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Astronomy & Astrophysics
Astronomy
Astronomy, which
etymologically means "law
of the
stars",
is a
science involving the
observation and explanation of
events occurring outside
Earth and its
atmosphere. Astronomy is often associated with
astrophysics.
Lunar astronomy: the far side of Earth's Moon. The
large impact basin pictured is Crater 308. It spans about 30 kilometers (19
miles) and was photographed by the crew of Apollo 11 as they circled the Moon
in 1969.
Astronomy is one of the few sciences where
amateurs still play an active role, especially in the discovery and
monitoring of transient
phenomena. This is not to be confused with
astrology, a
pseudoscience which 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; astronomy embraces the
scientific method, while astrology, with no basis in science, does not.
Given its huge scope, astronomy is divided into different
branches. A first main distinction is between theoretical and
observational astronomy. Observers use a variety of means to
obtain data about different phenomena, data that is then used by theorists
to create and constrain theories and models, to explain observations and to
predict new ones. 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).
Planetary astronomy: a Martian dust devil.
Photographed by the NASA Global Surveyor in Mars orbit, the long dark streak
is formed by the movement of a swirling column of Martian atmosphere (with
similarities to a terrestrial tornado). The dust devil (the black spot) is
climbing the crater wall. Dust devils occur when the atmosphere is heated by a
warm surface and begins to spin as it rises. The streaks in the right hand
half of the picture are sand dunes on the crater floor.
In astronomy, the main way of obtaining information is
through the detection and analysis of
electromagnetic radiation,
photons, but information is also carried by
cosmic rays,
neutrinos, 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:
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Optical astronomy refers to the techniques used to detect and analyze
light in and slightly around the
wavelengths than can be detected with the
eyes
(about 400 - 800 nm). The most common tool is the
telescope, with
electronic imagers and
spectrographs.
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Infrared astronomy deals with the detection 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.
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Radio astronomy uses completely different instruments to detect
radiation of wavelengths of mm to cm. The receivers are similar to those
used in
radio broadcast transmission (which uses those wavelengths of radiation).
See also
Radio telescopes.
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High-energy astronomy
Galactic astronomy: gravitational lensing. This
Hubble Space Telescope image shows several blue, loop-shaped objects that
actually are multiple images of the same galaxy. They have been duplicated by
the gravitational lens effect of the cluster of yellow, elliptical and spiral
galaxies near the photograph's center. The gravitational lens is produced by
the cluster's tremendous gravitational field that bends light to magnify,
brighten 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 , and so observations can be carried out only from
balloons or
space observatories.
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.
The study of astronomy almost stopped during the middle ages,
except for the work of
Arabic astronomers. In the late 9th century the Islamic astronomer al-Farghani
(Abu'l-Abbas Ahmad ibn Muhammad ibn Kathir al-Farghani) wrote extensively on the
motion of celestial bodies. In the 12th century, his works were translated into
Latin, and it is said that Dante got his astronomical knowledge from al-Farghani's
books.
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, also known as
the tilt of the Earth's axis relative to the Sun. As we know today, the Earth's
tilt is approximately 23o34', and al-Khujandi measured it as being 23o32'19".
Using this information, he also compiled a list of
latitudes and
longitudes of major cities.
Omar Khayyam (Ghiyath al-Din Abu'l-Fath Umar ibn Ibrahim al-Nisaburi al-Khayyami)
was a great
Persian scientist, philosopher, and poet who lived from
1048-1131.
He compiled many astronomical tables and performed a reformation of the
calendar which was more accurate than the
Julian and came close to the
Gregorian. An amazing feat was his calculation of the year to be
365.24219858156 days long, which is accurate to the 6th decimal place.
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 which described
correctly the details of the motion of the planets with the Sun at the center.
However, Kepler did not understand the reasons 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 far away 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.
Stellar astronomy: The Ant planetary nebula. The
ejection of gas, from the dying star at the center, has intriguing symmetrical
patterns unlike the chaotic patterns expected from an ordinary explosion.
Scientists using Hubble would like to understand how a spherical star can
produce such prominent symmetries in the gas that it ejects.
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