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

1) Natural satellite of the Earth. Mass 7.35 x 10²⁵ g = 1/81 or 0.0123 Earth's. Mean radius 1740 km = ~ 1/4 the Earth's; this relatively small size ratio makes the Earth-Moon system unique in the → solar system. Mean density 3.34 g cm^-3. Mean distance from Earth 384,400 km. → Escape velocity 2.38 km s^-1. → Surface gravity 162.2 cm s^-2 = 0.165 Earth's. → Sidereal period 27d 7h 43m 11s. → Eccentricity 0.0549. → Inclination of → orbital plane to → ecliptic 5° 8' 43''. → Obliquity 6° 41'. → Synodic period 29d 12h 44m 2s.9. → Orbital velocity 1.02 km s^-1. The Moon's average visual → Albedo is 0.12, a factor of three smaller than that of Earth. The Moon's → center of mass is displaced about 2 km in the direction of Earth. The average temperature on the surface of the Moon during the day is 107 °C. During the night, the average temperature drops to -153 °C. Studies of lunar rock have shown that melting and separation must have begun at least 4.5 x 10⁹ years ago, so the → crust of the Moon was beginning to form a very short time after the → solar system itself. Thickness of crust ~ 60 km; of mantle ~ 1000 km. Temperature of core ~ 1500 K. It would have taken only 10⁷ years to slow the Moon's rotation into its present lock with its → orbital period. Because of this → synchronous rotation, the Moon revolves once on its axis each time it orbits the Earth, thus always presenting the same face, the nearside, toward Earth. The Moon may have formed during a collision between the early Earth and a Mars-sized rocky planet about 4.6 billion years ago; → Theia.
2) A large body orbiting a planet.

O.E. mona, from P.Gmc. *mænon- (cf. O.S., O.H.G. mano, O.Fris. mona, O.N. mani, Du. maan, Ger. Mond, Goth. mena "moon"), cognate with Pers. mâh, as below, from PIE *me(n)ses- "moon, month."

Mâh and mâng in Pers. are variants of the same term, the dominant form being mâh, while mâng (Av. from, see below) is used in classical literature as well as in some dialects: Tabari, Kurd. mâng, Laki, Tâti, Taelši mong, Šahmirzâdi, Sangesari mung; Mid.Pers. mâh "moon, month;" O.Pers. māha- "moon, month;" Av. māh- "month, moon," also māwngh-; cf. Skt. mās- "moon, month;" Gk. mene "moon," men "month;" L. mensis "month;" O.C.S. meseci, Lith. menesis "moon, month;" O.Ir. mi, Welsh mis, Bret. miz "month;" O.E. mona; E. moon, month; Ger. Mond, Monat; Du. maan; PIE base *me(n)ses- "moon, month."

Note: In Persian the same term, mâh, is used for two different, but related, concepts: moon and month. This was also the case for other IE languages, as shows the above etymology. However, other IE languages have evolved toward more accuracy by using different forms of the same initial term, as in E. moon / month or Ger. Mond / Monat. The Latin family uses two unrelated words, as in Fr. lune "moon" / mois "month" and Sp. luna / mes. An additional difficulty in present Pers. is that the adj. mâhi not only means "lunar" and "monthly" it also denotes "fish." For the sake of clarity and precision, this dictionary uses mângi for "lunar."

Any of several theories about how the → Moon originated, among which: → fission theory, → capture theory, → co-formation theory, and → giant impact hypothesis. The model that is best supported by all the available data is the giant impact hypothesis. See also → canonical model.

→ lunar; → formation.

Same as → lunar halo.

→ Moon; → halo.

1) The number of days that have elapsed since the last → conjunction of the Sun and Moon. It is 7 days at → first quarter, 15 days at → full moon, and 22 days at → third quarter.
2) The time past since the → formation of the Moon.

→ Moon; → age.

1) Kohan-ruzi literally "age in days," from kohan-ruz "old in days," from kohan "old, ancient," kohné "worn;" Mid.Pers. kahwan "old, aged, worn;" pir; Mid.Pers. pir "old, aged, ancient;" Av. parô (adv.) "before, before (of time)," in front (of space); cf. Skt. puáh, combining form of puras "before (of time and place), in front, in advance;" mâh, → Moon.
2) Senn, → age.

The → rotation of the Moon's → orbit within the → orbital plane, whereby the axes of the ellipse change direction. The Moon's → major axis makes one complete revolution every 8.85 Earth years, or 3,232.6054 days, as it rotates slowly in the same direction as the Moon itself (direct, or → prograde motion). The Moon's apsidal precession is a → relativistic effect, and should not be confused with its → axial procession.

→ Moon; → apsidal; → precession.

The line connecting the Moon's north pole with its south pole, and passing through the center of the lunar disk. The longitude of the Moon's prime meridian is zero degrees.

→ Moon; → prime meridian.

A rainbow that arises from the refraction and reflection of moonlight on rain drops or mist.

→ moon; → bow.

A very small natural or artificial satellite orbiting a planet. Saturn has dozens of moonlets often associated with its → planetary rings.

→ moon; → -let.

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 unique feature of → Pluto's large satellite → Charon. It appears as a dark reddish area about 475 km in diameter in Charon's north polar region, as revealed in → New Horizons' approach images. It has been proposed that the feature is due to gas from Pluto. A part of Pluto's → atmosphere (→ methane molecules) is transiently cold-trapped and processed at Charon's winter pole (W. M. Grundy et al., 2016, Nature, 14 September).

Unofficial name.

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.

The first part or period of the day, extending from dawn, or from midnight, to noon. Not a precise astronomical term.

From M.E. morn, morwen, from O.E. margen earlier morgen (cf. O.H.G. morgen, Goth. maurgins) + → -ing.

Bâmdâd, from Mid.Pers. bâmdâd "morning, dawn," from bâm "beam of light, splendor," bâmik "brilliant" (from Av. *bāma- "light," bāmya- "light, luminous, bright," vīspô.bāma- "all resplendent," from bā- "to shine;" cf. Skt. bhāti- "light, splendor") + dâd "given," from dâdan "to give" (O.Pers./Av. dā- "to give, grant, yield," dadāiti "he gives;" Skt. dadáti "he gives;" Gk. tithenai "to place, put, set," didomi "I give;" L. dare "to give, offer," facere "to do, to make;" Rus. delat' "to do;" O.H.G. tuon, Ger. tun, O.E. don "to do;" PIE base *dhe- "to put, to do").

Not actually a star, but the planet Venus shining brightly in the east just before or at sunrise. Opposed to → evening star.

→ morning; → star.

Rujâ "morning star" in Tabari, "star" in Gilaki. This word is a variant of official Pers. ruz "day," since in Tabari and Gilaki the phoneme z is sometimes changed into j, as in rujin = rowzan "window" and jir or jer = zir "under." Therefore it is related to rowšan "bright, clear," rowzan "window, aperture;" foruq "light," afruxtan "to light, kindle;" Mid.Pers. rôšn "light; bright, luminous," rôc "day;" O.Pers. raucah-rocânak "window;" O.Pers. raocah- "light, luminous; daylight;" Av. raocana- "bright, shining, radiant;" akin to Skt. rocaná- "bright, shining," roka- "brightness, light;" Gk. leukos "white, clear;" L. lumen (gen. luminis) "light," from lucere "to shine," related to lux "light," lucidus "clear," luna, "moon;" Fr. lumière "light;" O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- "light, brightness;" → morning; → star.

A → mapping between two → objects in a → category.

→ morphology; → -ism.

From rixt, → morphology, + -mandi, → ism.