An Etymological Dictionary of Astronomy and Astrophysics
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A uniform atomic time scale for apparent geocentric ephemerides defined by a 1979 IAU resolution, which replaced Ephemeris Time. TDT is independent of the variable rotation of the Earth, and the length of the tropical year is defined in days of 86,400 seconds of international atomic time. In 1991 it was replaced by Terrestrial Time.

→ terrestrial; → dynamical; → time.

A parameter representing the product of the → gravitational constant by the Earth's mass. It is 3.987 x 10¹⁴ m³s^-2 or 3.987 x 10⁵ km³s^-2.

→ terrestrial; → gravitational; → constant.

The four innermost planets in the solar system, Mercury, Venus, Earth and Mars. They are called terrestrial because they have a compact, rocky surface like the Earth's. The planets, Venus, Earth, and Mars have significant atmospheres while Mercury has almost none. These planets are approximately the same size, with the Earth the largest. They are considerably denser than the Jovian planets, ranging from a specific gravity of 4 for Mars to 5.5 for the Earth.

→ terrestrial; → planet.

The modern astronomical standard for the passage of time on the surface of the Earth. It is the → coordinate time scale consistent with the theory of general relativity for an observer on the surface of the Earth. TT was renamed from Terrestrial Dynamical Time (TDT) in 1991. The fundamental unit of TT is the day of 86,400 SI seconds. It is related to the International Atomic Time by the relation: TDT = TAI + 32.184 sec.

→ terrestrial; → time.

1) Third in order, rank, level, stage, formation, etc.
2) Chem.: Being or containing a carbon atom having bonds to three other carbon atoms.
3) Geology: (usually with capital initial) A term formerly used to distinguish a major period of → geologic time, from the end of the → Cretaceous period about 65 million years ago to the start of the Quaternary period about 1.6 millions years ago. The International Commission on Stratigraphy no longer sustains this term as part of the formal stratigraphic nomenclature. Instead, the → Paleogene and → Neogene periods are recommended as the primary subdivisions of the Cenozoic era.

From L. tertius, → third; → period.

The unit of → magnetic induction flux density in the SI system. It is the induction of a field in which each meter of conductor with a current of one → ampere and arranged perpendicular to the direction of the vector of induction is acted upon by a force of one → newton. 1 tesla = 1 → weber/m² = 10⁴→ gauss.

Named after Nikola Tesla (1856-1943), American physicist, electrical engineer, and inventor, born in Smiljan, Lika, which was then part of the Austo-Hungarian Empire, region of Croatia.

A procedure by which the presence, quality, or genuineness of anything is determined.

M.E. test, originally "a small vessel for refining or assaying metals," from O.Fr. test, from L. testum "earthen pot," related to testa "piece of burned clay, earthen pot, shell," related to texere "to weave;" → technique.

Âzmun, from âzmudan, âzmây- "to try, experiment, test;" Mid.Pers. uzmudan, ôzmutan "to test, try, prove;" from O.Pers./Av. *uz-mây-, from uz-, → ex-, + mť(y)- "to measure;" from PIE *me- "measure;" cf. Skt. mati "measures," matra "measure;" Gk. metra "lot, portion;" L. metri "to measure."

Any of a pair of identical 1.96 kg gold-platinum cubes measuring 4.6 cm on a side that are planned to be used in the → eLISA experiment to detect → gravitational waves. The test masses will be released in → free fall in the the → LISA Pathfinder spacecraft. The aim is to test methods and techniques that will be used in the eLISA experiment to detect gravitational waves. Even in space there are forces capable of disturbing the cubes, including the radiation and wind from the Sun. Hence the test masses need be isolated from all of these non-gravitational influences. To do so, LISA Pathfinder continually measures their positions and manoeuvres around them with microthrusters to avoid ever touching them.

→ test; → mass

Same as → significance testing and → rule of decision.

→ test; → significance.

In → plasma physics, → fluid dynamics, and → self-gravitating systems, a particle or ensemble of particles which are affected by the evolution of the rest of the system, but do not affect the rest of the system. The test particle concept is used, e.g., for revealing a flow, describing the path of fluid elements, and so on.

→ test; → particle.

That part of the working time of a telescope devoted to tests of coupled instruments or the telescope itself.

→ test; → time.

The state or fact of being → testable. See also → falsifiability.

→ testable; → -ity.

The quality of a claim, hypothesis, or theory that can be verified by tests or experiments. See also → falsifiable.

→ test; → -able.

The ninth of Saturn's known satellites. It it 1060 km across and orbits at a mean distance of 294,660 km from Saturn. It has a rotational period of 1.89 days that equals its orbital period. Tethys is thought to be composed almost entirely of water-ice because of its low density (1.21 gm/cm³). The western hemisphere of Tethys is dominated by a huge impact crater called Odysseus, whose 400 km diameter is nearly 2/5 of that of Tethys itself. Tethys has two moons named Telesto and Calypso. It was discovered by J.-D. Cassini in 1684.

In Gk. mythology Tethys was a Titaness and sea goddess who was both sister and wife of Oceanus.

Combining form meaning four.

From Gk. tetra-, combining form of tettares, tessares "four," cognate with Pers. cahâr, → four.

1) A group of four.
2) A sequence of four → total lunar eclipses occurring at approximately six month intervals. Tetrads are relatively rare events. The first → total eclipse of a tetrad occurred on 15 April 2014, the next eclipse of the series will happen on 8 October 2014 and the next two eclipses on 4 April and 28 September 2015, respectively. The previous tetrad happened in 2003-2004 and the next one will be in 2032-2033.

From Gk. tetrad-, stem of tetras "group of four."

Cahârgân, from cahâr, → four, + -gân suffix of plurality.

Same as → quadrangle.

From Gk. tetragonon, from → tetra- + -gon "angled, angular," from gonia "angle," related to gony "knee;" L. genu "knee;" Mod.Pers. zânu "knee;" Av. žnav-, žnu- "knee;" Skt. janu-; PIE base *g(e)neu-.

Cahârguš "four-cornered," from cahâr, câr "four," cognate with L. quattuor, → four, + gušé, guš "corner, angle;" Mid.Pers. gôšak "corner."

A solid figure having four plane faces.

→ tetra-; → -hedron.

1) The main body of matter in a manuscript, book, newspaper, etc., as distinguished from notes, appendixes, headings, illustrations, etc.
2) The actual wording of anything written or printed.
3) Any of the various forms in which a writing exists (Dictionary.com).

M.E., from L. textus "text, terms, structure," originally, "pattern of weaving," → texture, from texere "to weave."

Bâfé, from bâftan "to weave," → texture.

A type of hypothetical → topological defect in the structure of → space-time that forms when large, complicated symmetry groups are completely broken.

M.E., from M.Fr., from L. textura "web, texture, structure," from stem of textere "to weave;" PIE base *tek- "to make" → technique.

Bâft, verbal noun, past stem of bâftan "to weave;" Mid.Pers. waf- "to weave;" Av. ubdaēna- "consisting of woven texture," from *ubda- "woven," p.p. from vaf- "to weave; to sing," literally "to weave the songs;" cf. Skt. vabh- "to bind, fetter," ubhnāti "he laces together;" Gk. hyphainein "to weave," hyphos "web;" P.Gmc. *webanan (cf. O.N. vefa, M.L.G., M.Du., Du. weven, O.H.G. weban, Ger. weben; O.E. wefan; E. weave), from PIE *webh-/*wobh- "to weave."