DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Z
Zeeman effect The splitting of spectral lines
into groups of closely spaced lines in the presence
of a strong magnetic field. The Zeeman effect
occurs in the spectra of sunspots and stars.
This effect demonstrates the existence of magnetic
fields in celestial bodies, and since the degree
of the splitting depends on the magnitude
of the magnetic fields, it allows measurement of
the field to be made.
Zeldovich process A process of energy extraction
from a rotating black hole via incident
electromagnetic or gravitational radiation.
When radiation hits a black hole, part of it is
absorbed and part is carried away. Normally,
the absorbed part has positive energy and the
amplitude of the transmitted wave is reduced.
However, in the case of cylindrical waves, the
absorbed wave may carry negative energy, in
which case the transmitted wave is amplified.
This is the variant with electromagnetic radiation
of the Penrose process. See Penrose process.
zenith The point directly overhead an observer,
as defined by the local horizontal. Technically,
there are three zenith directions: geocentric
zenith, the direction directly away from
the Earth’s center; geodetic zenith, the direction
of the local upwards normal from the Earth
ellipsoid; and astronomical zenith, the direction
of the upwards normal from the geoid.
Normally, when used without another qualifier,
zenith refers to either the geodetic or astronomical
zenith, which are within a few arc seconds
of each other. See also altitude, nadir.
zenith angle The angle from the vertical, or
zenith, to a given direction. The zenith angle is
the complement of the altitude.
zero mode See fermionic zero mode.
Zwicky compact galaxies
zero up-crossing The point where the water
surface in a water wave profile crosses the
zero line and is trending upward. Used in the
definition of wavelength.
zodiac The apparent path of the sun through
the background stars over the course of the year;
the intersection of the plane of the ecliptic with
the celestial sphere.
zodiacal light A band of diffuse light seen
along the ecliptic near the sun immediately after
sunset or before sunrise. It is created by sunlight
reflecting off the interplanetary dust particles,
which are concentrated along the ecliptic plane.
To see the zodiacal light, you must have very
dark skies; under the best conditions it rivals
the Milky Way in terms of brightness. The material
within the zodiacal light is slowly spiraling
inward toward the sun, due to the Poynting–
Robertson Effect; hence, it must be continuously
replaced by asteroid collisions and the debris
constituting comet tails. The zodiacal light is
the name given to the band of light seen close
to the sun while gegenschein is the term applied
to the same band of light located 180 ◦ from the
sun.
zonal In the direction parallel to the equator,
i.e., east-west. See also meridional.
zooplankton Animal forms of plankton.
Z-string See embedded defect.
Zulu Time See Universal Time.
Zwicky compact galaxies Galaxies originally
defined by F. Zwicky as distinguishable
from stars on the Palomar 1.2 m Schmidt telescope
plate and with angular diameter between
2 and 5 sec of arc. “Compact” is, in astronomy,
a loose synonym of “not fully resolved”.
The notion of compactness of a galaxy therefore
depends on the resolving power of the observational
equipment, although compact galaxies
have, in general, high surface brightness and
sharp borders. F. Zwicky circulated seven lists
of compact galaxies including about 200 objects
in the mid-1960s. Most of them are of blue
color and show emission lines in their spectra.
© 2001 by CRC Press LLC
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