DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

principal null directions
principal null directions In pseudo-
Riemannian geometry and general relativity,
one of the null (i.e., lightlike) eigenvectors obtainedfromtheWeyl(conformal)tensor,
orfrom
an equivalent spinor representation. The behavior
(and possible coincidence of one or more of
these directions) is used in the Petrov classification.
See Petrov types.
principal spinors The spinors αA, βB ,···,
determined up to a complex multiplying factor,
in the symmetrized decomposition
ψAB...P =α(AβB,...πP)
of a symmetric p-index SL(2,C)spinor ψ.
principle of equivalence See equivalence
principle.
probable maximum hurricane A parameterized
storm used for design purposes or to predicthurricanedamage.
Seealsostandardproject
hurricane.
progressive vector diagram A diagram that
shows the displacement a particle would have if
it had the velocity observed at the fixed position
of the current meter at all times. For infinitesimal
motion, this coincides with the particle trajectory.
progressive wave A (water) wave that exhibits
a net horizontal translation, such as waves
in the open ocean, as opposed to a standing
wave.
Prometheus Moon of Saturn, also designated
SXVI. It was discovered by S. Collins and
others in Voyager photos in 1980. Its orbit has
an eccentricity of 0.003, an inclination of 0 ◦ ,
and a semimajor axis of 1.39 × 10 5 km. Its
size is 73× 43× 31 km, its mass is estimated at
2.7×10 17 kg, and its density at 0.7 g cm −3 . This
low density indicates that Prometheus is probably
a porous body. It has a geometric albedo
of 0.6, and orbits Saturn once every 0.613 Earth
days. It is the inner shepherd satellite of Saturn’s
F ring.
prominence A structure in the solar corona
consisting of cool dense plasma supported by
© 2001 by CRC Press LLC
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magnetic fields. Prominences are bright structures
when seen over the solar limb, but appear
dark when seen against the bright solar disk
(see filament). Prominences are characterized
as quiescent or active with the former being very
stable, lasting for months, and the latter being
mostly associated with solar flares and having
lifetimes of minutes or hours.
proper time The time measured by an observer
with the clock that the observer carries
with him/herself, i.e., time measured in a reference
frame in which the clock or process is at
rest. One of the effects predicted by special relativity
is that if an observer moves with respect
to another, with a velocity comparable to the
velocity of light, then the clock of the moving
observer goes more slowly than the observer at
rest measures. The effect is relative: each of the
observers sees that the other one’s clock slows
down. However, if one of the observers moves
noninertially (e.g., going to a distant star, reversing
and returning), then on comparison at return,
the moving clock shows less elapsed time. (See
time dilatation.) The incrementdτ of the proper
time τ is related to the coordinate time and the
velocity dxi
dt ,i= 1, 2, 3by c 2 dτ 2 =c 2 d 2
t− dx 2 +dy 2 +dz 2
,
where {dx α }={dt,dx,dy,dz} are the change
in the event coordinates during the interval dτ.
General relativity is locally equivalent to a
flat space (because of the equivalence principle).
In general, for any coordinate frame, even in
general relativity, one can compute: c 2 dτ 2 =
−gαβ dx α dx β , wheregαβ arecomponentsofthe
metricassumedtohavesignature(−+++). See
summation convention.
proplyd A proto-star or young stellar object
whose residual gas reservoir is being ionized
away by ultraviolet radiation from nearby
more massive stars.
Proteus Moon of Neptune, also designated
NIII. It was discovered in 1989 on Voyager photos.
Its orbit has an eccentricity of 0.00044, an
inclination of 0.04 ◦ , and a semimajor axis of
1.18 × 10 5 km. Its size is 218 × 208 × 201 km,
but its mass is unknown. It has a geometric