DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

arotropic instability A wave instability associated
with the horizontal shear in a jet-like
current and that grows by extracting kinetic energy
from the mean flow field.
barotropy ln fluid mechanics, the situation
in which there is no vertical motion, and the
gradients of the density and pressure field are
proportional, and the vorticity (as measured in
an inertial frame) is conserved.
barred galaxies Disk galaxies showing a
prominent, elongated feature, often streaked by
absorption lanes due to interstellar dust. Prominent
bars are observed in about 1 3 of disk galaxies;
approximately 2 3 of galaxies do, however,
showsomebar-likefeature. Abarcancontribute
to a substantial part, up to 1 3 , of the total luminosity
of a galaxy. The bar photometric profile
is quite different from the photometric profile
of galaxies: the surface brightness along the
bar major axis is nearly constant but decreases
rapidly along the minor axis. The bar occupies
the inner part of the galaxy rotation curve where
the angular speed is constant; bars are therefore
supposed to be rotating end over end, like rigid
bodies.
barrier island An elongated island separated
from a coast by a shallow bay or lagoon. Generally
much longer in the longshore direction
than cross-shore direction. The Outer Banks of
North Carolina and much of the east coast of
Florida provide good examples.
barriers When an earthquake is caused by a
rupture on a fault, inversions of seismic waves
indicate some portions of the fault do not rupture;
these are barriers.
Barycentric Coordinate Time (TCB) Barycentric
Dynamical Time has been deemed by
the International Astronomical Union (IAU) to
be an inferior measure of time in one sense:
Its progress depends on the mass of the sun
and the mean radius and speed of the Earth’s
motion around the sun, and, to a smaller extent,
on the mean gravitational perturbations of
the planets. Therefore, in 1991 the IAU established
a time standard representing what an
SI clock would measure in a coordinate sys-
© 2001 by CRC Press LLC
baryogenesis
tem, such that the barycenter of the solar system
was stationary in this nearly inertial system,
as was the clock, but the clock was so
far removed from the sun and planets that it
suffered no gravitational effect. That time is
TCB =TDB+LB·(JD−2443144.5)·86400
sec, whereLB = 1.550505 · 10 −8 by definition
as of mid-1999, and JD stands for the Julian
Date in TDT. Presumably, the “constant”, b, is
subject to revision when and if the mass content
of the solar system or the properties of the
Earth’s orbit are redetermined. See Barycentric
Dynamical Time.
Barycentric Dynamical Time (TDB) In
1977, Dynamical Time was introduced as two
forms, Terrestrial Dynamical Time (TDT) (q.v.)
and TDB, on the basis of a 1976 IAU resolution.
The difference between these two consists
of periodic terms due to general relativity. TDB
is commonly used for the determination of the
orbits of the planets and their satellites, except
those of the Earth. It is particularly suited to
this purpose because it is adjusted from TDT in
such a way as to represent what a clock on the
geoid would measure if the Earth orbited the sun
in a circular orbit of radius 1 astronomical unit
(q.v.), while TDT contains relativistic effects of
the eccentricity of the Earth’s orbit. See also
Barycentric Coordinate Time (TCB). Approximately,TDB
=TDT + 0.001652825 cos(g)s
whereg is the mean anomaly (q.v.) of the Earth
in its orbit. See Ephemeris Time, dynamical
time.
baryogenesis Period of the early evolution of
the universe when baryons were generated from
quarks. Observationally, the universe is made
of normal matter, containing baryons and leptons
with no observational evidence of significant
amounts of antimatter anywhere. Similarly
to the successful predictions of nucleosynthesis,
the unfulfilled goal is to build a scenario where,
starting from a baryon symmetric state, quark
and lepton interactions lead to an excess of matter
over antimatter as the photon temperature
drops. It will suffice to produce an excess of 1
baryon every 10 9 antibaryons to give rise to a
universe made of normal matter and a baryonto-photon
ratio η ∼ 10 −9 , as observed.