DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

true anomaly
activity and crustal movements associated with
subduction of an oceanic plate and formation of
accretionary prisms on the continental slope are
marked.
true anomaly In Newtonian dynamics, the
angle between an orbiting body and periapse.
true celestial equator The true celestial
equator or celestial equator of date is the great
circle on the celestial sphere perpendicular to the
true celestial pole. q.v. Its intersections with the
ecliptic define the vernal equinox of date and the
autumnal equinox of date.
true celestial pole The true north celestial
pole is the direction of Earth’s instantaneous
rotation pole. It differs from the mean
north celestial pole due to the short timescale
(days to decades) variations called nutation.
The nutation itself, of order 10 ′′ arc, is normally
described by a theory, such as the IAU
1980 nutation theory, which suffices to predict
the pole position within a few hundreths
of an arc second, and smaller observed corrections
called the celestial pole offset. See
http://hpiers.obspm.fr/webiers/general/Earthor/
precnut/PNUT.html.
true north Geographic north defined by the
rotational pole of the Earth, as opposed to magnetic
north defined by the geomagnetic north
pole.
true polar wander The Earth’s hot spot system
appears to define a nearly rigid deep mantle
frame. The motion of the hot spot system with
respecttotherotationalaxisoftheEarthiscalled
true polar wander, as opposed to the apparent
polar wander as a result of continental drift. See
continental drift, hot spot, polar wander.
tsunami Water wave in the ocean or large
lakes caused by underwater earthquakes, landslides,
or volcanic activity. In the open ocean,
tsunamis can have wave lengths of hundreds of
kilometers, traveling at speeds of several hundreds
of kilometers per hour. They are often
called tidal waves, although they have no relation
with tides.
© 2001 by CRC Press LLC
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tsunami earthquake Generally means an
earthquake accompanying tsunami (tidal wave),
or an earthquake that causes a large tsunami
for its magnitude. Typical examples are the
1896 Sanriku-oki, Japan earthquake and the
1946 Aleutian earthquake. Existence of tsunami
earthquakes indicates that we cannot predict
tsunami height at a coast precisely only from
the magnitude of earthquakes, which makes
tsunami difficult to forecast. Extremely slow
faulting and generation of submarine landslide
have been considered as mechanisms of tsunami
earthquakes.
TT See Terrestrial Time.
T Tauri object A class of variable stars
whose brightness varies irregularly, with broad
and very intense emission lines. T Tauris, also
known as class II protostars, are very young premain
sequence stars that gain mass through an
accretion disk as evinced by an excess of infrared
emission, but whose surface is optically
visible. Also characterized by extensive and violent
ejections of mass and magnetic activity, T
Tauri stars are named after the first of their class
and have masses of roughly 0.2 to 5 M⊙.
T Tauri star The last stage of stellar evolution
before the main sequence. T Tauri stars
are characterized by emission lines, rapid variability,
and X-ray emission, associated with gas
being both accreted and lost in bipolar outflows
and with relatively strong magnetic fields and
rapid rotation (see stellar activity). The name is
frequently, but not always, restricted to stars of
roughly 0.5 to 2.5 solar masses, more massive
ones being called Herbig Ze/Be stars, and the
most massive being seen only as radio sources
called compact HII regions.
Tully–Fisher law An empirical law which
relates the width of the 21-cm neutral hydrogen
spectral emission line to the luminosity of
a spiral galaxy, proposed by R.B. Tully and
J.R. Fisher (1977): the total galaxy luminosity
is proportional to the fourth power of the width
of the 21-cm line. The relationship is best (i.e.,
data points show less scatter) if the luminosity
is measured in the infrared. The infrared luminosity
depends little on Hubble type and is