DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Čerenkov radiation
γ -rays in pure water. In 1937 Čerenkov confirmed
the theoretical prediction by Frank and
Tamm about the sharp angular dependence of
this radiation: Frank and Tamm earlier in the
same year had given a complete classical explanation
of the Čerenkov effect. Its quantum
theory was given by Ginsburg in 1940. In 1958
Čerenkov, Tamm, and Frank were awarded the
Nobel Prize.
Čerenkov radiation occurs when a charged
particle is moving in a transparent medium
with a velocity greater than that of light in this
medium. This radiation is emitted with a conical
front, the direction of motion of the particle
being its axis making the angle θ with the cone’s
generatrix, while cos θ = v/u. Here v is the velocity
of light in the medium and u, the velocity
of the charged particle (u >v). This picture
is very similar to that of waves on the surface
of water when a boat is moving faster than the
waves propagate. In the theory of Čerenkov’s
radiation, an important role is played by the
medium’s dispersion (the frequency dependence
of the electric and magnetic properties of the
medium, hence of its refraction index).
In fact, the classical theory of what we now
call Čerenkov radiation, as well as of some similar
phenomena involving the superluminal (in
a vacuum or in refractive media), was developed
as early as 1888 by Heaviside: the first
publication in the Electrician, with subsequent
publications in 1889 and 1892, and especially
in 1912 (comprising more than 200 pages of the
third volume of his Electromagnetic Theory).
In 1904 and 1905 Sommerfeld published four
papers on the same effect, though only in a vacuum;
there he mentioned, at least once, the previous
work of Heaviside, as did Th. des Coudres
in 1900. Subsequently this early development
was lost until 1979, when the references were
again cited.
The Čerenkov effect is the basis of Čerenkov
counters of ultrarelativistic charged particles
which essentially consist of a (usually Plexiglas)
transparent dielectric cylinder and some
∗ Čerenkov then was a student of S.I. Vavilov, performing a
research in fluorescence as a part of his Ph.D. thesis under
Vavilov’s supervision.
© 2001 by CRC Press LLC
photomultipliers to detect the Čerenkov radiation
emitted in the dielectric by these particles.
Ceres The first observed asteroid, discovered
by Giuseppe Piazzi in 1801. It has a diameter
of 913 km. It orbits in the main asteroid belt.
Its average distance to the sun agrees with the
distance of the “missing planet” predicted by
Bode’s Law.
CGS (Centimeter-Gram-Second) The system
of measurement that uses these units for
distance, mass, and time. This system incorporates
the use of electrostatic units (esu) or
electromagnetic units (emu) in the description
of electromagnetic phenomena. The magnetic
permeability (µ) and dielectric constant (ɛ) are
dimensionless in the CGS system.
chalcophile Elements that display an affinity
for sulfur are called chalcophile elements. Such
elements are readily soluble in iron monosulfide
melts and tend to be found concentrated in
sulfide ores.
Challenger Deep Deepest part of the Marianas
Trench, deepest ocean water. Located
at approximately 142 ◦ 15 ′ E,11 ◦ 20 ′ N off the
coast of Guam. Bottom depth approximately
11,000 m (36,100 ft); various “deepest” measurements
range within 100 m of this value.
Chandler wobble Although the Earth’s pole
of rotation (i.e., the axis about which the Earth
is spinning) lies close to the axis of the solid
Earth’s largest principle moment of inertia, they
are generally not quite coincident because the
solid Earth has a small amount of angular momentum
L oriented perpendicular to the axis
of the largest principle moment of inertia. There
is a small angle between the two axes, and the
action of L as viewed by an observer on the
planet is to cause the axis of rotation to swivel
around the axis of the maximum moment of inertia.
This is the free precession of the planet,
as opposed to the forced precession associated
with tidal couples due to the moon and sun, and
is called the Chandler wobble. The theory of
the rotation of solid bodies suggests that the period
of the wobble should be approximately 300
days, but in fact the period is around 435 days.