DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

a semimajor axis of 5.38 × 10 4 km. It is the
outer shepherding satellite for Uranus’ epsilon
ring. Its surface is very dark, with a geometric
albedo of less than 0.1. Its mass has not been
measured. It orbits Uranus once every 0.376
Earth days.
ophiolites In general, the mantle of the Earth
lies beneath the crust. In some cases the more
dense and basic mantle rocks have been brought
to the surface by mountain building processes.
These are ophiolites. Important ophiolites occur
in Cyprus, Yemen, and other places.
Oppenheimer–Snyder (OS) model The
simplest general relativistic model of gravitational
collapse. It consists of a sphere of dust
(perfectfluidwithoutpressure)invacuumwhich
collapses under its own weight to a point-like
singularity (see Schwarzschild metric). Since
there are no forces other than the self-gravity
of dust, the surface of the sphere moves along
a geodesic on the external Schwarzschild metric
with an ADM mass given by the matching
conditions with the internal Robertson–Walker
comoving metric. Such mass equals the proper
energy of dust only if the surface of the sphere
starts collapsing from rest with infinite radius,
otherwise it is either bigger or smaller than the
rest mass depending on the velocity of the surface
at a given radius.
The absence of internal pressure makes the
OS model unrealistic for modeling the collapse
of a star, but does show the qualitatively expected
features. See ADM mass, black hole,
comoving frame, Robertson–Walker cosmological
models.
opposition Orientation of planets so that the
angle planet-Earth-sun equals 180 ◦ . For outer
planets, the planets are as close to the Earth as
possible in their orbits. Opposition is geometrically
impossible for inner planets.
optical closure in measurement Making
consistent measurements; examples include independently
verifying that the sum of the absorption
coefficient plus the scattering coefficient
equals the beam attenuation coefficient and
that the integral over all directions of the volume
© 2001 by CRC Press LLC
Optimum Working Frequency (OWF)
scattering function equals the scattering coefficient.
optical closure in models Development of
consistent analytical and/or numerical models
that make predictions consistent with observations.
optical closure in scale Making the transition
from small scale (e.g., single particle) data
to bulk scale data in a manner consistent with
observations.
optical constants The nonnegative real and
imaginary parts of the refractive index; the constants
often strongly depend on frequency.
optical density See absorbance.
optical depth See optical distance.
optical distance The integral of the dimensionless
product of the beam attenuation coefficient
[m −1 ] multiplied by an infinitesimal unit
of distance [m] along the direction of travel.
optical double star A pair of stars very close
to each other in the sky but at different distances,
so that they do not really make up a binary star
system. See binary star system.
optical oceanography The subdiscipline of
oceanography concerned with the propagation
and interaction of radiation, typically at wavelengths
between ∼350 and ∼750 nm with seawater.
Optimum Working Frequency (OWF) The
frequency that is exceeded by the operational
MUF, at a given time, between a specified pair
of terminals via any available propagation mode,
during 90% of a specified period, usually a
month. The operational OWF is an anticipated
performance parameter derived, possibly, from
an HF prediction program. Unlike the operational
MUF, the OWF is based on known statistical
properties of the ionosphere and offers a
safe estimate of a reliable frequency for a propagation
path. When planning HF services, the
OWF is often used. However, by its nature,
90% of the time it is possible to find frequencies
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