DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

state of saturation, i.e., the ratio of the actual vapor
pressure (e) to the saturation vapor pressure
(E). The relative humidityf is (as a percentage)
f= e
× 100% .
E
Relative humidity expresses the saturation status
of air. Generally the relative humidity decreases
duringthedaytimeastemperatureincreases, and
increases at night as the temperature falls.
relativistic jets Collimated ejection of matter
at a velocity close to the speed of light, giving
rise to highly elongated, often knotted structures
in radio-loud active galactic nuclei, for example,
radio quasars and powerful radio galaxies.
Radio jets usually originate from an unresolved
core and physically connect, or point, to extended
lobes. Linear sizes of jets mapped at radio
frequencies in external galaxies range from
several kpc or tens of kpc, down to the minimum
size resolvable with very long baseline
interferometers (∼ 1 parsec). Parsec-size jets
show several indications of relativistic motion,
including apparent superluminal motion, and jet
one-sidednessascribedtorelativisticbeamingof
radiation. It is less clear whether jets observed at
scales of several kiloparsecs are still relativistic.
It is thought that only the most powerful radio
galaxies, class II according to Fanaroff and Riley,
and quasars may sustain a relativistic flow
along kiloparsec-sized jets. Jets emit radiation
over a wide range of frequencies. This suggests
that the radio emission is electron synchrotron
radiation. Galactic objects, like the evolved binary
system SS 433, and galactic superluminal
sources are also believed to harbor relativistic
jets. See superluminal source.
relativistic time delay The elongation of the
travel-time of an electromagnetic signal, caused
by the signal’s passing through a gravitational
field. This typically occurs when a radar signal
is sent from the Earth, reflected back from another
planet to be received at the Earth, perhaps
passing near the sun on the way. When the path
of the signal does not come near the sun, the
time-lapse between the emission of the signal
and its reception on the Earth is almost the same
as calculated from Newton’s theory. However, if
the signal passes near the sun, the distance cov-
© 2001 by CRC Press LLC
residence time (replacement time or average transit time) (TR)
ered by the signal becomes longer because the
sun curves the spacetime in its neighborhood.
When the planet and the Earth are on opposite
sides of the sun, and the path of the signal just
grazes the sun’s surface, one finds for the relativistic
time delay t:
t = GM
c 3
ln 4RERp/d 2
,
where M is the mass of the sun, RE is the distance
from the sun to the Earth, Rp is the distance
from the sun to the planet, and d is the
radius of the sun. Here G is Newton’s gravitational
constant, and c is the speed of light. The
planets used in the measurement are Mercury,
Venus, the artificial satellites Mariner 6 and
7, and Mars (where artificial satellites orbiting
Mars or landed on Mars were used as reflectors).
The value oft predicted by Einstein’s relativity
theory for the situation described above is
240 ms (Shapiro, 1964). This is now one of the
standard experimental tests of the relativity theory
(along with perihelion shift, light deflection,
and the gravitational redshift).
relativity See general relativity, special relativity.
relaxation time Characteristic time to represent
rate of relaxation phenomena. For instance,
for a viscoelastic body which is characterized
by Maxwell model, stress decreases exponentially
with time when a deformation is applied
at a time and is subsequently kept constant. The
time that stress becomes 1/e (e is base of natural
logarithms) of the initial value is called relaxation
time. Each material has its own relaxation
time.
rem An obsolete unit of radiation dose equivalent,
equal to 100 ergs per gram.
remote sensing reflectance In oceanography,
the ratio of the “water-leaving” radiance in
air to the downward plane irradiance incident
onto the sea surface [sr −1 ].
residence time (replacement time or average
transit time) (TR) The average length of time
that a parcel of water spends in a hydrologic
reservoir; the minimum period required for all
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