|
Astronomy, which etymologically means "law of the stars", (from Greek: αστρονομία =
άστρον + νόμος) is a science involving the observation and
explanation of events occurring outside Earth and
its atmosphere. It studies the origins, evolution, physical
and chemical properties of objects that can be observed in the sky (and are outside the earth), as well as the processes
involving them.
Lunar astronomy: the large crater is Daedalus, photographed by the
crew of Apollo 11 as they circled the Moon
in 1969. Located near the center of the far side of Earth's Moon, its diameter is about 93 kilometers (58 miles). Astronomy is one of the few sciences where amateurs still play
an active role, especially in the discovery and monitoring of transient phenomena. Astronomy is not to be confused with astrology, a
pseudoscience that attempts to predict a person's destiny by tracking the
paths of astronomical objects. 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 its earliest days, going back to ancient Greece and other ancient
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 easily known, 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 getting data, which involves building and
maintaining instruments and processing the resulting data; 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 are also categorized in another two ways: by "subject", usually according to the region of space (e.g.
Galactic astronomy) or "problems addressed" (such as star formation or cosmology); or by the way used for obtaining
information.
By subject or problem addressed
Planetary astronomy, or Planetary Sciences: a dust devil on Mars. Photographed by Mars Global Surveyor, the long dark streak is formed by a moving swirling column of Martian atmosphere
(with similarities to a terrestrial tornado). The dust devil itself (the black spot) is climbing the crater wall. The streaks on
the right are sand dunes on the crater floor.
- Astrobiology: the study of the advent and evolution of biological
systems in the universe.
- 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 universe as a whole and its evolution.
- Galactic astronomy: the study of the structure and
components of our galaxy and of other galaxies.
- Extragalactic astronomy: the study of objects
(mainly galaxies) outside our galaxy.
- Galaxy formation and evolution: the
study of the formation of the galaxies, and their evolution.
- Planetary Sciences: the study of the planets of the solar system.
- Stellar astronomy: the study of the stars.
- 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.
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, 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:
Extragalactic astronomy: gravitational lensing. This
image shows several blue, loop-shaped objects that are multiple images of the same galaxy. They have been duplicated by the
gravitational lens effect of the cluster of yellow galaxies near the photograph's center. The lens is produced by the cluster's
gravitational field that bends light to magnify and distort the image of a more distant object. Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is
transparent at those wavelengths. 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 can be carried out
only from balloons or space observatories.
Short history
In the early part of its history, astronomy involved only the observation and predictions of the motions of the objects in the
sky that could be seen with the naked eye. 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 took the earth to spin on its axis and considered the motions of the planets with respect to the sun.
Astronomy was mostly stagnant in medieval Europe, but flourished meanwhile in the Arab world. The late 9th century Islamic astronomer al-Farghani (Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir 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, 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. In Persia, Omar Khayyam (Ghiyath
al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi al-Khayyami) compiled many tables and 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. 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.
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.
Stellar astronomy, Stellar Evolution: The Ant planetary nebula. The ejection of gas, from the
dying star at the center, has symmetrical patterns unlike the chaotic patterns expected from an ordinary explosion.
See also
Astronomy tools
External links
Organizations
References: Formulas and Constants
External Links
|