DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

B
Baade–Wesselink method A method of determining
the distance to pulsating stars, which
can also be observed spectroscopically. One estimates
the surface temperature and hence the
surface brightness from the color index (B −V
color), based on simultaneous measurements of
the blue and visual magnitude at bright and dim
epochs in the star’s pulsation. One computes the
square root of the ratio of the observed flux to the
surfacebrightnessforeachoftheseepochs. This
is an estimate of the angular diameter of the star.
Spectroscopy yields the surface velocity of the
star (via blue and red shifting of spectral lines),
which leads to a determination of the total difference
in radius of the star between the observations.
The combination of these observations
allows a determination of the physical size of
the star, and thus of its absolute magnitude and
distance. The method has also been applied to
the expanding envelope of type II supernovae.
Babinet point One of three points on the
sky in a vertical line through the sun at which
the polarization of skylight vanishes. Usually
located at about 20 ◦ above the sun. See Arago
point, Brewster point.
baby universe A theory regarding matter that
falls into a black hole that subsequently evaporates.
It states that this matter may go into a
separate space-time, which could detach from
the universe at one location and reattach elsewhere.
This may not be particularly useful for
space travel, but the existence of baby universes
introduces a randomness so that even a complete
unifiedtheorywouldbeabletopredictmuchless
than expected. However, averaging over ensembles
of universes containing baby universes may
lead to predictions via expected values of certain
measured quantities, such as the cosmological
constant.
backarc spreading In some subduction
zones (ocean trenches) an area of sea-floor
© 2001 by CRC Press LLC
c
Baily’s beads
spreading occurs behind the subduction zone,
thus creating new oceanic crust. The Sea of
Japan is an example. The foundering oceanic
plate pulls away from the adjacent continental
margin, and backarc spreading fills the gap.
back scattering Scattering through angles
greater than 90 ◦ .
backscattering coefficient The integral over
the hemisphere of backward directions of the
volume scattering function.
backscattering fraction The ratio of the
backscattering coefficient to the scattering coefficient.
backshore The relatively flat portion of a
beach profile which lies between the steeper
beachface and the dunes, cliffs, or structures
behind the beach. Subaerial during non-storm
conditions.
Backus effect A particular type of nonuniqueness
that can occur in the inversion of geomagnetic
data on a spherical surface, where instead
of knowing the full vector magnetic field
only the magnitude of the field B is known and
not its orientation. Historically, early satellite
measurements from platforms such as the PO-
GOs are of this form, as it was difficult to obtain
high quality measurements of satellite orientation.
The effect occurs even if there is perfect
knowledge of B on a spherical surface but can
be alleviated by knowledge ofB in a shell or by
knowledge of the position of the magnetic equator.
The source of the error is the existence of
magnetic fields that are perpendicular to Earthlike
fields everywhere on a spherical surface,
and which, therefore, can be added or subtracted
from the Earth’s field without changing B. As
the Earth’s field is predominantly axial dipolar,
the error terms associated by the Backus effect
have strong sectoral variation in a band around
the equator. See nonuniqueness.
Baily’s beads A phenomenon appearing
at the onset and at the conclusion of a solar
eclipse, in which the photosphere is almost totally
eclipsed, except for a few locations on the
limb of the moon, which allow view of the pho