An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A ratio representing the number of ionizing photons to the number of electrons in a nebular emitting region.

→ ionization; → parameter.

A unit of length, equal to 1000 meters.,

→ kilo-; → meter.

An optical instrument using laser → beams to form → interference pattern. There are two types of laser interferometers: → homodyne and → heterodyne. A homodyne interferometer, like → Michelson interferometer, uses a single-frequency laser source. A → heterodyne interferometer uses a laser source with two close frequencies.

→ laser; → interferometer.

A facility dedicated to the detection and measurement of cosmic → gravitational waves. It consists of two widely separated installations, or detectors, within the United States, operated in unison as a single observatory. One installation is located in Hanford (Washington) and the other in Livingston (Louisiana), 3,000 km apart. Funded by the National Science Foundation (NSF), LIGO was designed and constructed by a team of scientists from the California Institute of Technology, the Massachusetts Institute of Technology, and by industrial contractors. Construction of the facilities was completed in 1999. Initial operation of the detectors began in 2001. Each LIGO detector beams laser light down arms 4 km long, which are arranged in the shape of an "L." If a gravitational wave passes through the detector system, the distance traveled by the laser beam changes by a minuscule amount -- less than one-thousandth of the size of an atomic nucleus (10^-18 m). Still, LIGO should be able to pick this difference up. LIGO directly detected gravitational waves for the first time from a binary → black hole merger (GW150914) on September 14, 2015 (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102). The Nobel Prize in physics 2017 was awarded to three physicists (Rainer Weiss, Barry C. Barish, and Kip S. Thorne) for decisive contributions to the LIGO detector and the observation of gravitational waves. LIGO had a prominent role in the detection of → GW170817, the first event with an → electromagnetic counterpart.

→ laser; → interferometer; → gravitational; → wave; → observatory.

A collaborative project between → NASA and → ESA to develop and operate a space-based gravitational wave detector sensitive at frequencies between 0.03 mHz and 0.1 Hz. LISA detects gravitational-wave induced strains in → space-time by measuring changes of the separation between fiducial masses in three spacecraft 5 million km apart. Ultimately, NASA and ESA decided in 2011 not to proceed with the mission. LISA was not the highest ranked mission in the 2010 Decadal Survey and funding constraints prevented NASA from proceeding with multiple large missions (http://lisa.nasa.gov). → LISA pathfinder.

→ laser; → interferometer; → space; → antenna.

The real physical diameter, as opposed to angular diameter.

→ linear; → diameter.

Any of a variety of devices used to measure the strength and direction of a magnetic field.

From → magneto- + → -meter.

The → SI unit of length; symbol m. It is defined by taking the fixed numerical value of the → speed of light in vacuum, c, to be 299 792 458 when expressed in the unit m/s, where the → second is defined in terms of Δν_Cs.

From Fr. mètre, from Gk. metron "measure," from PIE base *me- "to measure" (cf. O.Pers., Av. mā- "to measure;" Skt. mati "measures;" L. metri "to measure").

Metr, loan from Fr.

An apparatus that produces interference fringes by splitting a beam of monochromatic light so that one beam strikes a fixed mirror and the other a movable mirror. When the reflected beams are brought back together, an interference pattern results. It is used to measure very precise lengths, such as the wavelength of light, and for high-resolution spectroscopy.

Named after Albert Abraham Michelson (1852-1931), German-American physicist, who built the interferometer for the → Michelson-Morley experiment of 1887; → interferometer.

Andarzanešsanj, → interferometer.

1) A screw thread device used to make accurate physical linear measurements.
2) Same as → micron.

→ micro-; → -meter.

Microwaves with wavelengths between 1 and 10 millimeter, corresponding to frequencies between 300 GHz to 30 GHz. → millimeter-wave astronomy.

→ milli-; → meter; → wave.

That part of radio astronomy which uses electromagnetic waves in the range 1-10 millimeter to study various components of the Universe, in particular the chemistry of interstellar matter.

→ millimeter wave; → astronomy.

In the → mixing length theory, a parameter, α, that relates the → mixing length, l, to the → pressure scale height: α = l/H_P. It is usually supposed that α is of order unity. Changes in α correspond to variations in the efficiency of the → convection, hence the transfer of heat.

→ mixing; → length; → parameter.

A unit of length equal to 10^-9 m, or 10 Å.

→ nano-; → meter.

A dimensionless parameter expressed by h(z) = H(z)/H₀, where H(z) is the → Hubble parameter at → redshift z and H₀ is the → Hubble constant.

→ normalized; → Hubble; → parameter.

Any of various instruments for measuring refractive errors of the eye.

From opto- "vision; eye; optis," combining form from Gk. optos "seen, visible," from opsesthai "be going to see," related to ops "eye;" → -meter.

Did, → vision; -sanj, → -meter.

→ orbital element.

→ orbital; → parameter.

General: Any of a set of physical properties whose values determine the characteristics or behavior of something. → impact parameter; → ionization parameter.
Math.: A constant or variable term in a function that determines the specific form of the function but not its general nature, as a in f(x) = kx, where k determines only the slope of the line described by f(x).

Mod.L. parametrum, from Gk. → para- + metron "measure," → meter.

Parâmun, from parâ-, → para-, + mun/mân "measure," as in Pers. terms pirâmun "perimeter," âzmun "test, trial," peymân "measuring, agreement," peymâné "a measure; a cup, bowl," from O.Pers./Av. mā(y)- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure."

The total outer boundary of a two-dimensional figure.

→ peri- + → -meter.

An instrument for measuring the amount of light.

→ photo- + → -metry.