T

TAI

See International Atomic Time (TAI).

 

TCB

See Barycentric Coordinate Time (TCB).

 

TCG

See Geocentric Coordinate Time (TCG).

 

TDB

See Barycentric Dynamical Time (TDB).

 

Teph

The independent argument of the JPL planetary and lunar ephermerides DE405/LE405; in the terminology of General Relativity, a barycentric coordinate time scale. Teph is a linear function of Barycentric Coordinate Time (TCB) and has the same rate as Terrestrial Time (TT) over the time span of the ephemeris. Teph is regarded as functionally equivalent to Barycentric Dynamical Time (TDB). (See Barycentric Coordinate Time (TCB); Barycentric Dynamical Time (TDB);

Terrestrial Time (TT).)

 

Terminator

The boundary between the illuminated and dark areas of a celestial body.

 

Terrestrial Ephemeris Origin (TEO)

The original name for the Terrestrial Intermediate Origin (TIO). Obsolete.

 

Terrestrial Intermediate Origin (TIO)

The non-rotating origin of the Terrestrial Intermediate Reference System (TIRS), established by the International Astronomical Union (IAU) in 2000. The TIO was originally set at the International Terrestrial Reference Frame (ITRF) origin of longitude and throughout 1900–2100 stays within 0.1 mas of the ITRF zero-meridian. Formerly referred to as the Terrestrial Ephemeris Origin (TEO).

 

Terrestrial Intermediate Reference System (TIRS)

A geocentric reference system defined by the intermediate equator of the Celestial Intermediate Pole (CIP) and the Terrestrial Intermediate Origin (TIO) on a specific date. It is related to the Celestial Intermediate Reference System by a rotation of the Earth Rotation Angle, θ, around the Celestial Intermediate Pole.

 

Terrestrial Time (TT)

An idealized form of International Atomic Time (TAI) with an epoch offset; in practice TT=TAI+32s.184. TT thus advances by SI seconds on the geoid. Used as an independent argument for apparent geocentric ephemerides. (See second, Système International (SI).)

 

Topocentric

With reference to, or pertaining to, a point on the surface of the Earth.

 

Transit

1. The passage of the apparent center of the disk of a celestial object across a meridian. 2. The passage of one celestial body in front of another of greater apparent diameter (e.g., the passage of Mercury or Venus across the Sun or Jupiter’s satellites across its disk); however, the passage of the Moon in front of the larger apparent Sun is called an annular eclipse. (See eclipse, annular; eclipse, solar.)

 

Transit, shadow

The passage of a body’s shadow across another body; however, the passage of the Moon’s shadow across the Earth is called a solar eclipse.

 

True equator and equinox

The celestial coordinate system defined by the orientation of the Earth’s equatorial plane on some specified date together with the direction of the dynamical equinox on that date. The true equator and equinox are affected by both precession and nutation. (See mean equator and equinox; nutation; precession.)

 

TT

See Terrestrial Time (TT).

 

Twilight

The interval of time preceding sunrise and following sunset during which the sky is partially illuminated. Civil twilight comprises the interval when the zenith distance, referred to the center of the Earth, of the central point of the Sun’s disk is between 90° 50′ and 96°, nautical twilight comprises the interval from 96° to 102°, astronomical twilight comprises the interval from 102° to 108°. (See sunrise, sunset.)