DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Celsius, Anders
tions from the Earth. The celestial sphere thus
surrounds the Earth, and locations on the sphere
are given by the two angles necessary to define
a given direction.
Celsius, Anders Astronomer (1701–1744).
Proposed the Centigrade temperature scale.
Celsius scale Also called Centigrade Scale,
scale for measuring temperature in which the
melting point of ice is 0 ◦ , and the boiling point
of water is 100 ◦ . This definition has been superseded
by the International Temperature Scale
1968, which is expressed both in Kelvin and
degrees Celsius. Named after Anders Celsius
(1701–1744).
Centaur An “outer planet crosser.” A minor
body whose heliocentric orbit is between
Jupiter and Neptune and typically crosses the
orbits of one of the other outer giant planets
(Saturn, Uranus, Neptune). The orbits of the
Centaurs are dynamically unstable due to interactions
with the giant planets, so they must
be transition objects from a larger reservoir of
small bodies to potentially active inner solar system
objects. The Kuiper belt is believed to be
this source reservoir.
Centaurus A (Cen A) Active galaxy at
RA13 h 25 m 28 s , dec −43 ◦ 01 ′ 11 ′′ in constellation
Centaurus. Also classified as NGC5128. Distance
approximately 3.4Mpc. Large, elliptical
galaxy with strong dust lanes seen in the visible
and infrared, strong jets seen in radio (double
lobed) and in the X-ray (single lobed). Also
visible in the gamma ray range.
center of figure To a first approximation the
earth is a sphere. However, the rotation of the
earth creates a flattening at the poles and an
equatorial bulge. The shape of the earth can be
representedasanoblatespheroidwithequatorial
radius larger than the polar radius by about a factor
of 1/300. The center of the best fit spheroid
to the actual shape of the earth is the center of
figure. There is an offset between the center of
figure and the center of mass of a few kilometers.
center of mass In Newtonian mechanics, the
“average” location X of the mass, given in com-
© 2001 by CRC Press LLC
ponents by
X a =
m(i)x a (i)
m(i)
,a = 1, 2, 3 ,
where the sums indicated by are over all the
masses, labeled (i). In relativistic mechanics
there are many inequivalent formulations which
all reproduce this nonrelativistic result in the
limit of small velocities.
Centigrade scale See Celsius scale.
centimeter burst A transient solar emission
of radiation at radio wavelengths ∼1 to
10 cm. Centimeter bursts provide a powerful
diagnostic of energetic electrons in the solar
atmosphere, especially during solar flares.
The production mechanisms include thermal
bremsstrahlung, gyrosynchrotron radiation, and
collective plasma processes.
central meridian passage The passage of a
solar feature across the longitude meridian that
passes through the apparent center of the solar
disk. Useful for identifying a characteristic time
during the transit of a solar feature (e.g., an active
region) across the solar disk.
central peak A mound of deformed and fractured
rock found in the center of many impact
craters. This material originally existed under
the crater floor and was uplifted by the stresses
associated with the impact event. Central peaks
are believed to form by hydrodynamic flow during
crater collapse. The target material behaves
as a Bingham fluid, which displays properties
of viscous fluids yet has a definite plastic yield
stress. As the crater formed in this target material
collapses, shear stresses cause material to
be jetted up in the center of the crater. When the
shear stresses fall below the cohesion of the target
material, the motion of this central jet ceases,
and the material freezes into a central peak.
Central peaks are common features of complex
craters but are associated with the smaller complex
craters. As crater size increases, central
peaks tend to be replaced first by craters with a
ring of central peaks (called a peak ring), then
by a combination of central peaks surrounded
by a peak ring, and finally a multiple-ring basin.