DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

to (1), the ratio of accelerations is twice the ratio
of densities (in the case of the central body, as
if it were spread out to the orbital radius).
Small solid bodies such as rocks or artificial
satellites are held together by molecular bonds
and hence can survive the tidal stresses, but bodies
with little internal strength (such as “rubble
pile” asteroids) are easily pulled apart. The destruction
of bodies with low internal strength by
movement within the Roche limit may explain
the formation of some ring systems. The accretion
of very small satellites into a larger moon
is also inhibited in this region.
Roche lobe (Also inner Lagrangian surface.)
In a binary system, in a corotating frame,
the lowest-lying equipotential surface which envelopes
both members. If one of the stars fills its
Roche lobe, then mass will flow from it towards
the companion star. Stars that form or move
so close together that both stars fill their Roche
lobes are contact binaries or W Ursa Majoris
stars. In the more common case, an evolved star
which has already shed a good deal of matter,
continues to transfer material from the smaller
side of the lobe through a gas stream to the more
massive companion, perhaps through an accretion
disk. If the accretor is a main sequence star,
we would see an Algol system. If it is a white
dwarf, we would see a cataclysmic variable.
Rocket effect (string loop) One way of diminishing
the mass-energy density of a network
of cosmic strings is by the continuous generation
of loops. These will form through string
interactions, like intercommutation.
Loops will oscillate with characteristic frequencies
given by the inverse of their size (in
appropriate units). This allows energy to be radiated
away in the form of gravity waves. These
waves carry not only energy but also momentum
and therefore highly asymmetric loops might
be propelled like rockets, reaching velocities as
high as a tenth of the velocity of light. See cosmic
string, intercommutation (cosmic string),
scaling solution (cosmic string).
Rosalind Moon of Uranus also designated
UXIII. Discovered by Voyager 2 in 1986, it is
a small, irregular body, approximately 27 km
in radius. Its orbit has an eccentricity of 0, an
© 2001 by CRC Press LLC
Rossby wave
inclination of 0.3 ◦ , a precession of 167 ◦ yr −1 ,
and a semimajor axis of 6.99 × 10 4 km. 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 0.559 Earth days.
Rossby, Carl-Gustav (1898–1957)Swedish-
American meteorologist. He founded the first
meteorology department in the U.S. at the Massachusetts
Institute of Technology in 1928. He
was among the first to recognize the important
role of transient mid-latitude wave disturbances
in the atmospheric general circulation. He established
many of the basic principles of modern
meteorology and physical oceanography during
the 1930s. Some of the theoretical ideas that
he developed during that time were instrumental
in the development of numerical prediction
models during the following decades.
Rossby number The Rossby number ɛ is
a non-dimensional parameter defined as the ratio
between the horizontal advection scale and
the Coriolis scale in the horizontal momentum
equation, that is,ɛ= U
fL
where U and L are the
velocity scale and length scale of motion and
f is the Coriolis parameter. When this number
is much less than unity, the flow is said to be
geostrophic.
Rossby radius See Rossby radius of deformation.
Rossby radius of deformation (Rossby,
1938) This is the horizontal scale at which the
Earth’s rotation effects become as important as
buoyancy effects and is defined by LR = C/|f |
where C is the gravity wave speed and f is the
Coriolis parameter. This has been called the
Rossby radius or the radius of deformation. In
baroclinic flow, C is the wave speed of each
baroclinic mode in a non-rotating system. It
describes the radius Ro = u/f[m] of an inertial
circle which a freely moving water parcel
would follow with a speed u under the effect of
the Coriolis parameter f.
Rossby wave A wave in a rotating fluid body,
involving horizontal motion, for which the sole
restoring force is the Coriolis effect. In neutron
stars the retrograde mode, which rotates
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