DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

clei. In the synchrotron self-Compton scheme,
low-energy photons are produced in the radio
domain by synchrotron emission by relativistic
elections. If the source is very compact, the
same relativistic electrons then turn the radio
photons into higher energy photons by inverse
Compton scattering. The synchrotron self-
Compton mechanism suggests that the spectral
shape of the seed synchrotron photons is maintained
in the scattered spectrum. This prediction
has been apparently confirmed by observations
of the blazar 3C 279; however, the general validity
of the synchrotron self-Compton mechanism
for active galactic nuclei is as yet controversial.
synodical month Synodic month.
synodic month The time from one full moon
to the next (about 29.53 days).
synodic period In astronomy, the amount
of time taken for a celestial body (usually the
moon, but the concept can apply to other objects)
to rotate once with respect to the Earth-sun line;
the time between successive conjunctions of an
orbiting body. Thus, the period of time between
new moons (about 29.53 days) is the synodic
month.
synoptic scale In the atmosphere, the synoptic
scale is referred to as the scale of
© 2001 by CRC Press LLC
syzygy
moving weather systems associated with cyclones
or anticyclones. This scale is larger than
the mesoscale and less than the planetary scale.
synthetic aperture radar A radar imaging
technique in which the angular resolution is improved
by combining the returns from an image
as seen when illuminated by the radar transmitter
at different points as it moves past the target.
The essential point allowing this combination
is precise geometric reconstruction based on the
Doppler shift associated with specific target features.
These signals are reduced to zero Doppler
shift (maintaining phase coherence) and combined
to produce the effect of a large aperture
radar (an aperture, d, as large as the maximum
distance between the source transmitter locations
illuminating a particular target). This process
thus, gives enhanced angular resolution, θ,
according to the standard formula:
θ ∼ λ
d .
syzygy An alignment of three celestial bodies
in a straight line; in particular the position of the
moon when it is new or full is a syzygy between
the Earth, moon, and sun.
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