An Etymological Dictionary of Astronomy and Astrophysics
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A → transit circle that defines the position of the → Greenwich Meridian since the first observation was taken with it in 1851. Airy's transit circle lies at longitude 0°, by definition, and latitude 51° 28' 38'' N.

Named after Sir George Biddell Airy (1801-1892), Astronomer Royal, at the Royal Observatory in Greenwich from 1835 to 1881. Airy transformed the observatory, installing some of the most advanced astronomical apparatus of his day and expanded both staff numbers and their workload; → transit; → circle.

A change in the → energy level or → state of an → atom in which a → quantum of energy is either gained or lost. See also → forbidden transition; → permitted transition; → semiforbidden transition.

→ atomic; → transition.

A transition between two energy levels of an electron bound to a nucleus. The electron remains tied to the nucleus before and after the transition. → bound-free transition; → free-free emission.

Bound, p.p. of → bind; → transition.

A transition in which a bound electron is liberated. → free-bound emission; → free-free emission.

Bound, p.p. of → bind; → free.

A photon generation mechanism in an atom in which a transition initiates a series of secondary transitions from lower electronic levels.

→ cascade; → transition.

A transition between two quantum-mechanical energy levels. See also → discrete spectrum.

→ discrete; → transition.

The → transfer of an → electron from one → energy level to another.

→ electronic; → transition.

The passage of a celestial body or point across the → ephemeris meridian.

→ ephemeris; → transit.

The passage of an → exoplanet across the face its star.

→ exoplanetary; → transit.

A transition between two quantum mechanical → states that → violates the quantum mechanical → selection rules.

→ forbidden; → transition.

An → atomic transition involving a → hyperfine structure.

→ hyperfine; → transition.

A change in the → energy level or state of an atomic → nucleus involving a → quantum of energy.

→ nuclear; → transition.

A transition between two quantum mechanical states that does not violate the quantum mechanical selection rules.

→ permitted; → transition.

The changing of a substance from one phase to another, by → freezing, → melting, → boiling, → condensation, or → sublimation. Also known as phase transformation. A well known phase transition is the transition from → water to → ice. Phase transitions are often associated with → symmetry breaking. In water there is a complete symmetry under rotations with no preferred direction. Ice has a crystal structure, in which certain orientations in space are preferred. Therefore, in transition from water to ice the continuous rotational symmetry is lost.

→ phase; → transition.

The passage of an → inferior planet against the disk of the Sun, as viewed from Earth. Mercury and Venus pass in front of the Sun only when they are close to one of their → orbital nodes, at → inferior conjunction. For Mercury this occurs at the beginning of November (the → ascending node) or at the beginning of May (the → descending node), while for Venus it takes place at the beginning of December (the ascending node) or at the beginning of June (the descending node). See also → transit of Mercury, → transit of Venus.

→ planetary; → transit.

A space observatory under development by the → European Space Agency for launch around 2024. Its objective is to detect and characterize → exoplanets by means of their → transit signature in front of a very large sample of → bright stars, and measure the seismic oscillations (→ asteroseismology) of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems.

→ planetary; → transit; → oscillation; → star.

A phase transitions that occurs at zero temperature as a function of a non-thermal parameter like → pressure, → magnetic field, or → chemical composition. In contrast to ordinary → phase transitions, which are associated with passage through a critical temperature, quantum phase transitions are associated with → quantum fluctuations, a consequence of → Heisenberg's uncertainty principle. For example, see → Bose-Einstein condensation.

→ quantum; → phase; → transition.

A phase transition, predicted by cosmological models, to have occurred at approximately 10^-5 seconds after the Big Bang to convert a plasma of free quarks and gluons into hadron.

→ quark; → hadron; → phase; → transition.

A transition between two states of an atomic or molecular entity, the energy difference being emitted or absorbed as photons.

→ radiative; → transition.

A slight change in the energy level of a molecule due to the rotation of its constituent atoms about their center of mass.

→ rotational; → transition.