An Etymological Dictionary of Astronomy and Astrophysics
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Local disturbances in the ring structure caused by the gravitational influence of embedded satellites. If the satellite (moonlet) is large enough to clear a gap in the rings, the moonlet wakes become edge waves that precede the satellite on the inner edge and trail the satellite on the outer edge. For smaller satellites, the "gap-less" wakes have been nicknamed propellors (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ moonlet; → wake.

The light of the Moon.

From → moon + → light.

Mahtâb (Gilaki mângtâb) from mah, mâh (mâng), → moon, + tâb "light," from tâbidan, tâftan "to shine," tafsidan "to become hot" (Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth;" cf. Skt. tap- "to spoil, injure, damage; to suffer; to heat, be/become hot," tapati "burns;" L. tepere "to be warm," tepidus "warm;" PIE base *tep- "warm").

A → seismic event occurring on the → Moon; the lunar equivalent of an → earthquake. Moonquakes were first detected by the → seismometers placed on the Moon by Apollo astronauts from 1969 through 1972. The instruments placed by the Apollo 12, 14, 15, and 16 missions were functional until 1977. Unlike earthquakes, moonquakes are not believed to be caused by → tectonic plate movement, but by → tidal forces between Earth and the Moon. There are at least four different kinds of moonquakes: (1) deep moonquakes about 700 km below the surface. They occur at monthly intervals at about 100 distinct sites, indicating that these moonquakes are caused by → stresses from changes in lunar tides as the Moon orbits the Earth; (2) vibrations from the impact of → meteorites; (3) thermal quakes caused by the expansion of the frigid crust when first illuminated by the morning sun after two weeks of deep-freeze lunar night; and (4) shallow moonquakes only 20 or 30 km below the surface (science.nasa.gov/science-news).

→ moon; → quake.

The times at which the apparent upper limb of the ascending Moon is on the astronomical horizon.

→ moon; → rise.

The crossing of the visible horizon by the upper limb of the descending Moon.

→ moon; → set.

The delay between → sunset and → moonset.

→ moonset; → lag.

A large-scale → shock wave observed in Hα on the Sun's → chromosphere that is generated by the impact of a → solar flare. Moreton waves expand outward at about 1,000 km/s, and may travel for several hundred thousand kilometers. They are accompanied by meter-wave radio bursts.

Named after the American astronomer Gail E. Moreton (1960, A.J. 65, 494); → wave.

A system of → spectral classification introduced in 1943 by William W. Morgan (1906-1994), Philip C. Keenan (1908-2000), and Edith M. Kellman (1911-2007) at Yerkes Observatory. Also known as the MK (or MKK) classification or the → Yerkes system.

Named after the two main astronomers, as above; → classification.

A classification scheme of galaxies based on their apparent shape. → Hubble classification.

→ morphological; → classification.

An observationally determined relationship between the → morphological classification of galaxies and the → environments in which they are located. Specifically, the morphology-density relation indicates that early-type galaxies (→ ETG) are preferentially located in high density environments, whereas late-type galaxies (→ LTG) are preferentially found in low density environments. Hence, spiral galaxies are rare in the high densities of clusters and are common in the lower density group environments. Early-type galaxies, on the other hand, are common in clusters and are rarely found in isolation.

→ morphology; → density; → relation.

The action or process of moving or of changing place or position; movement.

Verbal noun of → move.

An observatory situated on a mountain 1700 m above sea level near Pasadena, California. It was built in 1904 by American astronomer George Ellery Hale as a solar-observing station for the Yerkes Observatory, but it became an independent observatory funded by the Carnegie Institution of Washington. In 1908 a 60-inch (152-cm) reflector, then the largest in the world, was added for observations of stars and galaxies. Ten years later a 100-inch (254-cm) reflecting telescope was put into service. It was the most powerful telescope in the world until the construction of the Palomar 200-inch reflector in 1948. The 100-inch telescope's most important discovery was Edwin Hubble's determination of the distance to the Andromeda Nebula in 1924. He showed that the nebula lay beyond the bounds of the Milky Way Galaxy and hence was a galaxy in its own right. Then in 1929, following the work of Vesto Slipher, Hubble and his assistant Milton Humason demonstrated that galaxies were moving away from one another. This movement is the expansion of the Universe.

→ mountain; a peak of the San Gabriel Mountains, located in northern Los Angeles County, California, named after Benjamin D. Wilson (1811-1878) a California statesman and politician; → observatory .

In general, the process of repeatedly adding a quantity to itself a certain number of times, or any other process which has the same result.

Verbal noun of → multiply.

The sign used to indicate multiplication, either a times sign (×), a centered dot (·), or an asterisk. The multiplication sign was introduced by William Oughtred in 1631.

→ multiplication; → sign.

Same as → Charpak's detector.

→ multi-; → wire; → proportional; → chamber.

A short-lived → elementary particle with negative → electric charge, represented by the symbol μ^-. The muon was discovered in 1936 by Carl Anderson (1905-1991) in → cosmic rays. It shares several properties with the electron: it is a → lepton with the same charge and → spin as the electron. But it is heavier than the electron (105 MeV/c²), about 200 times more massive. The muon is instable and decays after 2.197 × 10^-6 s into → electron, → neutrino, and → antineutrino (μ^-→ e^- + ν_μ + anti ν_e) .

A shortening of mu meson, from mu the 12th letter of the Gk. alphabet, + → meson.

An → instrument used in → geophysics to determine the average → density of geological bodies by measuring the → attenuation produced by → rocks on the flux of → atmospheric muons. This density muon → radiography is or example used to study the physical conditions inside → volcanoes.

→ muon; → telescope.

A sudden, random change in the genetic material of a cell. → commute.

From L. mutationem (nominative mutatio) "a changing," from mutatus, p.p. of mutare "to change," from PIE base *mei- "to change, go, move;" cf. Av. miθô "inverted, false," miθaoxta- "wrong spoken;" Skt. methati "changes, alternates, joins, meets," mith- "to alternate, meet," mithás "opposite, in opposition;" L. meare "to go, pass," mutuus "done in exchange;" Goth. maidjan "to change;" E. prefix mis- (in mistake).

Muteš, verbal noun of mutidan, from L. mutare, cognate with Av. miθô, as above, + -idan infinitive suffix.

The act of mystifying or the condition of being mystified.

→ mystify; → -tion.

The conductivity in a semiconductor caused by a flow of electrons, whereas p-type conductivity is caused by a flow of holes.

n standing for → negative; → type; → conductivity.