DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

one instant. The crater produced would be about
10 km across. This event would raise matter
into the atmosphere that would cause dramatic
surface cooling by blocking sunlight for at least
several years.
There are 240 known Apollos. See Amor
asteroid, Aten asteroid.
apparent horizon A spacelike topological
2-sphere from which the outgoing null rays all
have zero expansion. In gravitational theories,
especially in general relativity, a horizon is a
boundary between events visible from infinity
and those that are not. The surface of a black
hole, for instance, consists of those marginally
trapped rays (which just fail escape to infinity);
these constitute the event horizon. Generators
of the event horizon are not truly identified until
the evolution of the spacetime is complete into
the future. A more local definition is the apparent
horizon, the outermost surface defined
by the null rays which instantaneously are not
expanding. See event horizon, trapped surface.
apparent magnitude See magnitude.
apparent optical property (AOP) A ratio
of radiometric quantities that depends both on
the inherent optical properties and on the directional
nature of the ambient light field and
which is spatially and temporally stable. Applied
in oceanography to describe a water body;
examples include the average cosine of the light
field, the irradiance reflectance, the remote sensing
reflectance, and the diffuse attenuation coefficients.
apparent solar time Time based on the diurnal
motion of the true (observed) sun, as opposed
to mean solar time, to which it is related
by the equation of time. The rate of diurnal motion
undergoes seasonal variations because of
the obliquity of the ecliptic, the eccentricity of
the Earth’s orbit, and irregularities in the Earth’s
orbit.
apse Line connecting the pericenter to the
apocenter of an orbit, the longest axis of the
orbit.
© 2001 by CRC Press LLC
arcade
Ap star A chemically peculiar star of temperature
classification A, which is a slow rotator
and has a strong gravitational field. Ap stars
have a pattern of overabundance including silicon,
chromium, strontium, and europium and
other rare earths. Their magentic fields are measured
by the polarization induced in their spectral
lines by the Zeeman effect; the fields have
been measured up to 34000 Gauss (compared to
≈ 1G for the sun). Present understanding is that
the slow rotation and the magnetic field together
suppress convection to allow chemical segregation
and enhancement in the surface layers of
the stars.
aquifer A highly pervious geological formation,
empirically defined as a geologic formation
saturated with water and sufficiently permeable
to transmit “significant” quantities of water under
normal field conditions. On land, water enters
an aquifer through precipitation or influent
streams and leaves an aquifer through springs
or effluent streams. An unconfined aquifer is a
geologic formation in which the upper boundary
of the saturated zone is the water table. A
confined aquifer is an aquifer that is overlain
by a confining bed with significantly lower hydraulic
conductivity (an aquitard); water in a
well or piezometer within a confined aquifer
will rise above the top of the confined aquifer to
the potentiometric surface. A perched aquifer
is a region in the unsaturated zone that may be
temporarily saturated because it overlies an area
with lower hydraulic conductivity such as an
aquitard or aquiclude.
aquitard A semipervious geological formation
that transmits water very slowly as compared
to an aquifer.
Arago point One of three points on the sky
in a vertical line through the sun at which the polarization
of skylight vanishes. Usually located
at about 20 ◦ above the antisolar point (the point
opposite the sun on the sky). See Babinet point,
Brewster point.
arcade A configuration of coronal loops
spanning a magnetic neutral line. The loops are
often perpendicular to the neutral line but can be
sheared due to the forces of differential rotation.