DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

anabatic wind
layers of the star, the currently accepted explanation
for their strange appearance.
anabatic wind A wind that is created by air
flowing uphill, caused by the day heating of the
mountain tops or of a valley slope. The opposite
of a katabatic wind.
analemma The pattern traced out by the position
of the sun on successive days at the same
local time each day. Because the sun is more
northerly in the Northern summer than in Northern
winter, the pattern is elongated North-South.
It is also elongated East-West by the fact that
civil time is based on the mean solar day. However,
because the Earth’s orbit is elliptical, the
true position of the sun advances or lags behind
the expected (mean) position. Hence, the
pattern made in the sky resembles a figure “8”,
with the crossing point of the “8” occurring near,
but not at, the equinoxes. The sun’s position is
“early” in November and May, “late” in January
and August. The relation of the true to mean motion
of the sun is called the equation of time. See
equation of time, mean solar day.
Ananke Moon of Jupiter, also designated
JXII. Discovered by S. Nicholson in 1951, its
orbit has an eccentricity of 0.169, an inclination
of 147 ◦ , and a semimajor axis of 2.12×10 7 km.
Its radius is approximately 15 km, its mass,
3.8 × 10 16 kg, and its density 2.7 g cm −3 . Its
geometric albedo is not well determined, and it
orbits Jupiter (retrograde) once every 631 Earth
days.
Andromeda galaxy Spiral galaxy (Messier
object M31), the nearest large neighbor galaxy,
approximately 750 kpc distant, centered at RA
00 h 42.7 m , dec +41 ◦ 16 ′ , Visual magnitude 3.4 ,
angular size approximately 3 ◦ by 1 ◦ .
anelastic deformation Solids creep when a
sufficiently high stress is applied, and the strain
is a function of time. Generally, the response of
a solid to a stress can be split into two parts: elastic
part or instantaneous part, and anelastic part
or time-dependent part. The strain contributed
by the anelastic part is called anelastic deformation.
Part of the anelastic deformation can be
recovered with time after the stress is removed
© 2001 by CRC Press LLC
(retardation strain), and part of it becomes permanent
strain (inelastic strain). Anelastic deformation
is usually controlled by stress, pressure,
temperature, and the defect nature of solids.
Two examples of anelastic deformation are the
attenuation of seismic waves with distance and
the post-glacial rebound.
anemometer An instrument that measures
windspeed and direction. Rotation anemometers
use rotating cups, or occasionally propellers,
and indicate wind speed by measuring
rotation rate. Pressure-type anemometers include
devices in which the angle to the vertical
made by a suspended plane in the windstream
is an indication of the velocity. Hot wire
anemometers use the efficiency of convective
cooling to measure wind speed by detecting temperature
differences between wires placed in the
wind and shielded from the wind. Ultrasonic
anemometers detect the phase shifting of sound
reflected from moving air molecules, and a similar
principle applies to laser anemometers which
measure infrared light reemitted from moving
air molecules.
angle of repose The maximum angle at
which a pile of a given sediment can rest. Typically
denoted by φ in geotechnical and sediment
transport studies.
angle-redshift test A procedure to determine
the curvature of the universe by measuring the
angle subtended by galaxies of approximately
equal size as a function of redshift. A galaxy of
size D, placed at redshift z will subtend an angle
θ = D2o (1 + z)2
2cH −1
o
oz + (o − 2) (oz + 1) 1/2 −1 − 1 ,
in a universe with mean density o and no cosmological
constant. In models with cosmological
constant, the angle also varies in a defined
manner but cannot be expressed in a closed form.
However, since galaxies are not “standard rods”
and evolve with redshift, this test has not been
successful in determining cosmological parameters.