An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ Earth; → mass.

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

→ earth; → radius.

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.

→ 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%.

→ 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.

→ 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.

→ Earth; → rotation.

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

→ 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.

→ Earth; → Moon; → system.

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.

→ 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.

→ 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.

→ earth; → shine.

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

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").

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.

From → east + -ern an adjective suffix occurring with names of directions.

Xâvari from xâvar, → east, + -i adj. suffix.

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

→ eastern; → elongation.

1) Not hard or difficult; requiring no great labor or effort.
2) Free from pain, discomfort, worry, or care (Dictionary.com).

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.

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.

Probably back-formation from → eavesdropper.

Gušârdan, from gušâr, → eavesdropper, + -dan infinitive suffix.

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

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."

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.

Noun from → eavesdrop.

A → tidal current that generally moves seaward and occurs during the part of the tide cycle when sea level is falling.

M.E. eb(be); O.E. ebba; cognate with O.Fr. ebba, Du. eb(be), Ger. Ebbe ebb, M.E. ebben, O.E. ebbian, derivative of the noun; akin to → off; PIE base *apo- "off, away;" → tide.

Bâzkeš, from bâz- a suffix denoting "reversal, repetition, opposition," → re-, + keš present stem of kešidan "to draw, drag, carry," → tide.

1) An orbit that has a high → eccentricity, i.e. is highly elliptical.
2) In Ptolemy's geocentric model, a → deferent which is slightly off-center from the Earth.

→ ex-; → center; → -ic.