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TANGENTIAL MOTION Tangential motion is motion that is perpendicular to radial motion.

TAURUS [Abbreviation: Tau] Taurus (the bull) is a constellation of the zodiac. This constellation is seen along the ecliptic between Aries and Gemini. The brightest star (Alpha Tau) in Taurus is Aldebaran, which is one of the bull's eyes (Aldebaran is the 13th brightest star in the sky). The second-brightest star in Taurus (Beta Tau) is Alnath, the third brightest (Gamma Tau) is Hyadum I.

TECTONIC ACTIVITY Tectonic activity is the shifting of a planet's (or moon's) surface because of changes deep inside the body. Earthquakes, fissures, rifts, and volcanoes are some results of tectonic activity.

TELEMETRY Telemetry is a process in which measurements or other data are collected at remote locations and are automatically transmitted back to an observer.

TELESCOPE A telescope is a device that makes faraway objects appear closer and larger, allowing us to see distant objects in space. The first refracting telescope was invented by Hans Lippershey in 1608. Early telescopes used glass lenses and/or mirrors to detect visible light. Later telescopes gathered electromagnetic radiation from the entire spectrum, from radio waves to gamma rays.

TELESTO Telesto is one of the 18 moons of Saturn. It was discovered by B. Smith, H. Reitsema, S. Larson, J. Fountain in 1980. It is irregularly-shaped and has a radius of 15x12.5x7.5 km. Telesto an orbit with Calypso (294,660 km from the center of Saturn); these 2 moons are also called the Tethys Trojans because they orbit Saturn in Tethys' orbit, Telesto is 60° ahead of Tethys, Calypso is 60° Tethys Titan.

Planet Average Temperature Mercury 100-700 K Venus 726K Earth 260-310 K Mars 150-310 K Jupiter 120 K Saturn 88 K Uranus 59 K Neptune 48 K Pluto 37 K TEMPERATURE Temperature is a measure of how hot or cold something is - how much heat energy it has. Temperature is essentially a measure of how fast the particles in a body are moving (or vibrating). There are many different temperature scales, including Fahrenheit, Celsius, and Kelvin. Fahrenheit is a scale in which 32 °F is the freezing point of water and 212 °F is the boiling point of water. Celsius is a scale in which 0 °C is the freezing point of water and 100 °C is the boiling point of water. Kelvin is a scale in which 0 K is absolute zero and the size of one degree is the same as for Celsius. Temperatures measured in the Celsius scale can be converted to Fahrenheit or Kelvin by means of the formulas F=9/5 * C +32 and K=C+273.1.

TENTH PLANET No tenth planet beyond Pluto has been directly observed. A few astronomers think that there might be a tenth planet (or companion star) orbiting the Sun far beyond the orbit of Pluto. This distant planet/companion star may or may not exist. The hypothesized origin of this hypothetical object is that a celestial object, perhaps a hard-to-detect cool, brown dwarf star (called Nemesis), was captured by the Sun's gravitational field. This tenth planet is hypothesized to exist because of the unexplained clumping of some long-period comet's orbits. The orbits of these far-reaching comets seem to be affected by the gravitational pull of a distant, Sun-orbiting object.

TERESHKOVA, VALENTINA Colonel-Engineer Valentina Vladimirovna Tereshkova (1937- ) was a Soviet cosmonaut and the first woman in space. She was on the Vostok 5 mission which launched on June 16, 1963, and orbited the Earth 48 times. The flight lasted 2.95 days. The spacecraft was recovered on June 19, 1963, in the Soviet Union.

TERMINATION SHOCK The solar wind termination shock is the shock that occurs as the solar wind hits the heliopause and its speed slows greatly (down to about 20 km/s).

TERMINATOR The terminator is the day-night line on a planet (or other body).

TERRA A terra is an extensive land mass on a planet.

TERRESTRIAL Terrestrial means about or pertaining to the Earth.

TETHYS Tethys is one of the larger of the 18 moons of Saturn. This icy moon is heavily cratered and has many cracks and plains. It has a diameter of about 600 miles (1000 km). One huge ice trench, the Ithaca Chasma, is 40 miles (65 km) wide and extends over 3/4 of the way around the moon. It orbits at a mean distance of about 180,000 miles (294,000 km) from Saturn. Its orbital period is 45.3 hours. It was discovered by Cassini in 1684. It has a water-ice interior composition (density =1.21 gm/cm3). The surface temperature is -187° C = -305° F.

THALASSA Thalassa is a tiny moon of Neptune. It was discovered using NASA's Voyager 2 mission in 1989. It orbits 50,070 km from the center of Neptune and is about 80 km in diameter.

THEBE Thebe is the fourth-closest moon of Jupiter's 16 moons. Thebe is 68 x 56 miles (110 x 90 km) in diameter and orbits 138,000 miles (222,000 km) from Jupiter. Amalthea and Thebe likely provide the dust for the Gossamer ring. Thebe has a mass of 8 x 1017kg. It orbits Jupiter in 0.6745 day (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Thebe was discovered by Stephen Synnott (Voyager 1) in 1980.

THEORY OF RELATIVITY Albert Einstein formulated the Theories of Special and General Relativity. Special relativity supplanted Newtonian mechanics, yielding different results for very fast-moving objects. The Theory of Special Relativity is based on the idea that speed has an upper bound; nothing can pass the speed of light. The theory also states that time and distance measurements are not absolute but are instead relative to the observer's frame of reference. Space and time are viewed as aspects of a single phenomenon, called space-time. Energy and momentum are similarly linked. As a result, mass can be converted into huge amounts of energy, and vice versa, according to the formula E=mc2. General Relativity expands the theory of special relativity to include acceleration and gravity, both of which are explained via the curvature of space-time.

THERMOSPHERE The thermosphere is a thermal classification. It is the layer of the atmosphere located between the mesosphere and outer space. In the thermosphere, temperature increases with altitude. The thermosphere includes the exosphere and part of the ionosphere.

THOLUS A tholus is a small mountain or hill that is dome-shaped.

THOMPSON, J. J. Joseph John Thompson was a British scientist who discovered the existence of the electron in 1897. Electrons are tiny, negatively-charged atomic particles. In an atom, they orbit around the nucleus.

THRUST Thrust is the reactive force that is exerted by a rocket, propellar, or jet engine, which propels a craft.

TIDAL COUPLING When one celestial body orbits another, tidal forces tend to equalize the rotational periods, so that, after a very long time, they may become identical. This end result is called tidal coupling. Pluto is tidally coupled to its moon Charon; if you were standing on Pluto, its moon Charon would either always be visible or never be visible. As a consequence of the tidal interactions with the Moon, the Earth's rotational period is slowly decreasing . Billions of years from now, the Earth and the Moon will have the same period of rotation, and these will also exactly equal the orbital period of the Moon around the Earth.

TIDAL FORCES Tidal forces are the gravitational pull exerted upon planetary objects by nearby planets, moons, or stars.

TIDAL HEATING Tidal heating is the heating of a planet or satellite due to the friction caused by tides. For example, as Io orbits Jupiter at a relatively close distance, the huge tidal forces heat the interior of Io enormously, making it the most seismically active body in the solar system.

TIDE A tide is a periodic rise and fall of large bodies of water. Tides are caused by the gravitational interaction between the Earth and the Moon. The gravitational attraction of the moon causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the opposite side, since the Earth is also being pulled toward the moon (and away from the water on the far side). Since the earth is rotating while this is happening, two tides occur each day. Isaac Newton was the first person to explain tides scientifically.

TIRION CHART NUMBER A Tirion Chart Number is a catalog number given to each star in the chart/catalog made by Wil Tirion in 1981. It was the first star atlas that uses equinox-2000.0 coordinates for each star, and soon became the standard intermediate-scale star atlas. The first edition plotted about 43,000 stars down to magnitude 8.0. The second edition of Try Sky Atlas 2000.0 plots about 80,000 down to magnitude 8.5 (each star is given a Tirion Chart Number).

TITAN Titan is the largest of the 18 moons of Saturn. It has a diameter of 3,200 miles (5,150 km). It is the only moon known to have an atmosphere; it consists mostly of nitrogen (with some methane) and an atmospheric pressure of 1.6 bars (60 percent greater than the Earth's). The atmosphere with its heavy clouds obscures the moon's surface. It is the second-largest moon in the solar system (after Jupiter's moon Ganymede). Titan is larger than the planets Mercury and Pluto. The surface temperature is about -178°C = -289°F. It was discovered by C. Huygens in 1655.

TITANIA Titania is the largest of the 18 moons of Uranus. It is about half the size of our moon. Titania is covered by many small craters, a few huge impact basins ice cliffs, and fault lines. Titania has a diameter of 1,578 km and a mass of 3.49x10+21 kg. It orbits Uranus at an average of 435,840 km. Titania was discovered by Wm. Herschel in 1787.

TITUS-BODE LAW The Titus-Bode "Law" (also known as Bode's Law) is an interesting mathematical coincidence, and not a physical law. It is a numerical series that matches planetary distances from the Sun. The Titus-Bode series predicts the positions of all the planets in our Solar System except Neptune (plus it predicts a planet where the asteroid belt is). This relationship is named for Johann Titus and the German astronomer Elert Bode, who did their work in the late 1700's. For the Titus-Bode series for our Solar System, start with 0.4 (A.U.), then form the series by adding 0, 0.3, 0.6, 1.2, 2.4, etc. (doubling each time after the first). The results it predicts are: Mercury: 0.4 A.U., Venus 0.7 A.U., Earth 1.0 A.U., Mars 1.6 A.U., Asteroid Belt 2.8 A.U., Jupiter 5.2 A.U., Saturn 10.0 A.U., Uranus 19.6 A.U.

TNO TNO stands for Trans Neptunian Object. These small planetesimals made of rock and ice orbit our Sun past the planet Neptune. They are Kuiper Belt objects left over from the formation of the solar system. Pluto may be a TNO, albeit a rather massive one.

TOMBAUGH, CLYDE Clyde Tombaugh (1906-1997) was an American astronomer who discovered Pluto in 1930. He also correctly predicted (in 1950) that the surface of Mars was covered with craters.

TORRINO SCALE A Torrino Scale is a risk-assessment scale for showing the potential dangers associated with asteroids and comets that may hit the Earth. The Torrino Scale notes various objects' kinetic energy (incorporating both mass and velocity) vs. the collision probability. The scale was created by Dr. Richard P. Binzel of MIT.

TOTAL ECLIPSE A total eclipse happens when the moon entirely blocks the Sun or the Earth's shadow totally blocks our view of the moon. A total eclipse can only happen during a new Moon. During a total eclipse, the Moon's shadow moves across the Earth at speeds over 1,000 mph (1,600 kph). Because of the relative size and distance of the Sun, Earth and the moon, the Earth is the only planet in our Solar System that experiences total solar eclipses.

TOTALITY Totality is the short part of an eclipse when the moon entirely blocks the Sun or the Earth's shadow totally blocks our view of the moon.

TRAIN A train is a trail of glowing ions left behind a large meteor as it falls through the atmosphere. The train is usually only visible for a few seconds before the ions combine with other atoms in the atmosphere and lose their charge.

TRANSIT 1. Transit is when a smaller body passes in front of a larger one (for example, when an object passes between the Sun and the Earth). During this time, the object seems to be crossing the disk of the Sun. The only planets that ever pass between the Earth and the Sun are Mercury and Venus (since they are closer to the Sun than the Earth). Therefore, the only planets that can produce a transit are Mercury and Venus.2. Transit is the passage of a moon in front of its primary. 3. Transit is the passage of an object across an observer's meridian.

TRANS NEPTUNIAN OBJECT Trans Neptunian Objects (TNO's) are small planetesimals made of rock and ice that orbit our Sun past the planet Neptune. They are Kuiper Belt objects left over from the formation of the solar system. Pluto may be a TNO, albeit a rather massive one.

TRANSLUNAR Translunar refers to the space beyond the orbit of the Moon. Compare to cislunar.

TRANSNEPTUNIANS Transneptunians are asteroids (minor planets) whose orbits have a semimajor axis beyond the orbit of the orbit of Neptune (but some of them have a perihelion inside the orbit of Neptune). The collection of transneptunians is known as the Transneptunian Bely or the Kuiper Belt. About 200 transneptunians have been discovered (but many more exist).

TRANSPIRATION Transpiration is the passage of water from a plant's roots, though the plant, and out to the atmosphere.

TRAPEZIUM Trapezium is a star cluster located in the center of the Orion nebula. There are four massive, energetic (young OB) stars in the center of Trapezium. These four stars form the shape of a trapezium, a quadrileteral (four-sided) geometric figure having no parallel sides.

TRAPEZOID A trapezoid is a quadrileteral (four-sided) geometric figure having two parallel sides.

TRITON Triton is the largest of the 8 moons of Neptune. Unlike all other large planetary moons, Triton has a retrograde orbit (it rotates opposite to Neptune's rotation). Triton was discovered by William Lassell in 1846. Triton is the coldest object that has been measured in our Solar System, with a temperature of -235° C (-391° F). This rocky moon has a polar ice cap at its south pole and many other varied geologic features including volcanoes, huge cracks in the surface, and geysers of gaseous nitrogen. It has a thin, hazy atmosphere (mostly nitrogen) and a windy surface covered with nitrogen ice. Triton is slowly spiraling in towards Neptune. Triton was a god of the sea in Greek mythology.

TROJAN ASTEROIDS Trojan asteroids are asteroids that orbit in gravitationally stable Lagrange points in a planet's orbit, either trailing it or preceding it (these places are where the gravitational attraction of the Sun and of the planet balance each other). Jupiter has the most Trojan asteroids; Mars also has some. Achilles was the first Trojan asteroid found. The asteroids preceding Jupiter in its orbit were named for Greek heroes; those following Jupiter in its orbit were named for Trojan heroes. The largest Trojan asteroid is Hektor.

TROPICS The tropics are a 3,000 mile (4800 km) wide band around the equator, between the Tropic of Cancer (23.5° N latitude) and the Tropic of Capricorn (23.5° S latitude).

TROPOSPHERE The troposphere is the lowest region in the Earth's (or any planet's) atmosphere. On the Earth, it goes from ground (or water) level up to about 11 miles (17 kilometers) high. The weather and clouds occur in the troposphere. In the troposphere, the temperature generally decreases as altitude increases.

TROPOPAUSE The tropopause is the boundary zone (or transition layer) between the troposphere and the stratosphere of the Earth's atmosphere. The tropopause is characterized by little or no change in temperature as altitude increases.

TSIOLKOVSKY Konstantin Tsiolkovsky (1857-1935) was a Russian schoolteacher who was the first person on record to propose the idea of explorating space using rockets. Tsiolkovsky proposed the idea in 1898 and published it in 1903. He suggested that the rockets should be powered by liquid propellants. Many people call Tsiolkovsky the father of modern astronautics.

TSUNAMI A tsunami is a huge wave, caused by undersea earthquakes, volcanic eruptions, or, more rarely, by asteroid or meteoroid impact (as in the case of the K-T extinction).

T TAURI STAR T Tauri stars are a kind of variable star whose brightness varies irregularly. Their spectrum has broad and very intense emission lines, probably indicative of violent surface activity. They may be protostars in the later stages of formation, young stars that are not yet stable main sequence stars in the Hertzsprung-Russell Diagram.

TULLY-FISHER RELATION The Tully-Fisher Relation is a correlation between the luminosity of spiral and irregular galaxies and their rate of rotation (rotational velocity, which is measured from the broadening of its spectral lines since the light from it is both red- and blue-shifted). This means that larger spiral galaxies (more luminous ones) rotate faster than smaller ones. If you know the rate of rotation of a spiral galaxy, you can calculate its intrinsic brightness (intrinsic brightness is how bright it really is - not how bright it is from Earth, which is its apparent magnitude). Then, by comparing the galaxy's intrinsic brightness with its apparent magnitude, you can determine how far away the galaxy is. The physical reason for this empirical relationship is unknown. The relationship was first realized by R. B. Tully and J. R. Fisher in 1977 (Tully, R. B. and Fisher, J. R.: 1977, Astron. & Astrophys. 54, 661).

TWINKLING The twinkling of stars - their seeming to blink on and off (also called stellar scintillation) is caused by the Earth's atmosphere. Scintillation in caused when the star's light is distorted by the Earth's atmosphere. Scintillation is greater for bright stars that are low on the horizon. It is also known as astronomical scintillation.