DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

idge push
ridge push See ridge slide.
ridge slide (ridge push) A mid-ocean ridge
is elevated compared to the surrounding ocean
basin. The oceanic plate is driven to move away
from the ridge by the component of gravitational
force parallel to the plate in the down slope direction.
This plate driving force is called ridge
slide or ridge push. See seafloor spreading.
Riemann-Christoffel tensor See Riemann
tensor.
Riemann tensor In geometry, the curvature
tensor of an affine space. In terms of a 1-form
basis {ωa} and its second covariant derivatives,
the Riemann tensor Rabc d is defined as
∇a∇bωc −∇b∇aωc =−Rabc d ωd.
In a coordinate basis, the componentsR a bcd may
be written as
R a bcd =Ɣa bd,c −Ɣa bc,d +Ɣa fc Ɣf
bd −Ɣa fd Ɣf
cb
where Ɣ a bc
is the connection, and the “,” de-
notes a partial derivative. Note that when Ɣ a bc
is symmetric (no torsion) Rabcd =−Rabdc and
Rabcd =Rcdab. See affine connection, covariant
derivative, sign convention.
Rigel (β Orionis), Spectral type B, RA 05 h
14 m 32 s .3, dec −08 ◦ 12 ′ 06 ′′ . The brightest star
in the constellation Orion and the seventh brightest
star in the sky (apparent magnitude +0.12).
Rigel is a blue (very hot) supergiant (absolute
magnitude -8.1), about 100 times bigger in diameter
than the sun. It is approximately 430 pc
distant.
right ascension An angle coordinate on the
celestial sphere corresponding to longitude, and
measured in hours thus dividing the circle of
360 ◦ into 24 hours, measured eastward from the
location of the vernal equinox (also called the
first point of Aries). At the time of the vernal
equinox the right ascension of the sun is 0
hours. The direction to the sun at this time is
then 0 h RA, 0 ◦ declination, and this direction
on the celestial sphere is referred to as the Vernal
Equinox.
© 2001 by CRC Press LLC
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right-hand coordinate system One in which
ˆz=ˆx׈y in which {ˆx,ˆy,ˆz} are the unit vectors
in the coordinate directions.
right-hand rule A prescription to determine
the direction in a right-hand coordinate system,
of a vector cross product A × B: right-hand fingers
in the direction of A, curl fingers towards
B; thumb points in the direction of product.
right-lateral strike-slip fault In geophysics,
a strike-slip fault such that an observer on one
side of the fault sees the other side moving to
the right. The San Andreas fault in California is
a right-lateral strike-slip fault.
rigidity See shear modulus.
rigid lid approximation A boundary condition
applied when the surface displacements are
small compared with interface displacements.
In the continuity equation of flow motion, one
neglects the time change in the surface displacements,
but retains the time change in the interface
displacements.
rille Channel formed by flowing lava. Rilles
are old lava tubes where the roof has collapsed
along part or all of the channel length. Lava
tubes (hence rilles) are formed by very low viscosity
lavas. On the moon, rilles have widths
from a few tens of meters to a few kilometers
and lengths of hundreds of kilometers. Longer
channels are seen on Venus, although the composition
of the flows that created these channels
is still debated. Rilles typically are widest at
the upstream end and narrow as they progress
downstream. Rilles show no delta-like deposits
at their termini — they typically just fade away
into a lava plain.
Rindler observer A uniformly accelerated
observer in flat Minkowski spacetime. The observer’s
4-dimensional trajectory is given by
x = ca −1 e aξ cosh(aτ/c)
t = c 2 a −1 e aξ sinh(aτ/c) ,
with y,z, constant. Here x,y,z are Cartesian
coordinates (x along the direction of motion), t
is the time of an inertial observer, τ is the observer’s
proper time, ξ is a parameter related to