DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

O
obduction In tectonic activity, the process in
which part of a subducted plate is pushed up onto
the overriding plate. The process responsible for
the emplacement of ophiolites.
Oberon Moon of Uranus, also designated
UIV. It was discovered by Herschel in 1787. Its
orbit has an eccentricity of 0.0008, an inclination
of 0.10 ◦ , a semimajor axis of 5.83×10 5 km, and
a precession of 1.4 ◦ yr −1 . Its radius is 762 km,
its mass is 3.03 × 10 21 kg, and its density is
1.54 g cm −3 . Its geometric albedo is 0.24, and
it orbits Uranus once every 13.46 Earth days.
objective grating A course grating, often
consisting of a parallel array of wires, which is
placed in front of the first element of a telescope
(the objective) which produces a low dispersion
spectrogram of each object in the field; used for
rapid survey of the properties of sources in a
relatively wide region of sky.
objective prism A prism of narrow apex angle
(a few degrees) located at the objective of
a telescope, most often of a Schmidt telescope.
The objective prism, acting as a disperser, provides
the spectrum of each object in the field of
the telescope. A large number of low resolution
spectra (up to ∼ 10 5 ) can be recorded on
a plate or electronic detector. Surveys based on
objective prism spectroscopy have been very efficient
in finding objects with peculiar spectral
energy distribution, such as galaxies with UV
excess, or objects with very strong and broad
emission lines, such as quasars. Spectral resolving
power(λ/λ) achieved in common usage
are ∼ 100. Higher spectral resolving power
can be achieved with wider apex angle prism.
oblateness For an oblate spheroid, the ratio
of the difference between equatorial and polar
radii to the equatorial radius. In general,
the oblateness ɛ of a rotating inhomogeneous
spheroid with north-south symmetry depends on
© 2001 by CRC Press LLC
oblique ionogram
the rotation parameter m = 2 R 3 /GM and the
zonal harmonic coefficients Jl(l = 2, 4, 6, ...)
in a complicated manner. is the rotational angular
speed, R is the equatorial radius, G is the
gravitational constant, M is the total mass of the
spheroid, and Jl =−(1/M) dr(r/R) l ρ(r,θ)
Pl(cos θ)where the mass density of the spheroid
ρ depends only on the polar radius r and the polar
angle θ measured from the north pole, and Pl
is the l-th order Legendre polynomial. In planetary
physics, analyses of velocity variations during
the spacecraft fly-bys at major gaseous giant
planets have permitted us to determine their
first few zonal harmonic coefficients to a relatively
high accuracy, which has in turn provided
us with their dynamical oblatenesses. To
the first order in J2 (quadrupole moment, i.e.,
anisotropy in moments of inertia), there is a simple
relation oblateness ɛ = (3J2 + m)/2, which
shows that the oblateness is a critical indicator of
how fast a body is rotating. For a model planet
consisting of a small dense core of mass Mc
clad with a homogeneous spheroidal envelope
of mass Me = M − Mc and equatorial radius
R, J2/m = (1 − δc)/(2 + 3δc) and hence we
find ɛ/m = 5/(4 + 6δc) where δc = Mc/M is
the fraction of core mass. Thus, with the total
mass M, the equatorial radius R, and the rotation
rate all being equal, a homogeneous
spheroid without a core is more oblate than an
inhomogeneous spheroid containing a core.
oblique ionogram The conventional display
obtained from an oblique ionosonde, containing
information about the ionosphere. The synchronized
transmitter and receiver for an oblique
ionosonde are separated each at the terminal of
an HF link. An oblique ionogram is constructed
by displaying the received signal as a function
of frequency (on the horizontal axis) and time
delay (on the vertical axis). The structure of
the oblique ionogram is highly dependent on the
path length between the receiver and transmitter.
For short paths, less than 100 km, the oblique
ionogram will look similar to a vertical ionogram.
However, as the path length increases, the
F modes will gradually become more obvious.
For frequencies below the maximum usable frequency
(MUF), for a single mode, the shape of
an oblique ionogram shows that two paths are
possible. These two paths are for a high and