DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

transformation. Examples of such apparent singularities
occur at the poles on the Earth. At
every other point on the surface of the Earth,
the geographical longitude can be determined.
However, at each pole the meridians converge
to a point and the distance between points on
two different meridians tends to zero. A coordinate
system nonsingular at the pole can be easily
constructed, for example, by placing the origin
of a 2-d rectangular map at the pole. Genuine
singularities signal the breakdown of the theory
used to describe the process; a physical system
emerging from a singularity has the initial
values of some of its parameters undetermined,
and some aspects of evolution of physical systems
will be unpredictable. Consequently, for
instance, the existence of the Big Bang implies
that general relativity cannot completely predict
the state of matter at very high densities. It is
expected that the adequate theory to describe the
universe in the vicinity of the Big Bang will be
quantum gravity, a theory that combines classical
relativity and quantum mechanics. In quantum
gravity, the expectation goes, the state of
infinitely large density would be replaced with a
very high, but finite density, through which the
evolution of the universe can be calculated. Localized
singularities are often discussed in the
context of the cosmic censorship hypothesis, at
present a vague conjecture that states that every
generic singularity is hidden in a black hole and
so no signal from it can propagate outside the
surface of the black hole. See cosmic censorship.
singularity theorems A set of precise mathematical
arguments that prove that a universe
will contain a singularity in the past or future
if a number of specific assumptions about its
structure are true.
sinistral fault Another term for a left-lateral
fault.
Sinope Moon of Jupiter, also designated JIX.
Discovered by S. Nicholson in 1914, its orbit
has an eccentricity of 0.275, an inclination of
153 ◦ , and a semimajor axis of 2.37 × 10 7 km.
Its radius is approximately 18 km, its mass
7.76 × 10 16 kg, and its density 3.2 g cm −3 . Its
geometric albedo has not been well determined,
© 2001 by CRC Press LLC
skip zone
and it orbits Jupiter (retrograde) once every 758
Earth days.
sinusoidal wave A wave that has a profile
defined by a simple sinusoid. Corresponds to
linear or Airy wave theory.
Sirius -1.46 magnitude star of spectral type
A1 at RA06 h 04 m 08.9 s , dec −16 ◦ 42 ′ 58 ′′ .
SI (Systeme International) The system of
units based on the meter, kilogram, and second.
skewness A measure of slant or preference
for one side. The third moment of a statistical
distribution about the mean.
skip fading When the operating frequency
is close to the MUF and above the maximum
observed overhead critical frequency, then skip
fading (sometimes called MUF fading) may occur.
Changes in the ionosphere may alter the
MUF, taking it below the operating frequency
resulting in a sharp drop in signal strength. It is
possible that traveling ionospheric disturbances
could cause skip fading, although the most common
time it is observed is near dawn and dusk,
when the MUF is changing more regularly. The
distance from the receiver to the point where
the transmitted frequency can first be observed
is the skip distance. See skip zone.
skip zone If the operating frequency of an
HF transmitter is higher than the highest frequency
that can be reflected from the overhead
ionosphere, then there will be a region about
the transmitter where signals cannot be received.
This is called the skip zone. As the elevation angle
for the radio waves drops from overhead, the
obliquity factor increases until a point is reached
when propagation is just possible. The distance
from this point to the transmitter is called the
skip distance. It is the minimum distance from
the transmitter for which a sky wave will return
to Earth when the operating frequency exceeds
the vertical incidence critical frequency. The
only way to reduce the skip distance is to lower
the operating frequency. See ionospheric radio
propagation path.
423