An Etymological Dictionary of Astronomy and Astrophysics
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A time interval lasting 235 → lunations, or about 19 → tropical years (235 = 19 x 12 + 7), after which → lunar phases recur on the same days of the year.

Named after Meton of Athens, a Gk. mathematician, astronomer, geometer, and engineer who used it in 432 B.C., but it was known to the Babylonians by around 500 B.C. and to the Chinese around 600 B.C.; → cycle.

1) A mathematical → expression consisting of an → array of → components which are needed for calculating → infinitesimally small → distances between two → points in some geometrical → space. More simply put, the → function used to define a distance between two points in a → metric space. Also called → distance function.
2) In → general relativity the metric describes the → space-time geometry and gives the interval between two neighboring → events.
3) Pertaining to the meter or to the → metric system.

From → meter + → -ic.

Any of the suffixes adopted by the International System of Units (→ SI units).
For numbers larger than 1: → deca- (10¹), → hecto- (10²), → kilo- (10³), → mega- (10⁶), → giga- (10⁹), → tera- (10¹²), → peta- (10¹⁵), → exa- (10¹⁸), → zetta- (10²¹), and → yotta- (10²⁴) .
For numbers smaller than 1: → deci- (10^-1), → centi- (10^-2), → milli- (10^-3), → micro- (10^-6), → nano- (10^-9), → pico- (10^-12), → femto- (10^-15), → atto- (10^-18), → zepto- (10^-21), and → yocto- (10^-24).

→ metric; → prefix.

An set of points such that the distance between every pair of points is defined by a → distance function with the following properties: 1) the distance from the first point to the second equals zero if and only if the points are the same, 2) the distance from the first point to the second equals the distance from the second to the first, and 3) the sum of the distance from the first point to the second and the distance from the second point to a third exceeds or equals the distance from the first to the third.
In mathematical language, the properties, for a nonempty set X, can be expressed as:
1) d(x,y)≥ 0 and d(x,y) = 0 if and only if x = y.
2) d(x, y) = d(y,x) for all x, y ∈ X.
3) d(x,z)≤ d(x,y) + d(y,z) for all x, y, and z ∈ X. Also called → triangle inequality.

→ metric; → space.

A standard system of measurement using decimal units, in which the units of length, time, and mass are meter, second, and kilogram respectively.

→ metric; → system.

The abstract tensor operation which is computed in a particular → reference frame using the → metric components. The metric tensor defines magnitude and direction of vectors about a point.

→ metric; → tensor.

A physical → measurement unit in the → metric system.

→ metric; → unit.

The science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology.

From metro-, a combining form meaning "measure," → meter, + → -logy.

Andâze-šenâsi, from andâzé, → measure, + -šenâsi, → -logy.

Mega (million) → electron volt. A unit of → energy used to describe the total energy carried by a → particle or → photon.

→ mega- + → electron volt.

→ ideal magnetohydrodynamics.

→ magnetohydrodynamics (MHD); → condition.

An older name for the unit of electrical → conductance, which is defined to be the reciprocal of the → ohm. It is now replaced by the → siemens.

Ohm spelt backward.

A shiny silicate mineral composed of varying amounts of → aluminum, → potassium, → magnesium, and → iron. Mica appears as thin, flexible layers in → granite and other rocks, or as → crystals. It is used as a thermal or electrical → insulator.

From L. mica "crumb, grain."

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.

An experiment performed in 1887 to establish the presence or absence of an → ether, a medium through which light was supposed to travel. The experiment aimed to measure the speed of light coming from different directions. However no → ether drift was found. The null results obtained showed that the ether hypothesis was incorrect. Consequently, the theory of → special relativity, with its hypothesis that the speed of light is the same in all → inertial frames, reconciled the results of the Michelson-Morley experiment with the rest of physics.

→ Michelson interferometer; Michelson received the Nobel Prize in 1907 for his work, the first American to receive the Prize in science. Edward Williams Morley (1838-1923), an American chemist; → experiment.

Same as → apogee full Moon.

→ micro-; → Moon.

1) A combining form for "small."
2) A prefix meaning one millionth (10^-6).

From Gk. mikros "small."

Riz-, from riz "very small."

The area of computer science dealing with the use and development of microcomputers, and related peripheral devices and softwares. Also microinformatics.

→ micro-; → compute.

Riz-, → micro-; azdâyik, → informatics.

Anything that is regarded as a world in miniature.
A unity that is an epitome of a larger unity; opposed to macrocosm.

→ micro-; → cosmos.

The state or condition where the force of → gravity is very weak, e.g. the → weightlessness experienced inside an orbiting spacecraft.

→ micro-; → gravity.

A type of → gravitational lens, where the foreground → lensing object is of low mass, and the multiple images produced are too close together on the sky to be observed as separate images. Gravitational microlensing occurs when a foreground star happens to lie very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the background source star into two or more images, which are typically unresolved. However, these images of the source are magnified, by an amount that depends on the angular separation between the lens and source. If with the passage of time the lens moves across the Earth-source, the amount of brightening changes. Typically the source will appear to brighten, reach a maximum and then fade symmetrically back to normal over the course of a few weeks or months; this is called a → microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived signature of the planet. Microlensing is used in the search for → dark matter in the → Milky Way galaxy and its nearest neighbours, as well as for → extrasolar planets (e.g. B. S. Gaudi, 2010, arXiv:1002.0332).

→ micro-; → lensing.