Astronomy
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.
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
-
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:
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.
Timelines in astronomy
See also
Astronomy tools
External links
Organizations
References: Formulas and Constants
External links
