DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

central pit A depression found in the center
of many impact craters. Central pits are particularly
common on bodies where ice is (or believed
to be) a major component of the upper crust,
such as Jupiter’s icy moons of Ganymede and
Callisto and on Mars. The formation of central
pits is not well understood but is believed to be
related to the vaporization of crustal ice during
crater formation.
centrifugal force The conservative force that
arises when Newton’s equations are applied in
a rotating frame; the apparent force acting on a
body of mass m that is rotating in a circle around
a central point when observed from a reference
system that is rotating with the body:
F = m ω × (ω × r)
where ω is the angular velocity vector, and r is
the radius vector from the origin. The magnitude
of the force can also be expressed in terms of the
distance d from the axis and the velocity v in a
circle or orthogonal to the axis: F = mv 2 /d;
the direction of this force is perpendicularly
outward from the axis. These terms appear in
dynamical equations when they are expressed
in a rotating reference system or when they
are expressed in cylindrical or spherical coordinates
rather than Cartesian coordinates. These
terms are generally treated as if they were actual
forces.
centroid moment tensor (CMT) In seismology,
a moment tensor obtained when a point
source is put at a centroid of a source region.
Dziewonski et al. began to determine CMT routinely
in 1981 for earthquakes with Ms ≥ 5.5.
Recently, CMT has been used to estimate seismic
moment and force systems on a source region
of an earthquake, replacing a fault plane
solution determined from polarity of P- and Swaves.
Six independent components of the moment
tensor and four point-source hypocentral
parameters can be determined simultaneously
through iterative inversion of a very long period
(T >40 s) body wave train between the P-wave
arrival and the onset of the fundamental modes
and mantle waves (T > 135 s). Not assuming
a deviatoric source, some focal mechanisms
have been found to have large non-double couple
components.
© 2001 by CRC Press LLC
Čerenkov radiation
Cepheid variable A giant or supergiant star
crossing the instability strip in the HR diagram
at spectral type F-G. The stars can be crossing
either from blue to red as they first leave the
main sequence or from red to blue in later evolutionary
phases. In either case, the stars are
unstable to radial pulsation because hydrogen
at their surfaces is partially ionized on average
and acts like a tap or faucet that turns the flow of
outward radiation up or down, depending on its
exact temperature. As a result, the stars change
their size, brightness, and color (temperature) in
very regular, periodic patterns 1 to 50 days, and
with a change of 0.5 to 1 magnitude. Because
the counterbalancing force is gravity, the pulsation
period, P , is roughly equal to (Gρ) −1/2 ,
where ρ is the star’s average density and G is
Newton’s constant of gravity. This, in turn,
means that there is a relationship between period,
luminosity, and color for whole populations
of Cepheids. Thus, if you measure the
period and color of a Cepheid, you know its real
brightness and can, in turn, learn its distance
from its apparent brightness. Cepheids can be
singled out only in relatively nearby galaxies,
and with the Hubble Space Telescope, they have
been measured out to galaxies in the Virgo Cluster.
Classical Cepheids are population I stars
that are massive young objects. The W Virginis
Cepheids belong to the older population II stars.
The prototype star, δ Cephei, was discovered
to vary in 1784. This period-luminosity relation
was originally discovered for the Cepheids
in the Large Magellanic Cloud by Henrietta S.
Leavitt in the first decade of the 20th century.
Once an independent measure of the distance
to a few nearby Cepheids was accomplished
(through spectroscopy and other methods), a
period-luminosity relation was defined. The calibration
of the period-luminosity relation is the
underpinning of our distance measurements to
the nearest galaxies (within 400 Mpc), which
then calibrates the Hubble constant, the proportionality
constant that relates red shift of distant
galaxies to the expansion of the universe.
Čerenkov radiation The Čerenkov effect (in
Russia: Vavilov–Čerenkov effect ∗ ) was first observed
with the naked eye by P.A. Čerenkov in
1934 as a “feeble visible radiation” from fast
electrons (Vavilov’s interpretation, 1934) due to