DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

TT+LG·(JD−2443144.5)·86400 sec, where
LG = 6.96929 · 10 −10 by definition as of 1992,
JD stands for the Julian Date in TDT, and TT
stands for Terrestrial Time, which is essentially
the same as TDT. Presumably, the “constant”,
LG, is subject to revision when and if the potential
on the geoid is redetermined. For details,
seeIERSTechnicalNote13, TheIERSStandards
(Ed.: D.D. McCarthy) (U.S. Naval Observatory,
Washington, 1992).
Terrestrial Dynamical Time (TDT) In
1977, Dynamical Time was introduced in two
forms, TDT and Barycentric Dynamical Time
(TDB). The difference in these two consists of
periodic terms due to general relativity and does
not exceed about 1.7 milliseconds in any year.
TDT is used for the determination of the orbits
of objects orbiting the Earth. It advances at the
same rate as International Atomic Time (TAI),
being equal, for all practical purposes, to TAI +
32.184 s. Both TAI and TDT contain relativistic
effects of the Earth’s motion around the sun,
which reflect the fact that any reference frame
centered on Earth’s center is not inertial. Any of
TDT, TAI, and TCG are thus subject to further
special relativistic corrections in their relationship
to any barycentric time standard, such as
TCB. For details, see IERS Technical Note 21,
The IERS Conventions (Ed.: D.D. McCarthy)
(U.S. Naval Observatory, Washington, 1996).
See dynamical time.
terrestrial heat flow Heat flux from the interior
of the Earth. It usually means the conductive
heat flux measured at the surface of the
Earth, and it is usually called heat flow or heat
flowdensity. Thepresentglobalaverageisabout
0.06 W/m 2 .
terrestrial planets Those planets that are
similar to the Earth in their characteristics.
There are four terrestrial planets in our solar system:
Mercury, Venus, Earth, and Mars. Characteristics
of the terrestrial planets include small
size (< 13,000 km in diameter), high density
(> 3000kg/m 3 , indicatingrockycompositions),
close to the sun (< 1.5 astronomical units), few
or no moons, and no rings. These characteristics
differ considerably from those of the Jovian
planets, which dominate the outer solar sys-
© 2001 by CRC Press LLC
Tethys
tem. Although Pluto is similar to the terrestrial
planets with its small size, one moon, and lack
of rings, its large distance from the sun and its
lower density (about 2000 kg/m 3 , indicating an
icy composition) place it in a category of its own,
where it shares characteristics with many of the
large icy bodies in the outer solar system. The
terrestrial planets are sometimes also called the
inner planets, since they are located in the inner
part of the solar system.
Terrestrial Time (TT) In 1991, the International
Astronomical Union (IAU) defined an
“ideal form of Terrestrial Dynamical Time”, and
designated Terrestrial Time. This definition appears
to be primarily intended to extend the
scope of TDT off the geoid, by imposing certain
restrictions on relativistic coordinate systems.
It is used in dynamical theories of solar
system motions, and it could take into consideration
the need to bring dynamical measures of
time (from the motions of spacecraft orbiting the
Earth) into accord with atomic time. For practical
purposes, it is identical with Terrestrial Dynamical
Time. For details, see IERS Technical
Note 13, The IERS Standards (Ed.: D.D. Mc-
Carthy) (U.S. Naval Observatory, Washington,
1992).
tesla A unit of magnetic flux equal to one
weber per square meter.
Tethys Moon of Saturn, also designated
SIII. Discovered by Cassini in 1684. Its surface
shows a crater, Odysseus, with a radius
of 200 km. This is some 40% of the radius of
Tethys and is probably at the limit of what the
moon could sustain without breakup. The crater
is now flattened and conforms to Tethys’ spherical
shape. There is also a large valley, Ithaca
Chasma, which is 2000 km long and extends 3/4
of the way around Tethys. It is some 100 km
wide and 3 to 5 km deep. Tethys’ orbit has an
eccentricity of 0, an inclination of 1.86 ◦ , a precession
of 72.25 ◦ yr −1 , and a semimajor axis of
2.95 × 10 5 km. Its radius is 530 km, its mass
7.55 × 10 20 kg, and its density 1.21 g cm −3 .It
has a geometric albedo of 0.9, and orbits Saturn
once every 1.888 Earth days.
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