Glossary -- T

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tangential velocity

the velocity perpendicular to the line of sight.

telescope

device used to gather and focus electromagnetic radiation. A telescope extends the power of human vision by making objects brighter, sharper, and larger, as well as, imaging objects in wavelengths that are not detectable by the human eye.

temperature

a measure of the random motion energy (the average kinetic energy) of a group of particles in a gas, liquid, or solid. The temperature is higher if the particles are moving faster.

theory

a model or idea that has undergone testing or validation from careful observations and can be used to make a variety of predictions of what will happen under different circumstances.

thermal spectrum

a spectrum that has energy at all wavelengths (a full rainbow). Produced by solids, liquids, and dense (high-pressure) gases.

time zone

interval of longitudes 15 degrees wide in which every clock is set to the same time (e.g., every clock in the Pacific time zone will give the same time).

total solar eclipse

a type of solar eclipse that happens when the Sun and Moon are exactly lined up and the Moon is close enough to the Earth to totally block the Sun's surface (contrast with annular eclipse).

trigonometric parallax

a method of determining distances to nearby stars that uses trigonometry to derive the distance from the size of the parallax angle and the distance between a planet and the Sun: distance in parsecs = (planet-Sun distance in A.U.)/(parallax angle in arc seconds).

tropical year

the time interval between two successive vernal equinoxes = 365.2422 mean solar days (contrast with sidereal year).

T-Tauri (star)

young star that is just beginning nuclear fusion and produces strong outflows of particles (winds) that clears away the gas and dust from which the star formed.

21-cm line radiation

emission line in the radio band by cool, neutral atomic hydrogen that is used to map the structure of the Milky Way and other galaxies because it passes easily through dust in the interstellar medium.

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last update: 06 August 1999