An Etymological Dictionary of Astronomy and Astrophysics

Moderately warm; tepid.

Etymology (EN): M.E. lukewarme “tepid,” from luke “tepid,” of unknown origin, + → warm.

Etymology (PE): Velarm “lukewarm, tepid,” of unknown origin.

Moderately warm; tepid.

Etymology (EN): M.E. lukewarme “tepid,” from luke “tepid,” of unknown origin, + → warm.

Etymology (PE): Velarm “lukewarm, tepid,” of unknown origin.

The → SI unit of → luminous flux, equal to the luminous flux emitted per unit solid angle by a standard point source having a → luminous intensity of 1 → candela.

Etymology (EN): L. lumen (gen. luminis) “light,” from lucere “to shine,” related to lux “light,” lucidus “clear,” luna, “moon;” Fr. lumière “light;” cf. Pers. ruz “day,” 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;”
O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- “light, brightness.”

Etymology (PE): Lumen loanword, as above.

The → SI unit of → luminous flux, equal to the luminous flux emitted per unit solid angle by a standard point source having a → luminous intensity of 1 → candela.

Etymology (EN): L. lumen (gen. luminis) “light,” from lucere “to shine,” related to lux “light,” lucidus “clear,” luna, “moon;” Fr. lumière “light;” cf. Pers. ruz “day,” 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;”
O.E. leoht, leht, from W.Gmc. *leukhtam (cf. O.Fris. liacht, M.Du. lucht, Ger. Licht), from PIE *leuk- “light, brightness.”

Etymology (PE): Lumen loanword, as above.

The luminous intensity in a given direction of a small element of surface area divided by the orthogonal projection of this area onto a plane at right angle to the direction. It is measured in candelas per square meter. Luminance is often called surface brightness of the object.

Etymology (EN): From lumin-, combining form of → lumen “light,” + -ance a suffix used to form nouns either from adjectives in -ant or from verbs.

Etymology (PE): Tâbâni, from tâbidan “to shine,” → luminous.

The luminous intensity in a given direction of a small element of surface area divided by the orthogonal projection of this area onto a plane at right angle to the direction. It is measured in candelas per square meter. Luminance is often called surface brightness of the object.

Etymology (EN): From lumin-, combining form of → lumen “light,” + -ance a suffix used to form nouns either from adjectives in -ant or from verbs.

Etymology (PE): Tâbâni, from tâbidan “to shine,” → luminous.

The emission of light at low temperatures by any process other than → incandescence, where a substance emits light without being strongly heated. Luminescence is a collective term for different phenomena, for example: → phosphorescence, → fluorescence, → chemiluminescence, → photoluminescence.

See also: From lumin-, from → lumen; → -escence.

The emission of light at low temperatures by any process other than → incandescence, where a substance emits light without being strongly heated. Luminescence is a collective term for different phenomena, for example: → phosphorescence, → fluorescence, → chemiluminescence, → photoluminescence.

See also: From lumin-, from → lumen; → -escence.

Capable of, suitable for, or exhibiting luminescence.

See also: From lumin-, from → lumen; → -escent.

Capable of, suitable for, or exhibiting luminescence.

See also: From lumin-, from → lumen; → -escent.

The → total → brightness of a star or other astronomical object. It is expressed in watts and represents the total amount of → energy that the object radiates each → second over all wavelength regions of the → electromagnetic spectrum. Because this quantity is independent of distance, it is an → intrinsic brightness.

See also:
→ absolute luminosity, → anomalous luminosity effect, → bolometric luminosity, → color-luminosity diagram, → Eddington luminosity, → H II region luminosity, → intrinsic luminosity, → luminosity class, → luminosity distance, → luminosity function, → luminosity problem, → luminosity-size relation, → mass-luminosity ratio, → mass-luminosity relation, → peak luminosity, → period-luminosity relation, → solar luminosity, → stellar luminosity, → wind luminosity.

See also: Verbal noun of → luminous.

The → total → brightness of a star or other astronomical object. It is expressed in watts and represents the total amount of → energy that the object radiates each → second over all wavelength regions of the → electromagnetic spectrum. Because this quantity is independent of distance, it is an → intrinsic brightness.

See also:
→ absolute luminosity, → anomalous luminosity effect, → bolometric luminosity, → color-luminosity diagram, → Eddington luminosity, → H II region luminosity, → intrinsic luminosity, → luminosity class, → luminosity distance, → luminosity function, → luminosity problem, → luminosity-size relation, → mass-luminosity ratio, → mass-luminosity relation, → peak luminosity, → period-luminosity relation, → solar luminosity, → stellar luminosity, → wind luminosity.

See also: Verbal noun of → luminous.

A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star’s → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars,
or → main-sequence stars).
Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.

See also: → luminosity; → class.

A classification of stellar spectra according to luminosity for a given → spectral type. The luminosity class is an indication of a star’s → surface gravity. It is shown by a Roman numeral as follows: I (→ supergiants), II (bright → giants), III (normal giants), IV (→ subgiants), and V (→ dwarf stars,
or → main-sequence stars).
Luminosity classes VI (→ subdwarfs) and VII (→ white dwarfs) are rarely used. Subclasses a, b, and c are especially used for supergiants, while the most luminous → hypergiants are assigned luminosity class Ia-0.

See also: → luminosity; → class.

1. Distance derived by comparison of → observed and → intrinsic luminosities. If an object has a known luminosity L, and the observed flux is S, the luminosity distance is defined by D_L = (L/4πS)^1/2.
   
   

2. In cosmology, the → expansion of the Universe results in a diminution of the photon flux and the above equation fails. The reason is that for a homogeneous and isotropic Universe (→ Robertson-Walker metric), the luminosity decreases by a factor (1 + z)⁴. Therefore, the luminosity distance is related to the → angular diameter distance (D_A) by: D_L = (1 + z)².D_A.

See also: → luminosity; → distance.

1. Distance derived by comparison of → observed and → intrinsic luminosities. If an object has a known luminosity L, and the observed flux is S, the luminosity distance is defined by D_L = (L/4πS)^1/2.
   
   

2. In cosmology, the → expansion of the Universe results in a diminution of the photon flux and the above equation fails. The reason is that for a homogeneous and isotropic Universe (→ Robertson-Walker metric), the luminosity decreases by a factor (1 + z)⁴. Therefore, the luminosity distance is related to the → angular diameter distance (D_A) by: D_L = (1 + z)².D_A.

See also: → luminosity; → distance.

Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.

See also: → luminosity; → function.

Number → distribution of → stars or galaxies (→ galaxy) with respect to their → absolute magnitudes. The luminosity function shows the → number of stars of a given intrinsic luminosity (or the number of galaxies per integrated magnitude band) in a given → volume of space.

See also: → luminosity; → function.

Low-mass → protostars are about an order of magnitude less luminous than expected. Two possible solutions are that → low-mass stars form slowly, and/or protostellar → accretion is episodic. The latter accounts for less than half the missing luminosity. The solution to this problem relates directly to the fundamental question of the time required to form a low-mass star (McKee & Offner, 2010, astro-ph/1010.4307).

See also: → luminosity; → problem.

Low-mass → protostars are about an order of magnitude less luminous than expected. Two possible solutions are that → low-mass stars form slowly, and/or protostellar → accretion is episodic. The latter accounts for less than half the missing luminosity. The solution to this problem relates directly to the fundamental question of the time required to form a low-mass star (McKee & Offner, 2010, astro-ph/1010.4307).

See also: → luminosity; → problem.

The relation between the stellar luminosity of a galaxy and its physical size. More at → mass-size relation.

See also: → luminosity; → size; → relation.

The relation between the stellar luminosity of a galaxy and its physical size. More at → mass-size relation.

See also: → luminosity; → size; → relation.

Radiating light or other types of electromagnetic energy.

Etymology (EN): From L. luminosus “shining, full of light,” from → lumen (gen. luminis) “light,” related to lucere “to shine.”

Etymology (PE): Tâbân “luminous,” verbal adj. of tâbidan “to shine,” variants tâftan “to shine,” tafsidan “to become hot,” related to tâb “heat, burning; heated iron; torment,” âftâb “sunshine,” tâbé “frying-pan,” tab “fever;” dialect of Gaz tôu-, tôwâ “to shine;” Khotanese ttav- “to be hot;” Mid.Pers. tâftan “to heat, burn, shine;” taftan “to become hot;” Parthian tâb- “to shine;”
Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf. Skt. tap- “to heat, be/become hot; to spoil, injure, damage; to suffer,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “to be warm.”

Radiating light or other types of electromagnetic energy.

Etymology (EN): From L. luminosus “shining, full of light,” from → lumen (gen. luminis) “light,” related to lucere “to shine.”

Etymology (PE): Tâbân “luminous,” verbal adj. of tâbidan “to shine,” variants tâftan “to shine,” tafsidan “to become hot,” related to tâb “heat, burning; heated iron; torment,” âftâb “sunshine,” tâbé “frying-pan,” tab “fever;” dialect of Gaz tôu-, tôwâ “to shine;” Khotanese ttav- “to be hot;” Mid.Pers. tâftan “to heat, burn, shine;” taftan “to become hot;” Parthian tâb- “to shine;”
Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf. Skt. tap- “to heat, be/become hot; to spoil, injure, damage; to suffer,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “to be warm.”

A high-luminosity variable star, which represents a transition phase in the life of a massive star when it evolves off the main sequence to become a supernova. Only about a dozen confirmed LBVs are presently known in our Galaxy. → Hubble-Sandage variable.

See also: → luminous; → blue; → variable.

A high-luminosity variable star, which represents a transition phase in the life of a massive star when it evolves off the main sequence to become a supernova. Only about a dozen confirmed LBVs are presently known in our Galaxy. → Hubble-Sandage variable.

See also: → luminous; → blue; → variable.

A measure indicating the ability of a light source to emit visible light using a given amount of → power. It is a ratio of the visible energy emitted to the power that goes into the bulb from the electrical line.

See also: → luminous; → efficacy.

A measure indicating the ability of a light source to emit visible light using a given amount of → power. It is a ratio of the visible energy emitted to the power that goes into the bulb from the electrical line.

See also: → luminous; → efficacy.

A measure of the rate of flow of luminous energy, evaluated according to its ability to produce a visual sensation. It is measured in lumens.

See also: → luminous; → flux.

A measure of the rate of flow of luminous energy, evaluated according to its ability to produce a visual sensation. It is measured in lumens.

See also: → luminous; → flux.

A galaxy that emits most of its energy in the infrared and
whose infrared luminosity (in the 8-1000 µm range) is more
than 10¹¹ solar luminosities. → ultraluminous infrared galaxy.

See also: → luminous; → infrared; → galaxy.

A galaxy that emits most of its energy in the infrared and
whose infrared luminosity (in the 8-1000 µm range) is more
than 10¹¹ solar luminosities. → ultraluminous infrared galaxy.

See also: → luminous; → infrared; → galaxy.

A measure of the amount of light that a point source radiates in a given direction. It is expressed by the luminous flux per unit leaving the source in the direction per unit of solid angle.

See also: → luminous; → intensity.

A measure of the amount of light that a point source radiates in a given direction. It is expressed by the luminous flux per unit leaving the source in the direction per unit of solid angle.

See also: → luminous; → intensity.

Ordinary baryonic matter that can emit electromagnetic radiation, as opposed to → dark matter.

See also: → luminous; → matter.

Ordinary baryonic matter that can emit electromagnetic radiation, as opposed to → dark matter.

See also: → luminous; → matter.

A stellar explosion thought to be caused by the → merger of stars in a → binary system. They are characterized by a distinct red color, and a → light curve that lingers with resurgent brightness in the → infrared. The luminosity of the explosion is between that of a → supernova and a → nova.

See also: → luminous; → red; → nova

A stellar explosion thought to be caused by the → merger of stars in a → binary system. They are characterized by a distinct red color, and a → light curve that lingers with resurgent brightness in the → infrared. The luminosity of the explosion is between that of a → supernova and a → nova.

See also: → luminous; → red; → nova

Of or pertaining to the moon.

Etymology (EN): From O.Fr. lunaire, from L. lunaris “of the moon,” from luna “moon” (with capital L) “moon goddess,” from *leuksna- (cf. O.C.S. luna “moon,” O.Pruss. lauxnos “stars,” M.Ir. luan “light, moon”), from the same source as lux, lumen “light;” cognate with Pers. ruz, → day, rowšan “bright, clear.”

Etymology (PE): 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, Taeši 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 precison, this dictionary uses mângi for “lunar.”

Of or pertaining to the moon.

Etymology (EN): From O.Fr. lunaire, from L. lunaris “of the moon,” from luna “moon” (with capital L) “moon goddess,” from *leuksna- (cf. O.C.S. luna “moon,” O.Pruss. lauxnos “stars,” M.Ir. luan “light, moon”), from the same source as lux, lumen “light;” cognate with Pers. ruz, → day, rowšan “bright, clear.”

Etymology (PE): 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, Taeši 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 precison, this dictionary uses mângi for “lunar.”

A calendar that is based on the cycles of the → lunar phase and involves the → lunar month and
→ lunar year. For example → Islamic calendar, → Vietnamese lunar calendar.

See also: → lunar; → calendar.

A calendar that is based on the cycles of the → lunar phase and involves the → lunar month and
→ lunar year. For example → Islamic calendar, → Vietnamese lunar calendar.

See also: → lunar; → calendar.

A → crater on the surface of the Moon.

See also: → lunar; → crater.

A → crater on the surface of the Moon.

See also: → lunar; → crater.

The interval between two successive sunrises for an observer standing on the Moon. This is not the rotational period of the Moon, because the Moon-Earth system has moved round the Sun during that period. It is equal to the length of a → synodic month (29.5306 days).

See also: → lunar; → day.

The interval between two successive sunrises for an observer standing on the Moon. This is not the rotational period of the Moon, because the Moon-Earth system has moved round the Sun during that period. It is equal to the length of a → synodic month (29.5306 days).

See also: → lunar; → day.

A fine, powder-like dust covering the Moon’s surface. → regolith. It is formed when meteoroids crash on the Moon’s surface, heating and pulverizing rocks, which contain silica and metals. Since there is no wind or water to smooth rough edges, the tiny grains are sharp and jagged, and cling to nearly everything. Their main chemical compositions are SiO₂ (about 45%) and Al₂O₃ (about 15%).
The dust grains have an average size of 19 microns (40% smaller than hair).

See also: → lunar; → dust.

A fine, powder-like dust covering the Moon’s surface. → regolith. It is formed when meteoroids crash on the Moon’s surface, heating and pulverizing rocks, which contain silica and metals. Since there is no wind or water to smooth rough edges, the tiny grains are sharp and jagged, and cling to nearly everything. Their main chemical compositions are SiO₂ (about 45%) and Al₂O₃ (about 15%).
The dust grains have an average size of 19 microns (40% smaller than hair).

See also: → lunar; → dust.

The → darkening of the → Moon which occurs when the Moon enters the → umbra of the → Earth’s shadow. This phenomenon can occur only when the → full Moon is near one of the → lunar nodes of its → orbit around the Earth. There will be a → total eclipse if the entire Moon enters the umbra, otherwise the eclipse will be partial when the Moon is somewhat to the north or south of the node and does not cross the shadow entirely. During the eclipse the Moon looks more or less dark, depending especially on the transparency of the Earth’s → atmosphere. The → refraction of Sun’s light through the atmosphere sometimes gives a red color to the eclipsed Moon. Colored fringes can be seen around the shadow edge during → partial eclipses. Because an eclipse of the Moon is due to the cutting off of the Sun’s light, it is visible from the entire hemisphere where the Moon is above the horizon. The maximum duration of a total lunar eclipse, when the Moon passes through the shadow centrally, is 1h 47m (M.S.: SDE).

See also: → lunar; → eclipse.

The → darkening of the → Moon which occurs when the Moon enters the → umbra of the → Earth’s shadow. This phenomenon can occur only when the → full Moon is near one of the → lunar nodes of its → orbit around the Earth. There will be a → total eclipse if the entire Moon enters the umbra, otherwise the eclipse will be partial when the Moon is somewhat to the north or south of the node and does not cross the shadow entirely. During the eclipse the Moon looks more or less dark, depending especially on the transparency of the Earth’s → atmosphere. The → refraction of Sun’s light through the atmosphere sometimes gives a red color to the eclipsed Moon. Colored fringes can be seen around the shadow edge during → partial eclipses. Because an eclipse of the Moon is due to the cutting off of the Sun’s light, it is visible from the entire hemisphere where the Moon is above the horizon. The maximum duration of a total lunar eclipse, when the Moon passes through the shadow centrally, is 1h 47m (M.S.: SDE).

See also: → lunar; → eclipse.

The farthest distance from a → lunar orbit node within which, if the Moon happens to be at full, a lunar eclipse may occur. The lunar ecliptic limit extends about 12° on each side of the node.

See also: → lunar; → ecliptic; → limit.

The farthest distance from a → lunar orbit node within which, if the Moon happens to be at full, a lunar eclipse may occur. The lunar ecliptic limit extends about 12° on each side of the node.

See also: → lunar; → ecliptic; → limit.

An extremely thin gathering of gas surrounding the → Moon. It is made up of → atoms and → ions generated at the Moon’s surface by interaction with → solar radiation, → plasma in the Earth’s → magnetosphere, or → micrometeorites.

See also: → lunar; → exosphere.

An extremely thin gathering of gas surrounding the → Moon. It is made up of → atoms and → ions generated at the Moon’s surface by interaction with → solar radiation, → plasma in the Earth’s → magnetosphere, or → micrometeorites.

See also: → lunar; → exosphere.

The Moon’s hemisphere which is not visible from the Earth. The Moon always shows the same face to the Earth, because Earth and Moon are → tidally locked. This means that the period of → lunar rotation on it axis is the same as its sidereal revolution period around the Earth (→ sidereal month). In other words, the Moon is in → synchronous rotation with the Earth. As a result, the same side always faces the Earth.

To be more precise, taking the lunar → libration into account, the Moon presents about 59% of its surface to Earth. → libration in longitude, → libration in latitude, → physical libration, → geometrical libration.

See also: → lunar; → far; → side.

The Moon’s hemisphere which is not visible from the Earth. The Moon always shows the same face to the Earth, because Earth and Moon are → tidally locked. This means that the period of → lunar rotation on it axis is the same as its sidereal revolution period around the Earth (→ sidereal month). In other words, the Moon is in → synchronous rotation with the Earth. As a result, the same side always faces the Earth.

To be more precise, taking the lunar → libration into account, the Moon presents about 59% of its surface to Earth. → libration in longitude, → libration in latitude, → physical libration, → geometrical libration.

See also: → lunar; → far; → side.

See → Moon formation.

See also: → lunar; → formation.

See → Moon formation.

See also: → lunar; → formation.

The study of the → Moon’s → crust, → rocks, strata (→ stratum), etc.

See also: → lunar; → geology.

The study of the → Moon’s → crust, → rocks, strata (→ stratum), etc.

See also: → lunar; → geology.

A light color area on the → Moon, as contrasted with → lunar maria. Also called terra.

See also: → lunar; → highland.

A light color area on the → Moon, as contrasted with → lunar maria. Also called terra.

See also: → lunar; → highland.

A very bright crescent of light glowing on the lunar horizon at → sunset or just before → sunrise. It has been suggested that → lunar dust is transported electrically high into sky, allowing sunlight to scatter and create glows. On the day side of the → Moon, solar → ultraviolet radiation is strong enough to kick → electrons from → dust grains in the lunar soil. Removal of electrons, which have a negative electric charge, leaves the dust with a positive electric charge. Since like charges repel, the positively charged dust particles get pushed away from each other, and the only direction not blocked by more dust is up. In the 1960s, Surveyor probes filmed a glowing cloud floating just above the lunar surface during sunrise. Later, Apollo 17 astronaut Gene Cernan, while orbiting the Moon, recorded a similar phenomenon at the sharp line where lunar day meets night, called the → terminator.

See also: → lunar; → horizon; → glow.

A very bright crescent of light glowing on the lunar horizon at → sunset or just before → sunrise. It has been suggested that → lunar dust is transported electrically high into sky, allowing sunlight to scatter and create glows. On the day side of the → Moon, solar → ultraviolet radiation is strong enough to kick → electrons from → dust grains in the lunar soil. Removal of electrons, which have a negative electric charge, leaves the dust with a positive electric charge. Since like charges repel, the positively charged dust particles get pushed away from each other, and the only direction not blocked by more dust is up. In the 1960s, Surveyor probes filmed a glowing cloud floating just above the lunar surface during sunrise. Later, Apollo 17 astronaut Gene Cernan, while orbiting the Moon, recorded a similar phenomenon at the sharp line where lunar day meets night, called the → terminator.

See also: → lunar; → horizon; → glow.

One of the 28 divisions of the sky, identified by the prominent stars in them, that the Moon passes through during its monthly cycle, as used in ancient Chinese, Hindu, and Arab astronomy/astrology.

Etymology (EN): From O.Fr. mansion, from L. mansionem (nom. mansio) “a staying, a remaining, night quarters, station,” from manere “to stay, abide” (Fr. maison, ménage; E. manor, mansion, permanent); cf. Pers. mân “house, home,” mândan “to remain, stay, relinquish, leave;” Mid.Pers. mândan “to remain, stay;” O.Pers. mān- “to remain, dwell;” Av. man- “to remain, dwell; to wait;” Gk. menein “to remain;” PIE base *men- “to remain, wait for.”

Etymology (PE): Manzel, from Ar. “dwelling, habitation, mansion.”

One of the 28 divisions of the sky, identified by the prominent stars in them, that the Moon passes through during its monthly cycle, as used in ancient Chinese, Hindu, and Arab astronomy/astrology.

Etymology (EN): From O.Fr. mansion, from L. mansionem (nom. mansio) “a staying, a remaining, night quarters, station,” from manere “to stay, abide” (Fr. maison, ménage; E. manor, mansion, permanent); cf. Pers. mân “house, home,” mândan “to remain, stay, relinquish, leave;” Mid.Pers. mândan “to remain, stay;” O.Pers. mān- “to remain, dwell;” Av. man- “to remain, dwell; to wait;” Gk. menein “to remain;” PIE base *men- “to remain, wait for.”

Etymology (PE): Manzel, from Ar. “dwelling, habitation, mansion.”

An area on the surface of the → Moon that appears darker and smoother than its surroundings.
Once thought to be seas, lunar maria are now known to be basaltic basins created by volcanic → lava floods; plural maria.

See also: → lunar; L. mare “sea,” plural form maria, because
Galileo thought the dark featureless areas on the Moon were → seas.

An area on the surface of the → Moon that appears darker and smoother than its surroundings.
Once thought to be seas, lunar maria are now known to be basaltic basins created by volcanic → lava floods; plural maria.

See also: → lunar; L. mare “sea,” plural form maria, because
Galileo thought the dark featureless areas on the Moon were → seas.

Plural of → lunar mare.

See also: → lunar mare.

Plural of → lunar mare.

See also: → lunar mare.

The mass of the → Moon, which is 7.35 x 10²² kg, about 1/81 of the Earth’s mass.

See also: → lunar; → mass.

The mass of the → Moon, which is 7.35 x 10²² kg, about 1/81 of the Earth’s mass.

See also: → lunar; → mass.

The average time between successive new or full moons. Also called → synodic month, → lunation.

See also: → lunar; → month.

The average time between successive new or full moons. Also called → synodic month, → lunation.

See also: → lunar; → month.

One of the two points of intersection of the orbit of the Moon with the plane of → ecliptic. Indeed, the lunar orbit is tilted by about 5 degrees relative to the ecliptic. The revolution period of a lunar node in ecliptic is 18.61 years. Due to perturbation by the Sun, the lunar nodes slowly regress westward by 19.3° per year.

See also → ascending node; → descending node.

Etymology (EN): → lunar; → node.

Etymology (PE): Gereh, → node; gowzahri, related to gowzahr, → draconic month.

One of the two points of intersection of the orbit of the Moon with the plane of → ecliptic. Indeed, the lunar orbit is tilted by about 5 degrees relative to the ecliptic. The revolution period of a lunar node in ecliptic is 18.61 years. Due to perturbation by the Sun, the lunar nodes slowly regress westward by 19.3° per year.

See also → ascending node; → descending node.

Etymology (EN): → lunar; → node.

Etymology (PE): Gereh, → node; gowzahri, related to gowzahr, → draconic month.

Same as → lunar node.

See also: → lunar; → orbit; → node.

Same as → lunar node.

See also: → lunar; → orbit; → node.

The apparent shift in the → Moon’s position relative to the background stars when observed from different places on Earth. The first parallax determination was for the Moon, by Hipparchus (150 B.C.). He determined that one-fifth of the Sun’s angular diameter corresponded to the lunar parallax between Hellespont and Alexandria.

See also: → lunar; → parallax.

The apparent shift in the → Moon’s position relative to the background stars when observed from different places on Earth. The first parallax determination was for the Moon, by Hipparchus (150 B.C.). He determined that one-fifth of the Sun’s angular diameter corresponded to the lunar parallax between Hellespont and Alexandria.

See also: → lunar; → parallax.

One of the various changes in the apparent shape of the Moon, because as the Moon orbits the Earth different amounts of its illuminated part are facing us. The phases of the Moon include: the → new moon, → waxing crescent, → first quarter, → waxing gibbous, → full moon, → waning gibbous, → last quarter, → waning crescent, and → new moon again.

See also: → lunar; → phase.

One of the various changes in the apparent shape of the Moon, because as the Moon orbits the Earth different amounts of its illuminated part are facing us. The phases of the Moon include: the → new moon, → waxing crescent, → first quarter, → waxing gibbous, → full moon, → waning gibbous, → last quarter, → waning crescent, and → new moon again.

See also: → lunar; → phase.

A probe for exploring and reporting on conditions on or about the Moon.

See also: → lunar; → probe.

A probe for exploring and reporting on conditions on or about the Moon.

See also: → lunar; → probe.

The process whereby the → Moon gradually moves out into a slightly larger orbit. The → gravitational attraction of the Moon on the → Earth creates two ocean → tidal bulges on the opposite sides of our planet.

The Earth rotates faster than the Moon revolves about the Earth. Therefore, the tidal bulge facing the Moon advances the Moon with respect to the line joining the centers of the Earth and the Moon. The Moon’s gravity pulls on the bulge and slows down the → Earth’s rotation. As a result, the Earth loses → angular momentum and the days on Earth are gradually increasing by 2.3 milliseconds per century. Since the angular momentum in the → Earth-Moon system is conserved, the Earth must impart the loss in its own angular momentum to the Moon’s orbit. Hence, the Moon is being forced into a slightly larger orbit which means it is receding from the Earth. However, eventually this process will come to an end. This is because the Earth’s own rotation rate will match the Moon’s orbital rate, and it will therefore no longer impart any angular momentum to it. In this case, the planet and the Moon are said to be tidally locked (→ tidal locking). This is a stable situation because it minimises the energy loss due to friction of the system. Long ago, the Moon’s own rotation became equal to its orbital period about the Earth and so we only see one side of the Moon. This is known as → synchronous rotation and it is quite common in the solar system. The Moon’s average distance from Earth in increasing by 3.8 cm per year. Such a precise value is possible due to the Apollo laser reflectors which the astronauts left behind during the lunar landing missions (Apollo 11, 14, and 15). Eventually, the Moon’s distance will increase so much that it will be to far away to produce total eclipses of the Sun.

See also: → lunar; → recession.

The process whereby the → Moon gradually moves out into a slightly larger orbit. The → gravitational attraction of the Moon on the → Earth creates two ocean → tidal bulges on the opposite sides of our planet.

The Earth rotates faster than the Moon revolves about the Earth. Therefore, the tidal bulge facing the Moon advances the Moon with respect to the line joining the centers of the Earth and the Moon. The Moon’s gravity pulls on the bulge and slows down the → Earth’s rotation. As a result, the Earth loses → angular momentum and the days on Earth are gradually increasing by 2.3 milliseconds per century. Since the angular momentum in the → Earth-Moon system is conserved, the Earth must impart the loss in its own angular momentum to the Moon’s orbit. Hence, the Moon is being forced into a slightly larger orbit which means it is receding from the Earth. However, eventually this process will come to an end. This is because the Earth’s own rotation rate will match the Moon’s orbital rate, and it will therefore no longer impart any angular momentum to it. In this case, the planet and the Moon are said to be tidally locked (→ tidal locking). This is a stable situation because it minimises the energy loss due to friction of the system. Long ago, the Moon’s own rotation became equal to its orbital period about the Earth and so we only see one side of the Moon. This is known as → synchronous rotation and it is quite common in the solar system. The Moon’s average distance from Earth in increasing by 3.8 cm per year. Such a precise value is possible due to the Apollo laser reflectors which the astronauts left behind during the lunar landing missions (Apollo 11, 14, and 15). Eventually, the Moon’s distance will increase so much that it will be to far away to produce total eclipses of the Sun.

See also: → lunar; → recession.

The loose, fragmentary material on the Moon’s surface. The lunar regolith has resulted from → meteorite collisions all along the Moon’s history. It is the → debris thrown out of the → impact craters. The composition of the lunar regolith varies from place to place depending on the rock types impacted. Generally, the older the surface, the thicker the regolith. Regolith on young → maria may be only 2 meters thick; whereas, it is perhaps 20 meters thick in the older → highlands.

See also: → lunar; → regolith.

The loose, fragmentary material on the Moon’s surface. The lunar regolith has resulted from → meteorite collisions all along the Moon’s history. It is the → debris thrown out of the → impact craters. The composition of the lunar regolith varies from place to place depending on the rock types impacted. Generally, the older the surface, the thicker the regolith. Regolith on young → maria may be only 2 meters thick; whereas, it is perhaps 20 meters thick in the older → highlands.

See also: → lunar; → regolith.

The Moon’s motion around its axis, which takes place in 27.321 661 days (→ sidereal month). Since the Moon and the Earth are → tidally locked our satellite has a → synchronous rotation. This means that it rotates once on its axis in the same length of time it takes to revolve around Earth. That is why the Moon always shows the same face to us. However, over time we can see up to 59 percent of the lunar surface because the Moon does not orbit at a constant speed (→ libration in longitude) and its axis is not perpendicular to its orbit (→ libration in latitude).

The Moon also creates tides in Earth oceans. As the Earth rotates, the rising and falling sea waters bring about friction within the liquid itself and between the water and solid Earth. This removes energy from Earth’s rotation and causes it to spin more slowly. As a result, days are getting longer, at about 2 milliseconds per century. On the other hand, since the → angular momentum of the → Earth-Moon system must be conserved, the Moon gradually moves away from the Earth. This, in turn, requires its orbital period to increase and, because the Moon is tidally locked to Earth, to spin more slowly.

See also: → lunar; → rotation.

The Moon’s motion around its axis, which takes place in 27.321 661 days (→ sidereal month). Since the Moon and the Earth are → tidally locked our satellite has a → synchronous rotation. This means that it rotates once on its axis in the same length of time it takes to revolve around Earth. That is why the Moon always shows the same face to us. However, over time we can see up to 59 percent of the lunar surface because the Moon does not orbit at a constant speed (→ libration in longitude) and its axis is not perpendicular to its orbit (→ libration in latitude).

The Moon also creates tides in Earth oceans. As the Earth rotates, the rising and falling sea waters bring about friction within the liquid itself and between the water and solid Earth. This removes energy from Earth’s rotation and causes it to spin more slowly. As a result, days are getting longer, at about 2 milliseconds per century. On the other hand, since the → angular momentum of the → Earth-Moon system must be conserved, the Moon gradually moves away from the Earth. This, in turn, requires its orbital period to increase and, because the Moon is tidally locked to Earth, to spin more slowly.

See also: → lunar; → rotation.

Same as → sidereal month.

See also: → lunar; → sidereal; → orbital; → period.

Same as → sidereal month.

See also: → lunar; → sidereal; → orbital; → period.

→ lunar highland.

See also: → lunar; terra “earth,” → terrestrial.

→ lunar highland.

See also: → lunar; terra “earth,” → terrestrial.

A year based solely on the Moon’s motion, containing 12 synodic months, each of 29.5306 days, that is a year of 354.3672 days. Used by Hebrews, Babylonians, Greeks, and Arabs.

See also: → lunar; → year.

A year based solely on the Moon’s motion, containing 12 synodic months, each of 29.5306 days, that is a year of 354.3672 days. Used by Hebrews, Babylonians, Greeks, and Arabs.

See also: → lunar; → year.

The rocks that make up the bright portions of the lunar surface.

See also: From → lunar + ite a suffix used to form the names of minerals, such as hematite and malachite.

The rocks that make up the bright portions of the lunar surface.

See also: From → lunar + ite a suffix used to form the names of minerals, such as hematite and malachite.

The interval of a complete lunar cycle, between one new Moon and the next, that is 29 days, 12 hours, 44 minutes, and 2.8 seconds. or 29.5306 days. → synodic month.

Etymology (EN): M.E. lunacyon, from M.L. lunation-.

Etymology (PE): Mahâyand, literally “coming, arrival of the Moon,” from mâh→ moon + âyand “coming, arrival,” present stem of âmadan “to come”; O.Pers. aitiy “goes;” Av. ay- “to go, to come,” aēiti “goes;” Skt. e- “to come near,” eti “arrival;” Gk ion " going," neut. pr.p. of ienai “to go;” L. ire “to go;” Goth. iddja “went,” Lith. eiti “to go;” Rus. idti “to go;” from PIE base *ei- “to go, to walk.”

The interval of a complete lunar cycle, between one new Moon and the next, that is 29 days, 12 hours, 44 minutes, and 2.8 seconds. or 29.5306 days. → synodic month.

Etymology (EN): M.E. lunacyon, from M.L. lunation-.

Etymology (PE): Mahâyand, literally “coming, arrival of the Moon,” from mâh→ moon + âyand “coming, arrival,” present stem of âmadan “to come”; O.Pers. aitiy “goes;” Av. ay- “to go, to come,” aēiti “goes;” Skt. e- “to come near,” eti “arrival;” Gk ion " going," neut. pr.p. of ienai “to go;” L. ire “to go;” Goth. iddja “went,” Lith. eiti “to go;” Rus. idti “to go;” from PIE base *ei- “to go, to walk.”

A calendar in which the → solar year consists of 12 or 13 lunar → synodic months. Lunisolar calendars are → solar calendars, but use the lunar month as the basic unit rather than the → solar day. The 13th → embolismic month is to keep lunar and solar cycles in pace with each other. The reason is that the solar year has about 365 days, but 12 lunar months amount to 354
days, which is about 11 days short of a year. The most well-known lunisolar calendars are the Babylonian, Hebrew, and Chinese.

See also: From luni-, from → lunar, + → solar; → calendar.

A calendar in which the → solar year consists of 12 or 13 lunar → synodic months. Lunisolar calendars are → solar calendars, but use the lunar month as the basic unit rather than the → solar day. The 13th → embolismic month is to keep lunar and solar cycles in pace with each other. The reason is that the solar year has about 365 days, but 12 lunar months amount to 354
days, which is about 11 days short of a year. The most well-known lunisolar calendars are the Babylonian, Hebrew, and Chinese.

See also: From luni-, from → lunar, + → solar; → calendar.

→ precession of the equator.

See also: From luni-, from → lunar, + → solar;
→ precession.

→ precession of the equator.

See also: From luni-, from → lunar, + → solar;
→ precession.

The Wolf. A constellation in the southern hemisphere, located at about 15h right ascension, 45° south declination. Abbreviation: Lup; genitive: Lupi.

Etymology (EN): L. lupus “wolf,” PIE *wlqwos/*lukwos; cf. Pers. gorg, as below; Gk. lykos; Albanian ulk; O.C.S. vluku; Rus. volcica; Lith. vilkas “wolf;” P.Gmc. *wulfaz (cf. O.S. wulf, O.N. ulfr, O.Fris., Du., O.H.G., Ger., E. wolf).

Etymology (PE): Gorg “wolf,” Aftari dialect varg, M.Pers. gurg, O.Pers. Varkana- “Hyrcania,” district southeast of the Caspian Sea, literally “wolf-land,” today Iran Gorgân; Khotanese birgga-; Sogdian wyrky;
Av. vəhrka-; Skt. vrka-.

The Wolf. A constellation in the southern hemisphere, located at about 15h right ascension, 45° south declination. Abbreviation: Lup; genitive: Lupi.

Etymology (EN): L. lupus “wolf,” PIE *wlqwos/*lukwos; cf. Pers. gorg, as below; Gk. lykos; Albanian ulk; O.C.S. vluku; Rus. volcica; Lith. vilkas “wolf;” P.Gmc. *wulfaz (cf. O.S. wulf, O.N. ulfr, O.Fris., Du., O.H.G., Ger., E. wolf).

Etymology (PE): Gorg “wolf,” Aftari dialect varg, M.Pers. gurg, O.Pers. Varkana- “Hyrcania,” district southeast of the Caspian Sea, literally “wolf-land,” today Iran Gorgân; Khotanese birgga-; Sogdian wyrky;
Av. vəhrka-; Skt. vrka-.

Any of the several → dark clouds lying in the direction of the constellation → Lupus between → Galactic longitudes 334° < l < 352° and → Galactic latitudes +5° < b < +25°. In terms of angular extent the whole group is one of the largest low-mass star forming complexes on the sky, and it also contains one of the richest associations of → T Tauri stars. An average distance of about 150 pc places it among the nearest star forming regions, together with those in Corona Australis, Ophiuchus, Taurus-Auriga, and Chamaeleon (Comeron, 2008, in Handbook of Star Forming Regions Vol. II, PASP, Reipurth, ed.).

See also: → Lupus; → dark; → cloud.

Any of the several → dark clouds lying in the direction of the constellation → Lupus between → Galactic longitudes 334° < l < 352° and → Galactic latitudes +5° < b < +25°. In terms of angular extent the whole group is one of the largest low-mass star forming complexes on the sky, and it also contains one of the richest associations of → T Tauri stars. An average distance of about 150 pc places it among the nearest star forming regions, together with those in Corona Australis, Ophiuchus, Taurus-Auriga, and Chamaeleon (Comeron, 2008, in Handbook of Star Forming Regions Vol. II, PASP, Reipurth, ed.).

See also: → Lupus; → dark; → cloud.

An large nonthermal radio source in the constellation → Lupus, identified as a very old supernova remnant. It is also an extended source of soft X-rays.

See also: → Lupus; → loop.

An large nonthermal radio source in the constellation → Lupus, identified as a very old supernova remnant. It is also an extended source of soft X-rays.

See also: → Lupus; → loop.

A large → main belt  → asteroid that belongs to a sub-type of hydrated → M-type asteroids. It is an elongated body with its longest side around 130 km.
The → Rosetta space probe flew by Lutetia and gathered data on it in 2008. Lutetia was discovered on November 15, 1852, by Hermann Goldschmidt (1802-1866) from the balcony of his apartment in Paris.

See also: Named → Lutetia from L. Lutetia Parisiorum, literally “Parisian swamps,” the Gallo-Roman city that was the ancestor of present-day Paris.

A large → main belt  → asteroid that belongs to a sub-type of hydrated → M-type asteroids. It is an elongated body with its longest side around 130 km.
The → Rosetta space probe flew by Lutetia and gathered data on it in 2008. Lutetia was discovered on November 15, 1852, by Hermann Goldschmidt (1802-1866) from the balcony of his apartment in Paris.

See also: Named → Lutetia from L. Lutetia Parisiorum, literally “Parisian swamps,” the Gallo-Roman city that was the ancestor of present-day Paris.

A systematic error that can be introduced when → trigonometric parallaxes are used to calibrate a luminosity system. The bias arises when stars are selected by a lower limit in the observed parallax values. This favors the stars for which the measured parallax result is relatively too large.

See also: Named after Th. Lutz & D.H. E. Kelker, 1973, PASP 85, 573; → bias.

A systematic error that can be introduced when → trigonometric parallaxes are used to calibrate a luminosity system. The bias arises when stars are selected by a lower limit in the observed parallax values. This favors the stars for which the measured parallax result is relatively too large.

See also: Named after Th. Lutz & D.H. E. Kelker, 1973, PASP 85, 573; → bias.

SI unit of illumination equal to a luminous flux of 1 lumen per square meter.

SI unit of luminous incidence or illuminance, equal to 1 lumen per square meter.

See also: From Gk. lux “light,” → lumen.

SI unit of illumination equal to a luminous flux of 1 lumen per square meter.

SI unit of luminous incidence or illuminance, equal to 1 lumen per square meter.

See also: From Gk. lux “light,” → lumen.