DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Ångström (Å) A unit of length used in spectroscopy,
crystallography, and molecular structure,
equal to 10 −10 m.
angular diameter distance Distance of a
galaxy or any extended astronomical object estimated
by comparing its physical size to the angle
subtended in the sky: ifD is the diameter of
the galaxy and δ the angle measured in the sky,
then dA =D/tanδD/δ. For a Friedmann
model with density o in units of the critical
density, and zero cosmological constant, the angular
diameter dA of an object at redshift z can
be given in closed form:
dA =
2 o
2cH −1
o
(1 +z)2
oz+(o − 2)
(oz+ 1) 1/2
− 1 .
Other operational definitions of distance can be
made (see luminosity distance) depending on
the intrinsic (assumed to be known) and the observed
properties to be compared.
angular momentum L = r × p, where ×
indicates the vector cross product, r is the radius
vector from an origin to the particle, and p is the
momentum of the particle. L is a pseudovector
whose direction is given by r, p via the righthand
rule, and whose magnitude is
|L| =|r||p| sinθ,
whereθ is the angle between r and p. For a body
or system of particles, the total angular momentum
is the vectorial sum of all its particles. In
this case the position is generally measured from
the center of mass of the given body. See pseudovector,
right-hand rule, vector cross product.
angular velocity (ω) The angle through
which a body rotates per unit time; a pseudovector
with direction along the axis given by the
right-hand rule from the rotation.
anisotropic A material whose properties
(such as intrinsic permeability) vary according
to the direction of flow.
anisotropic scattering Scattering that is not
spherically symmetric.
© 2001 by CRC Press LLC
anomalistic month
anisotropic turbulence See isotropic turbulence.
anisotropic universe A universe that expands
at different rates in different directions.
The simplest example is Kasner’s model (1921)
which describes a space that has an ellipsoidal
rate of expansion at any moment in time. Moreover,
the degree of ellipticity changes with time.
The generic Kasner universe expands only along
two perpendicular axes and contracts along the
third axis.
anisotropy The opposite of isotropy (invariance
under rotation), i.e., variation of properties
under rotation. For example, if a rock has a fabric
such as layering with a particular orientation,
then phases of seismic waves may travel at different
speeds in different directions through the
rock, according to their alignment with the fabric.
The wave speed along an axis varies when
the axis is rotated through the rock with respect
to the fabric, i.e., it is anisotropic. In terms of
the material properties of the rock, this would
be associated with an elasticity tensor that varies
under rotation. This occurs in the real Earth: for
example, wave speeds are observed to be faster
in the upper mantle under the ocean in the direction
perpendicular to the mid-ocean ridges.
The Earth’s inner core has been determined to
be anisotropic, with (to a first approximation)
faster wave speeds parallel to the Earth’s rotation
axis than in directions perpendicular to
it. Many other physical properties may also be
anisotropic, such as magnetic susceptibility, diffusivity,
and turbulence.
annual flood The maximum discharge peak
flow during a given water year (October 1
through September 30) or annual year.
annular eclipse A solar eclipse in which the
angular size of the moon is slightly too small
to obscure the entire solar photosphere. As a
result, a ring (“annulus”) of visible photosphere
surrounds the dark central shadow of the moon.
Annular eclipse occurs when the moon is near
apogee, giving it a smaller angular size.
anomalistic month See month.