DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

astronomical refraction
such as mountains, lakes, and large ore deposits,
which cause the plumb line to deviate slightly
from the normal to the ellipsoid.
astronomical refraction The apparent angular
displacement toward the zenith in the position
of a celestial body, due to the fact that
the atmosphere over any observer is apparently
a planar slab with density decreasing upward.
The effect vanishes overhead and is largest near
the horizon, where it becomes as much as 30 ′ .
The fact that the sun is refracted to appear above
its true angular position contributes measurably
to the length of the apparent day. Also called
atmospheric scintillation.
astronomical scintillation Any irregular
scintillation such as motion, time dependent
chromatic refraction, defocusing, etc. of an image
of a celestial body, produced by irregularities
in the Earth’s atmosphere. The effects
have periods of 0.1 to 10 sec and are apparently
caused by atmospheric irregularities in the centimeter
to decimeter and meter ranges, within
the first 100 m of the telescope aperture.
astronomical tide Fluctuations in mean water
level (averaged over a time scale of minutes)
that arise due to the gravitational interaction of
(primarily) the earth, moon, and sun. May also
be used to refer to the resulting currents.
astronomical twilight See twilight.
astronomical unit (AU) The mean distance
between the sun and the Earth (1.4959787 ×
10 8 km). This is the baseline used for trigonometric
parallax observations of distances to
other stars.
astronomy, infrared The observation of astronomical
objects at infrared (IR) wavelengths,
approximately in the range from 1 to 200 µm,
that provide information on atomic motions that
cause changes in charge distribution. The midinfrared
spans approximately the range from 2.5
to 25 µm and includes fundamental transitions
for bond stretching and bending of most interstellar
molecules. Longer and shorter wavelengths,
known as the far and near IR, respectively,
correspond to low frequency motions of
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groups of atoms and overtones of far and mid-IR
features.
astronomy, infrared: interstellar grains,
comets, satellites, and asteroids Absorption,
reflection, and emission at infrared (IR)
wavelengths provide astronomers with unique
molecular information for molecules not visible
atotherwavelengths, suchasradio, becausethey
lack a permanent dipole moment, or are solids,
such as ices on interstellar grains or solar system
bodies. IR spectroscopy of these solid materials,
measured in absorption and reflection, respectively,
have supplied most remotely measured
information about the mineralogy and chemical
composition of interstellar grains and solar system
surfaces. Most spectra of outer solar system
bodies have been measured in reflected sunlight
in the near IR because solar radiation diminishes
with increasing wavelength so they are dark in
the mid-IR.
astronomy, ultraviolet: interstellar The
observation of astronomical objects and phenomena
at ultraviolet (UV) wavelengths, approximately
in the range from 100 to 4000 Å,
provide information on the electronic transitions
of materials, molecules, and reactive species.
UV absorption of interstellar materials
have helped to put constraints on the form and
distribution of most carbon bearing species in
thegalaxy. Seediffuseinterstellarbands(DIBs).
asymmetry factor In scattering, the mean
cosine of the scattering angle.
asymmetry parameter Asymmetry factor.
asymptotic The (normalized) angular shape
of the radiance distribution at depths far from the
boundary of a homogeneous medium; the directional
and depth dependencies of the asymptotic
radiance distribution decouple and all radiometric
variables (e.g., irradiances) vary spatially at
the same rate as the radiance, as governed by
the inherent optical properties only. See diffuse
attenuation coefficient.
asymptotically simple space-time A spacetime
(M,g)is said to be asymptotically simple