Glossary -- T

Glossary links (select a letter for definitions of astronomy terms beginning with that letter):
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tangential velocity

the velocity perpendicular to the line of sight.

tectonics

any stretching or compression of the lithosphere.

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 logical, systematic set of principles or explanation that has undergone testing or validation from careful observations and has stood up against attempts to prove it false. A scientific theory 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.

thermosphere

upper layer a planet's atmosphere where the temperature rises with increasing altitude because the gases absorb X-rays and ultraviolet light. X-rays ionize gas particles. Aurorae occur in this layer.

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).

transit technique

a method of finding exoplanets by looking for a drop in the brightness of a star as the planet passes in front of the star.

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).

troposphere

lowest layer of a planet's atmosphere (closest to the surface) where the temperature decreases with increasing altitude and where convection is important. Clouds form in here. Greenhouse effect is present.

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.

Glossary links (select a letter for definitions of astronomy terms beginning with that letter):
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last updated: June 10, 2010