Astronomical dictionary

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This is by no means a comprehensive glossary of astronomical terms however it should provide some assistance to novices in particular. Click on the symbol for a diagram or picture associated with that word. Cross references within the glossary are indicated by hyperlink text. Click on the letters below to go to the first entry of alphabetical sections.

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T association An group of low mass T-Tauri stars.
T-Tauri stars An early stage in the evolution of a star. During this stage, the star is shrinking under the influence of gravity and it sends out a strong stellar wind. This 'blows away' the cloud of dust and gas that is left over from the original nebula that the star formed from. In our Solar systen, it is thought that this stage may have lasted for about 30 million years and the resulting Solar wind was strong enough to have stripped the original atmosphere from the Earth.
Telluric A little used term meaning something that is to do with the Earth. When light from stars is altered by passing through the atmosphere, any lines that are thus created in the spectrum are referred to as telluric lines.
Terminator The dividing line between the illuminated and shadowed parts of a planetary or lunar disc.
Terrae Extensive highland areas on the planet Venus. Smaller areas are referred to as 'regiones'. Click here for a summary of named planetary features.
Tesserae Highland areas of the planet Venus that are criss-crossed with intersecting ridges and grooves. Originally called 'parquet terrain'. Click here for a summary of named planetary features.
Tektites
There is some debate about the origin of these small glassy objects. They appear to have been heated twice, suggesting they are meteoric in origin. It is generally accepted, however, that they are of terrestrial origin. tektites photograph
Third contact This is the moment in time that an eclipse ends. For Solar eclipses, it is when the Sun starts to reappear from behind the Moon and can be accompanied by the diamond ring effect. For a Lunar eclipse, it is when the Moon starts to emerge from the Earth's umbra.
TLP TLP stands for 'Transient Lunar Phenomenon'. These are temporary appearances of colour on the surface of the Moon. Sometimes no colour is observed but a normally distinct object appears obscured. They seem to appear most often in the craters Aristarchus, Gassendi and Alphonsus. There is no real explanation for these events, one idea is that they are a release of gas from the interior of the Moon whilst another suggests that they are caused by unusual observing conditions.
Total eclipse Can refer to either the Sun or the Moon. See eclipse.
Transit

When a small celestial object e.g. Mercury or Venus moves in front of a larger one e.g. the Sun. The smaller object appears as a black speck moving against the brighter background. The shadow of a satellite across the disc of the primary object is also termed a transit e.g. the shadows of the satellites (moons) of Jupiter.

Transit also means the passage of a celestial body (star, planet, galaxy and so on) or a point on the celestial sphere (first point of Aries and so on) across the meridian of an observer. Thus, at the time of transit, the body or point on the celestial sphere must be at its highest point in the sky.

Trojan Asteroids A group of minor planets (asteroids) that orbit the Sun at the same distance as Jupiter. One group is 60 degrees ahead and the other 60 degrees behind Jupiter. They are held in place by the gravity of Jupiter.
The Tunguska Event

Early in the twentieth century, a massive explosion devastated a large area of forest in Siberia. It has never been fully explained. Independent thinkers subscribe to the alien spacecraft crash theory! Since there has been no trace of a meteorite, or other material e.g. fragments from an asteroid, reported from the site, the generally accepted explanation is that it was the result of a comet impact with the Earth. it is believed that the comet exploded before it hit the ground.

Books:

Comet and Asteroid Impact Hazards on a Populated Earth

Rogue Asteroids and Doomsday Comets : The Search for the Million Megaton Menace That Threatens Life on Earth

Impact! The Threat of Comets and Asteroids

Tycho (Brahe) Tycho Brahe was a Danish astronomer. He did not believe in the Sun centered Solar System. He realised that the system proposed by the Greek astronomer Ptolemy was wrong but could not bring himself to put the Earth anywhere but at the centre of things. He had the Sun and Moon orbiting the Earth with the other planets orbiting the Sun. Not many people were actually convinced by this solution to the problem of planetary motion. Tycho was a superbly accurate observer, his star catalogue was far better than anything that had gone before. His observatory was called Uraniborg. He made such accurate observations of the planets, particularly Mars, that when he died in 1601, his last assistant, Johannes Kepler, was able to use his observations to formulate his three Laws of planetary motion. This is rather ironic given Tycho's belief of an Earth centred System. Tycho's system of the Solar SystemFor more detailed information on Tycho Brahe, use the "Ask Jeeves" searchbox above. Type in 'Tycho Brahe biography' to pull up loads of informative sites
Ultraviolet This is a form of radiation that has a shorter wavelength than that of visible light but longer than that of X-rays. It is generated by very hot objects. The Sun is a strong source of ultra violet radiation but, luckily for life on Earth, ozone in the Earth's atmosphere absorbs UV. This means that telescopes designed to observe faint UV sources in the sky have to be placed in Earth orbit. The satellite IUE (International Ultraviolet Explorer) was launched on 26 Jan 1978 and mapped the entire sky using UV wavelengths. This reliable and sturdy satellite way exceeded it design lifetime of 3 years. Extreme UV wavelengths are close to being X-rays and ROSAT (Röntgen satellite) was designed to investigate both. The Faint Object Camera of the Hubble Space Telescope has been used to image stars using UV wavelengths - it has been used to image the centre regions of globular clusters, 47 Tucanae for example. In the latter, several hundred stars have now been imaged and 21 or more extremely hot stars have been newly identified because they are emitting high levels of UV. IUE satellite picture Click for a diagram of the electromagnetic spectrum
Umbra The darker part of a shadow during an eclipse. An observer standing in the umbra of a Solar eclipse will see a total eclipse. This word can also mean the darker part of a sunspot. See also penumbra
Universal Time (UT) The same as Greenwich Mean Time (GMT)
Vallis The term applied to a valley on a planet other than Earth. Click here for a basic table of selected Vallis of the Solar System
Van Allen belts Two doughnut shaped volumes of space around the Earth. They were discovered by the US satellite Explorer 1, launched I believe in January 1958. One experiment was designed to measure the particle density in space around the Earth. At one point, the detectors were seriously overloaded, indicating a much higher density than expected. The Van Allen belts are named after the scientist who was running that experiment, James Van Allen. They are created where the magnetic field of the Earth interacts with the particles of the Solar wind. The particles are trapped in the Van Allen belts. When there is a solar flare, the Van Allen belts overflow. The trapped particles enter the Earth's atmosphere at the poles and form aurorae as they interact with the gases of our atmosphere.
Vastitas A term applied to an extensive plain on a planet other than Earth.
Venus The second planet out from the Sun in our Solar System. For more detailed information click here.
Waning When the phase of an object is reducing. It is most usually encountered when describing the phases of the Moon.
Wavelength This is defined as the distance from a given point on a wave to the same point on the next wave. There are a variety of different ways of expressing wavelengths, for short wavelength waves, nanometres or angstroms (one tenth of a nanometre) are often used. You can work out the wavelength by dividing the speed of the wave by the frequency of the wave. The speed of a wave can be calculated by multiplying the frequency by the wavelength or the frequency can be calculated by dividing the speed by the wavelength.
Waxing When the phase of an object is increasing. It is most usually encountered when describing the phases of the Moon.
White dwarf

A tiny, hot star that marks the end of the life of a star similar in size to the Sun. They contain a similar mass of matter to the Sun but compressed into a volume comparable to that of the Earth. All nuclear reactions have ceased in white dwarf stars so they gradually cool down.

Widmanstätten pattern This is a characteristic pattern that is found when an iron meteorite is cut open, etched with acid and polished. It is named after Alois Josep Widmanstätten, a Viennese scientist who discovered it in 1808. The pattern is caused by the crystal structure of the meteorite. The crystals are made from nickel-iron alloys named kamacite and taenite. Widmanstätten pattern
WIMP Acronym for Weakly Interacting Massive Particle. A theoretical particle that contributes to the 'missing mass' of the universe.
Zeeman effect When taking a spectrum of a star, sometimes it is observed that the spectral lines are duplicated. This splitting of the lines is called the Zeeman effect. It is caused by a magnetic field. The lines can be split into ,ore than two if the field is strong enough.
Zenith The point directly overhead of an observer.
Zenith distance The angular distance of an astronomical object from the zenith.
Zodiac The belt along the ecliptic through which the Sun, Moon and brighter planets always appear to travel. It is 8 degrees either side of the ecliptic and passes through 13 constellations or part constellations.
Zodiacal constellations The 12 constellations that are used in the false science of astrology. This tends to ignore the 13th constellation through which the zodiac passes - Ophiuchus
Zodiacal light This is best seen on clear Moonless evenings or mornings. It is a cone shaped faint light that rises from the horizon and stretches along the ecliptic. It is visible only when the Sun is a little below the horizon. It is thought to be caused by particles that are spread throughout the plane of the Solar System.

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