An Etymological Dictionary of Astronomy and Astrophysics

A prominent → H II region in the constellation → Serpens lying about 2 kpc away from the Sun. It measures about 30’ across and surrounds the open cluster → M16 (NGC 6611), which contains at least 20 stars of spectral type B0.5 or earlier, including one 05V. At a projected distance from the cluster of about 2 pc, where the H II region has expanded into surrounding → molecular cloud, a striking → elephant trunk morphology or → pillar results. The nebula is the site of ongoing → star formation, especially in these pillar structures. The Eagle Nebula is often erroneously given the cluster’s designation (M16).

Etymology (EN): Eagle, because the nebula’s shape is vaguely reminiscent of an eagle in flight; from M.E. egle, from O.Fr. egle, from O.Prov. aigla, from L. → aquila “black eagle;” → nebula.

Etymology (PE): Miq, → nebula; &#352âhin, → aquila.

The vertebrate organ of hearing, which is also responsible for maintaining equilibrium. It is usually composed of three parts: the outer ear, middle ear, and inner ear.

Etymology (EN): M.E. ere, O.E. eare “ear” (O.N. eyra, Dan. øre, M.Du. ore, Du. oor, O.H.G. ora, Ger. Ohr); cf. Gk. aus; L. auris; Av. usi “both ears;” Lith. ausis; O.C.S. ucho; O.Ir. au; PIE *ous-.

Etymology (PE): Guš “ear” (gušidan “to hear,” niyušidan “to listen”);
Mid.Pers. gôš “ear,” gôšidan “to hear, listen;” O.Pers. gauša-; Av. gaoša- “ear,” gaoš- “to hear;”
cf. Skt. ghosa- “noise, tumult,” ghus- “to sound, cry or proclaim aloud;” PIE *ghous-.

Occurring at or near the beginning of a period of time, process, or sequence of events.

Etymology (EN): M.E. erlich (adj.), erliche (adv.); O.E. ærlice, from ær “soon, ere” (from P.Gmc. *airiz, from PIE *āier- “day, morning,” cf. Av. ayar- “day, day time,” ayarə.drājah- “duration of one day,” ayarə.bara- “day’s ride,” Mid/Mod.Pers. parēr “the day before yesterday,” from *parā.ayer- “the day before”)

• -lice “-ly,” adverbial suffix.

Etymology (PE): Âqâzin adj. of âqâz “beginning,” from Proto-Iranian *āgaHz-, from prefix ā- + *gaHz- “to run, start,” cf. Sogdian āγāz “beginning, start” (Cheung 2007).

A fairly long-lived step in the evolution of → low-mass
and → intermediate-mass stars when helium burning shifts from the center to a shell around the core. At this phase the stellar luminosity is provided almost entirely by → helium shell burning. The He-shell burning generally adds mass to the growing carbon/oxygen core, resulting in → degenerate matter due to its increasing density.

See also: → early; → AGB; → phase.

A period in the evolution of the → solar system when the planets and satellites were in the process of formation.

See also: → early; → solar; → system.

A star near the beginning of the → spectral classification sequence. A star of → spectral type O, B, A, or F0 to F5. Same as → early-type star.

See also: → early; → spectral; → class; → star.

A qualitative term used to describe a phase in the history of the Universe, from the → Big Bang event to the apparition of the first structures (seeds of future galaxies), at a → redshift around 30.

See also: → early; → universe.

In the → Hubble classification, galaxies on the left part of the → Hubble sequence.
Early-type galaxies tend to have redder colors, higher average surface brightnesses, and lower → neutral hydrogen content than → late-type galaxies. This terminology is based on the obsolete and erroneous idea that → elliptical and → lenticular galaxies might be evolutionary precursors to → spiral and → barred spiral
galaxies.

See also: → early; → type; → galaxy.

Hot, luminous stars of → spectral type O, B, A, and F0 to F5. They were originally thought, wrongly, to be at an earlier stage of evolution than → late-type stars. See also → spectral classification.

See also: → early; → type; → star.

The third planet from the Sun. At → perihelion, it is 147,099,590 km from the Sun, and at → aphelion it is 152,096.150 km, whereas its mean distance from the Sun (→ astronomical unit) is 149,598 × 10⁶ km. Its orbital period is 365.2563 days (→ sidereal year) and its → eccentricity 0.017. Other characteristics: → axial inclination 23.44°; rotation period 23.934 h (→ sidereal day); mean density 5.52 g/cm³; mass 5.974 × 10²⁴ kg; → escape velocity 11.18 km/s; average → albedo 0.37.

The Earth’s atmosphere consists of 78% nitrogen, 21% oxygen and 0.9% argon, plus carbon dioxide, hydrogen, and other gases in much smaller quantities. The atmospheric pressure at sea level is about 1,000 mbar. The surface average temperature is 15° C, but it varies, on the average, between -50° C (winter, Siberia) and up to + 40° C (summer, Sahara).

Liquid water covers 71% of the surface. Over 5,000 active volcanoes have been registered throughout man’s known history. The outer layer of the planet, the → lithosphere, is covered with the → crust. In the → upper mantle and beneath the lithosphere, lies → asthenosphere.
Convection in the asthenosphere has caused → plate tectonics motions and continent drifts. The densest layer of the Earth is its → core, about 3000 km to 6400 km beneath the surface, consisting primarily of iron and nickel. This core is believed to be at the origin of the magnetic field, which reaches about 3 × 10^-5 tesla near the equator. It has only one natural satellite, the Moon (M.S.: SDE).

Etymology (EN): M.E. erthe, from O.E. eorðe “ground, soil, dry land;” cf. O.N. jörð, M.Du. eerde, O.H.G. erda, Goth. airþa; from PIE base *er-.

Etymology (PE): Zamin, variant zami “earth, floor, land,” Mid.Pers. zamig, Av. zam- “the earth;” cf. Skt. ksam- “the ground, earth;” Gk. khthôn, khamai “on the ground;” L. homo “earthly being” (as in homo sapiens,
homicide, humble, humus, exhume), humus “the earth;” O.Russ. zemi “land, earth;” PIE root *dh(e)ghom “earth”.

The mass of our planet Earth, which is 5.9736 × 10²⁴ kg (3 × 10^-6 → solar masses), 317.83 times smaller than the → Jupiter mass. The Earth mass is in particular used to describe the mass of → super-Earth  → extrasolar planets.

See also: → Earth; → mass.

The distance from the Earth’s center to its surface, about 6,371 km.

See also: → earth; → radius.

An → asteroid or → comet whose → orbit occasionally brings it relatively close to the Earth. → near-Earth object.

See also: → Earth; → grazer.

A physical system composed on the → Earth and the → Moon in which both objects directly influence each other. The total energy in the Earth-Moon system is conserved. The most notable influence that the two objects have on each other is → tides.

See also:

→ tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal disruption, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching.

See also: → Earth; → Moon; → system.

The innermost part of the Earth consisting of a solid → inner core, mainly composed of → iron, and a → liquid → outer core. The → pressure and → temperature are so extreme that the molten iron solidifies. The temperature at the inner core boundary is expected to be close to the
→ melting point of iron at 330 gigapascal (GPa). From static laser-heated diamond anvil cell experiments up to 200 GPa, using synchrotron-based fast → X-ray diffraction as a primary melting diagnostic, S. Anzellini et al. (2013, Science 340, 484)
conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 K. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the → mantle. The inner core, 2,400 km in diameter, is suspended in the molten metal of the → outer core, which is about 2,240 km thick. The temperature difference between the mantle and the core is the main engine for large-scale thermal movements, which coupled with the → Earth’s rotation, function as a generator for the planet’s → magnetic field.

See also: → Earth; → core.

The rocky outermost layer of the Earth,
ranging from about 10 to 65 km in thickness.

It is distinguished from the underlying the → Earth’s mantle layer by its more → silicon- and → aluminium-rich composition, lower density, and the lower velocity at which it conducts seismic energy.

It includes → continental crust (about 40 km thick) and → oceanic crust (about 7 km thick). The crust and the topmost layer of the mantle form the → lithosphere.

The five most abundant → chemical elements in the Earth’s crust are, in percentage by weight of the Earth’s crust: → oxygen (O) 46%, silicon (Si) 28%, aluminium (Al) 8%, → iron (Fe) 5%, and → calcium (Ca) 4%.

See also: → Earth; → crust.

A major subdivision of Earth’s internal structure, located beneath the → Earth’s crust and above the central → core. On average, the mantle begins 35 km below the surface and ends at a depth of about 2,900 km. See also → upper mantle and → lower mantle, → asthenosphere, → lithosphere.

See also: → Earth; → mantle.

The natural motion of the Earth around its own axis, which takes place once in a → sidereal day. The Earth rotates toward the → east, in the same direction as it revolves around the Sun. If viewed from the north celestial pole, the Earth turns → counterclockwise. The opposite is true when the Earth is viewed from the south celestial pole. The Earth’s rotation is responsible for the diurnal cycles of day and night, and
also causes the apparent movement of the Sun across the sky. The Earth’s rotation velocity at the → equator is 1,673 km h^-1 or about 465 m s^-1.
More generally, at the → latitude  φ it is given by: v_φ = v_eq cos φ, where v_eq is the rotation velocity at the equator.

The Earth’s rotation is gradually slowing down under the action of the → tides, which are generated by the → gravitational attraction of the → Moon. As the result of this → tidal friction, the day is becoming
longer at a rate of about 2 milliseconds, or 0.002 seconds, per century (or one second every 50,000 years). Moreover, the loss of the Earth’s → rotational angular momentum increases the Moon’s → orbital angular momentum, because the angular momentum of the → Earth-Moon system is conserved. In consequence,
the Moon slowly recedes from the Earth by about 4 cm per year, which leads to increasing its orbital period and the length of a month as well.

See also: → Earth; → rotation.

The illumination of the dark part of the Moon’s disk by the light reflected from the Earth’s surface and atmosphere. Also called → earthshine.

See also: → earth; → light.

Sudden shaking of the → Earth’s surface caused by the passage of a → seismic wave whose mechanical effects can be destructive. See also → starquake.

See also: → earth; → quake.

The visibility of that part of the Moon not illuminated by the Sun. The phenomenon is caused by the solar light reflected by the Earth. It was explained correctly for the first time by Leonardo da Vinci (M.S.: SDE). Same as → earthlight.

See also: → earth; → shine.

The point on the → celestial horizon 90° → clockwise from the → north point. The point where the Sun rises at the → equinoxes.

Etymology (EN): O.E. east, from P.Gmc. *aus-to-, *austra- “east, toward the sunrise” (cf. Du. oost, Ger. Ost, O.N. austr “from the east”), from PIE *aus- “dawn” (cf. Av. uš-, ušah- “dawn,” Skt. usas-, usah- “dawn,” Gk. aurion “morning,” Lith. auszra “dawn,” L. aurora “dawn,” auster “south”).

Etymology (PE): Xâvar “east,” originally “west,” from Mid.Pers. *xvar barân “where the Sun is led,” from xvar “sun” (Av. hū-, hvar- “sun”, cf. Skt. surya-, Gk. helios, L. sol, O.H.G. sunna, Ger. Sonne, E. sun; PIE base *sawel- “sun”) + barân, pr.p. of bar-, bordan “to carry, lead” (Mid.Pers. burdan,
O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”)

• time and place suffix -ân.

Lying toward or situated in the east.

Etymology (EN): From → east + -ern an adjective suffix occurring with names of directions.

Etymology (PE): Xâvari from xâvar, → east, + -i adj. suffix.

The position of a planet when it can be seen in the western sky just after sunset.

See also: → eastern; → elongation.

1. Not hard or difficult; requiring no great labor or effort.
   

2. Free from pain, discomfort, worry, or care (Dictionary.com).

Etymology (EN): M.E. aisie, esy, from O.Fr. aisie “comfortable, at ease, rich, well-off,” p.p. of aisier “to put at ease,” from aise “comfort, pleasure, well-being,” of unknown origin.

Etymology (PE): Âsân “easy,” from Mid.Pers. âsân “calm, quiet, at rest, peaceful, easy,” related to âsudan “to rest, repose;” Av. ā- + saē- (saii-) “to lie down, go to sleep,” → holiday.

To listen secretly to a private conversation.

Etymology (EN): Probably back-formation from → eavesdropper.

Etymology (PE): Gušârdan, from gušâr, → eavesdropper,

• -dan infinitive suffix.

A person or thing that secretly listens to or monitors the private conversation or data of others.

Etymology (EN): From M.E. eavesdrop, from O.E. yfesdrype “place around a house where the rainwater drips off the roof,” literally “one who stands on the eavesdrop in order to listen to conversations inside the house,” from eaves “the lower border of a roof that overhangs the wall”

• drip, drop “to fall in small portions.”

Etymology (PE): Gušâr in Tabari “eavesdropper,” from guš, → ear, + -âr probably contraction of dâr (as in dustâr/dustdâr) agent noun from dâštan “to hold, have, possess; appoint, erect,” → property.

The monitoring and/or examining the data that is passed over the network without sender and receiver’s permission and/or knowledge. For example, a user on the Internet could eavesdrop on someone’s phone conversation or e-mail.

See also: Noun from → eavesdrop.