ATLAS OF THE UNIVERSE
A
Aberration of starlight:The apparent displacement
of a star from its true position in the sky due to the fact
that light has a definite velocity (299,792.5 kilometres
per second). The Earth is moving around the Sun, and
thus the starlight seems to reach it ‘at an angle’. The
apparent positions of stars may be affected by up to
20.5 seconds of arc.Absolute magnitude: The apparent magnitude that
a star would have if it were observed from a standard
distance of 10 parsecs, or 32.6 light-years. The absolute
magnitude of the Sun is 4.8.Absolute zero: The lowest limit of temperature:
273.16 degrees C. This value is used as the starting
point for the Kelvin scale of temperature, so that
absolute zero 0 Kelvin.Absorption of light in space: Space is not
completely empty, as used to be thought. There is
appreciable material spread between the planets,
and there is also material between the stars; the
light from remote objects is therefore absorbed and
reddened. This effect has to be taken into account in
all investigations of very distant objects.Absorption spectrum: A spectrum made up of dark
lines against a bright continuous background. The Sun
has an absorption spectrum; the bright background
or continuous spectrum is due to the Sun’s brilliant
surface (photosphere), while the dark absorption
lines are produced by the solar atmosphere. These
dark lines occur because the atoms in the solar
atmosphere absorb certain characteristic wavelengths
from the continuous spectrum of the photosphere.Acceleration:Rate of change of velocity.
Conventionally, increase of velocity is termed
acceleration; decrease of velocity is termed
deceleration, or negative acceleration.Aerolite: A meteorite whose composition is stony.Aeropause:A term used to denote that region of the
atmosphere where the air-density has become so slight
as to be disregarded for all practical purposes. It has
no sharp boundary, and is merely the transition zone
between ‘atmosphere’ and ‘space’.Airglow:The faint natural luminosity of the night sky
due to reactions in the Earth’s upper atmosphere.Air resistance: Resistance to a moving body caused
by the presence of atmosphere. An artificial satellite
will continue in orbit indefinitely only if its entire orbit
is such that the satellite never enters regions where
air resistance is appreciable.Airy disk: The apparent size of a star’s disk produced
even by a perfect optical system. Since the star can
never be focused perfectly, 84 per cent of the light will
concentrate into a single disk, and 16 per cent into a
system of surrounding rings.Albedo:The reflecting power of a planet or other
non-luminous body. A perfect reflector would have
an albedo of 100 per cent.Altazimuth mount: A telescope mounting in which
the instrument can move freely in both altitudeand
azimuth. Modern computers make it possible to drive
telescopes of this sort effectively, and most new large
telescopes are on altazimuth mountings.Altitude: The angular distance of a celestial body
above the horizon, ranging from 0 degrees at the
horizon to 90 degrees at the zenith.Ångström unit: The unit for measuring the
wavelength of light and other electromagnetic
vibrations. It is equal to 100 millionth part of
a centimetre. Visible light ranges from about
7500 Å (red) down to about 3900 Å (violet).Antenna: A conductor, or system of conductors,
for radiating or receiving radio waves. Systems of
antennae coupled together to increase sensitivity,
or to obtain directional effects, are known as antenna
arrays, or as radio telescopes when used inradio
astronomy.Apastron:The point in the orbit of a binary system
where the stars are at their furthest from each other.
The closest point is known as the periastron.Aphelion: The orbital position of a planet or other
body when it is furthest from the Sun. The closest
point is known as the perihelion.Apogee:The point in the orbit of the Moon or an
artificial satellite at which the body is furthest from
the Earth. The closest point is known as the perigee.Arc, degree of: One 360th part of a full circle (360°).Arc minute, arc second: One 60th part of a degree
of arc. One minute of arc (1’) is in turn divided into
60 seconds of arc (60”).Ashen light: The faint luminosity of the night side of
the planet Venus, seen when Venus is in the crescent
stage. It is probably a genuine phenomenon rather than
a contrast effect, but its cause is not certainly known.Asteroids:The minor planets, most of which move
around the Sun between the orbits of Mars and Jupiter.
Several thousands of asteroids are known; much the
largest is Ceres, whose diameter is 1003 kilometres.
Only one asteroid (Vesta) is ever visible with the
naked eye.Astrology: A pseudo-science which claims to link
the positions of the planets with human destinies.
It has no scientific foundation.Astronomical unit: The distance between the Earth
and the Sun. It is equal to 149,597,900 kilometres,
usually rounded off to 150 million kilometres.Astrophysics: The application of the laws and
principles of physics to all branches of astronomy. It
has often been defined as ‘the physics and chemistry
of the stars’.Atmosphere: The gaseous mantle surrounding a
planet or other body. It can have no definite boundary,but merely thins out until the density is no greater
than that of surrounding space.Atom:The smallest unit of a chemical element which
retains its own particular character. (Of the 92 elements
known to occur naturally, hydrogen is the lightest and
uranium is the heaviest.)Aurorae (polar lights):Aurora Borealis in the
northern hemisphere, Aurora Australis in the southern.
They are glows in the upper atmosphere, due to
charged particles emitted by the Sun. Because the
particles are electrically charged, they tend to be
attracted towards the magnetic poles, so that aurorae
are seen best at high latitudes.Azimuth: The horizontal direction or bearing of a
celestial body, reckoned from the north point of the
observer’s horizon. Because of the Earth’s rotation,
the azimuth of a body is changing all the time.B
Background radiation: Very weak microwave
radiation coming from space, continuously from all
directions and indicating a general temperature of
3 degrees above absolute zero. It is believed to be
the last remnant of the Big Bang, in which the universe
was created about 15,000 million years ago. The
Cosmic Background Explorer satellite (COBE) has
detected slight variations in it.Baily’s Beads: Brilliant points seen along the edge
of the Moon’s disk at a total solar eclipse, just before
totality and again just after totality has ended. They are
due to the Sun’s light shining through valleys between
mountainous regions on the limb of the Moon.Barycentre: The centre of gravity of the Earth–Moon
system. Because the Earth is 81 times more massive
than the Moon, the barycentre lies within the terrestrial
globe.Binary star: A star made up of two components
that are genuinely associated, and are moving around
their common centre of gravity. They are very common.
With some binaries the separations are so small
that the components are almost touching each
other, and cannot be seen separately, although
they can be detected by means of spectroscopy
(see spectroscope). See also eclipsing binary;
spectroscopic binary.Black hole: A region of space surrounding a very
massive collapsed star, or ‘collapsar’, from which
not even light can escape.Bode’s Law: An empirical relationship between the
distances of the planets from the Sun, discovered by
J. D. Titius in 1772 and made famous by J. E. Bode.
The law seems to be fortuitous, and without any real
scientific basis.Bolide:A brilliant meteor, which may explode during
its descent through the Earth’s atmosphere.Bolometer: A very sensitive radiation detector, used
to measure slight quantities of radiation over a very
wide range of wavelengths.Glossary
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