
Ways of obtaining information
In astronomy, information is mainly received from the detection and
analysis of electromagnetic radiation and
photons, but information is also carried by cosmic rays, neutrinos, meteors, and, in the near future, gravitational
waves (see LIGO and LISA).
A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:
- Infrared astronomy deals with the detection and analysis of
infrared radiation (wavelengths longer than red light). The most common tool is the telescope but with the instrument optimized for infrared. Space telescopes are also used to eliminate noise (electromagnetic interference) from the atmosphere.
- High-energy astronomy includes X-ray astronomy,
gamma-ray astronomy, and extreme UV astronomy, as well as studies of neutrinos and cosmic rays.
Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is
transparent at the wavelengths being detected. Infrared light is heavily absorbed by water vapor, so infrared observatories have to be located in high, dry places or in space.
The atmosphere is opaque at the wavelengths used by X-ray
astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") Far infrared astronomy, so observations must be carried out
mostly from balloons or space observatories. Powerful gamma rays can, however be
detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.
Short history
In early times, astronomy involved only the observation and predictions of the motions of the naked-eye objects. The Rigveda refers to the 27 constellations associated with the motions of the sun and also the 12 zodiacal divisions of the sky. The ancient
Greeks made important contributions to astronomy, among them the definition of the magnitude system. The Bible contains a number of statements on
the position of the earth in the universe and the nature of the stars and planets, most of which are poetic rather than literal;
see Biblical cosmology. In 500
AD, Aryabhata presented a mathematical system that described the earth as
spinning on its axis and considered the motions of the planets with respect to the sun.
Observational astronomy was mostly stagnant in medieval Europe, but flourished in the Arab world. The late
9th century Islamic astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th
century. In the late 10th century, a huge observatory was built near Tehran, Persia (now Iran), by the astronomer al-Khujandi, who observed a series of meridian transits of the Sun,
which allowed him to calculate the obliquity of the ecliptic. Also in Persia, Omar Khayyam performed a
reformation of the calendar that was more accurate than the Julian and came close to the Gregorian.
During the Renaissance, Copernicus proposed a heliocentric model of
the Solar System. His work was defended, expanded upon, and corrected by
Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the
details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind
the laws he wrote down. It was left to Newton's invention of
celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton
also developed the reflecting telescope.
Stars were found to be faraway objects. With the advent of spectroscopy
it was proved that they were similar to our own sun, but with a wide range of temperatures, masses, and sizes. The existence of our galaxy, the Milky Way, as a separate group of
stars was only proven in the 20th century, along with the existence of
"external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background
radiation, Hubble's Law, and cosmological abundances of elements.
For a more detailed history of astronomy, see the history
of astronomy.
Timelines in astronomy
Astronomy tools
External links
Organizations
References: Formulas and Constants
Other External links
|
