An Etymological Dictionary of Astronomy and Astrophysics
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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.

→ 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.

→ 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.

→ 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.

→ 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).

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-.

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.

→ 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 interval of time (346.620 03 days) between two successive passages of the Sun through the same node of the Moon's orbit. It takes less than a solar year to complete an eclipse year because the Moon's orbit and the lunar nodes are slowly regressing.

→ eclipse; → year.

In ancient calendars, a year that contains an → embolismic month.

→ embolismic month; → year.

An → exoplanet similar to Earth.

→ exo- + → earth.

The contradiction between a colder Sun (about 30% less luminous) some 4 billion years ago, as predicted by models, and the warm ancient Terrestrial and Martian climates derived from geological evidence.

→ faint; → early; → sun; → paradox.