An Etymological Dictionary of Astronomy and Astrophysics
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The instant when the axis of the Moon's → shadow cone passes closest to Earth's center. For → total eclipses, the instant of greatest eclipse is virtually identical to the instants of greatest magnitude and greatest duration. However, for → annular eclipses, the instant of greatest duration may occur at either the time of greatest eclipse or near the sunrise and sunset points of the eclipse path (F. Espenak, NASA).

Superlative of → great; → eclipse.

The largest → elongation of an inferior planet from the Sun. It may be → greatest eastern elongation or → greatest western elongation. The greatest elongation of Mercury is about 28°, and thus Mercury can only be observed 112 minutes after sunset or before sunrise. For Venus, it is about 47°, making it visible at most about 3 hours after sunset or before sunrise.

Superlative of → great; → eastern; → elongation.

The Greatest → elongation of an inferior planet occurring before sunrise.

Superlative of → great; → western; → elongation.

A → numeral system in which letters represent numbers. In an earlier system, called acrophonic, the symbols for numerals came from the first letter of the number name. Subsequently, the numerals were based on giving values to the letters of alphabet. For example α, β, γ, and δ represented 1, 2, 3, and 4; while ι, κ, λ, and μ stood for 10, 20, 30, and 40, and ρ, σ, τ, and υ for 100, 200, 300, and 400. The Greek also used the additive principle. For example 11, 12, 13, 14, and 374 were written ια, ιβ, ιγ, ιδ, and τοδ. The numbers between 1000 and 9000 were expressed by adding a subscript or superscript ι (iota) to the symbols for 1 to 9. For example ιA and ιΘ for 1000 and 9000. Numbers greater than 9999 were expressed using M, which was the myriad, 10,000. Therefore, since 123 was represented by ρκγ, 123,000 was written as M^ρκγ.

→ numeral; → system.

A color intermediate in the spectrum between yellow and blue (wavelength between 5000 and 5700 Å). The color of most grasses and leaves while growing.

Green, from O.E. grene, related to growan "to grow," from W.Gmc. *gronja- (cf. Dan. grøn, Du. groen, Ger. grün), from PIE base *gro- "to grow."

Sabz "green," from Mid.Pers. sabz "green, fresh," related to sabzi "grass."

A brilliant green color that occasionally appears on the upper limb of the Sun as it rises or sets.

→ green; → flash.

A pea harvested and eaten while still green, soft, and unripe; a garden pea; usually in plural.

→ green; → pea.

A member of a class of galaxies of relatively small size (→ compact galaxy) having very strong → emission lines especially the → [O III] doublet and an unusually large → equivalent width of up to 1000 Å. They were first noted because of their peculiar bright green color and small size, unresolved in → Sloan Digital Sky Survey imaging. Green Peas are similar to high-→ redshift  → Lyman alpha emitting galaxies (LAEs) in many respects (small sizes, low → stellar masses, 10^8-10 → solar masses (Msun), low metallicities for their stellar masses, high → specific star formation rates (sSFR), and large [O III] λ5007/[O II]λ3727 ratios. Green Peas are relatively luminous and massive galaxies compared to the faint-end → dwarf starburst galaxies and LAEs (See Yang et al, 2017, arxiv/1706.02819, and references therein).

Such called because of their appearance and green color (mainly due to very strong optical emission line [O III] 5007 Å) in composite images; → green; → pea; → galaxy.

A building with transparent walls and roof, usually of glass, for the cultivation and exhibition of plants under controlled conditions (Dictionary.com).

→ green; → house.

Garmxâné, literally "warm house," from garm, → warm, + xâné, → house.

An increase in → temperature caused when incoming → solar radiation is passed but outgoing → thermal radiation is trapped by the → atmosphere. The major factors for this effect are → carbon dioxide and → water vapor. The greenhouse effect is very important on Venus and Earth but very weak on Mars. On average, about one third of the solar radiation that hits the Earth is reflected back to space. The Earth's surface becomes warm and emits → infrared radiation. The → greenhouse gases trap the infrared radiation, thus warming the atmosphere. Without the greenhouse effect the Earth's average global temperature would be -18° Celsius, rather than the present 15° Celsius. However, human activities are causing greenhouse gas levels in the atmosphere to increase.

→ greenhouse; → effect.

Gases responsible for the greenhouse effect. These gases include: water vapor (H₂O), carbon dioxide (CO₂); methane (CH₄); nitrous oxide (N₂O); chlorofluorocarbons (CF_xCl_x); and tropospheric ozone (O₃).

→ greenhouse; → gas.

The → Greenwich Mean Sidereal Time corrected for → nutation. Therefore, it is measured with respect to the → true vernal equinox. GAST and GMST differ by the → equation of the equinoxes.

→ Greenwich Meridian; → apparent; → sidereal; → time.

The → sidereal time related to the angle between the → prime meridian and the → mean vernal equinox, measured in the plane of the equator.

→ mean; → Greenwich Meridian; → sidereal; → time.

The → prime meridian that separates east from west in the same way that the Equator separates north from south. It is defined by the position of the → Airy transit circle.

A borough in southeast London, England, on the Thames River. It is the site of the original Royal Observatory, through which passes the prime meridian, or longitude 0°; → meridian.

The number and fraction of → mean sidereal days elapsed on the → Greenwich meridian since 12h January 1, 4773 BC (mean sidereal).

→ Greenwich meridian; → sidereal; → date.

The integral part of the → Greenwich sidereal date.

→ Greenwich; → sidereal; → day; → number.

A → solar calendar in which the year length is assumed to be 365.2425 solar days. It is now used as the civil calendar in most countries. The Gregorian calendar is a revision of the → Julian calendar instituted in a papal bull by Pope Gregory XIII in 1582. The reason for the calendar change was to correct for drift in the dates of significant religious observations (primarily Easter) and to prevent further drift in the dates.

Named after Pope Gregory XIII (1502-1585), an Italian, born Ugo Boncompagni, Pope from 1572 to 1585, who ordered the reform of the Julian calendar; → calendar.

A reflecting telescope in which the light rays are reflected from the primary mirror to a concave secondary mirror, from which the light is reflected back to the primary mirror and through the central hole behind the primary mirror. Compare with the → Cassegrain telescope, in which the secondary mirror is convex.

Named after the Scottish mathematician and astronomer James Gregory (1638-1675), who devised the telescope, but did not succeed in constructing it; → telescope.

A theoretical limit of approximately 6 × 10¹⁹  → electron-volts for the energy of → cosmic rays above which they would lose energy in their interaction with the → cosmic microwave radiation background photons. Cosmic ray protons with these energies produce → pions on blackbody photons via the Δ resonance according to: γ_CMB + p → p + π⁰, or γ_CMB + p → n + π⁺, thereby losing a large fraction of their energy. These interactions would reduce the energy of the cosmic rays to below the GZK limit. Due to this phenomenon, → Ultra-high-energy cosmic rays are absorbed within about 50 Mpc.

Named after Kenneth Greisen (1966), Physical Review Letters 16, 748 and Georgiy Zatsepin & Vadim Kuzmin (1966), Journal of Experimental and Theoretical Physics Letters 4, 78; → limit.

1) A → grating of crossed bars; gridiron.
2) A → network of → horizontal and → perpendicular lines, uniformly spaced, for locating points on a map, chart, or aerial photograph by means of a system of coordinates.
3) Electricity: i) A metallic framework employed in a storage cell or battery for conducting the electric current and supporting the active material.
ii) A system of electrical distribution serving a large area, especially by means of high-tension lines.

Shortening of gridiron "a utensil consisting of parallel metal bars on which to broil meat or other food," from M.E. griderne, from gridel, from O.Fr. gredil, gridil, from L. craticula "gridiron, small griddle," diminutive of cratis "wickerwork."

Šabâk, from Laki šowâk "a net woven from goat fleece used for carrying chaff or fruits like melon," variants šâvâk (Lori), šavak (Nahâvand).