DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

deep water wave A wave in water that has a
depth at least half of one wavelength. Thenc=
√ gL/(2π), where c is the wave speed, g is the
acceleration of gravity, andL is the wavelength.
See shallow water wave.
defect See cosmic topological defect.
deferent In the Ptolemaic theory of the Earthcentered
universe, the large orbital circle around
a point between the Earth and the equant, followed
by the center of a planet’s circular epicycle.
deficit angle (cosmic string) Cosmic strings
are inhomogeneities in the energy density field
that form during cosmic phase transitions. In
the limit of energy below the Planck energy
(10 19 GeV), strings can be well described in the
weak gravity and thin string limit. The first condition
allows simplification of the relevant equations,
while the second assumes that dimensions
transversaltothestringareeffectivelynegligible
comparedtothelengthofthestring. Inthislimit,
the Einstein equations predict a metric with the
usual Minkowski aspect in cylindrical coordinates
ds 2 =dt 2 −dz 2 −dr 2 −r 2 dθ 2
but where the azimuthal angleθ varies between
0 and 2π(1 − 4GU), withG andU being Newton’s
constant and the energy per unit length of
the string, respectively. While space-time looks
locally flat around the string, globally, however,
it is non-Euclidean due to the existence of this
missing angle δθ ≡ 8πGU (called the deficit
angle) which, in usual simple models, is small,
of order 10 −5 radian. This peculiar feature implies
that constant-time surfaces perpendicular
to different segments of a string will have the
shape of a cone. See Abelian string, cosmic
string, cosmic topological defect.
deflation A term used to denote the reduction
in elevation of a beach or other area subject to
sedimenttransport, duetotransportofsediments
by wind.
deformation radius See Rossby radius of
deformation.
© 2001 by CRC Press LLC
degree (temperature)
degeneracy The condition in which some
fermion (a particle with angular momentum =
1/2¯h and subject to the Pauli exclusion principle)
is packed as tightly as quantum mechanical
considerations permit. Electrons are degenerate
in white dwarfs and neutrons in neutron stars
and pulsars. Neutrinos, if they have nonzero
rest mass and contribute to hot dark matter, may
be degenerate in dwarf galaxies. The ignition of
a nuclear reaction in degenerate matter leads to
an explosion because the reaction heats the gas.
The gas does not expand (because the pressure
in degenerate matter depends only on the density,
not on the temperature), and so it cannot
cool. The reaction goes faster at the higher temperature
and releases more energy. The gas gets
hotter, and so forth, until finally it is no longer
degenerate, and it expands explosively.
degree (temperature) On the Celsius thermometer
scale, under standard atmosphere pressure,
the freezing point of water is 0 degrees, and
the boiling point of water is 100 degrees. The
space between these two temperature points is
separated into 100 parts. Each part represents 1
degree, i.e., 1 ◦ C. On the Fahrenheit thermometer
scale, under standard atmosphere pressure,
the freezing point of water is 32 degrees, and
the boiling point is 212 degrees. Thus,
or
degree ◦ F = 9
5 degree◦ C + 32
degree ◦ C = 5
9
degree ◦ F − 32 .
The thermodynamic temperature scale, also
called Kelvin temperature scale or absolute temperature
scale, is an ideal temperature scale
based on Carnot cycle theory. It was chosen
as the basic temperature scale in 1927. Additionally,
the international practical temperature
scale has been established, based on the thermodynamic
temperature scale. Currently the standard
international practical temperature scale in
use is the International Practical Temperature
Scale 1968, IPTS-68. Symbol is T68 and unit is
K. The relation between international practical
Celsius temperature scale (t68, unit is ◦ C) and
international practical temperature scale is
t68 = T68 − 273.15