An Etymological Dictionary of Astronomy and Astrophysics
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The slow rotation of the major axis of a planet's orbit in the same direction as the revolution of the planet itself, due mainly to gravitational interactions with other planets. The perihelion of the planet Mercury advances about 9'.6 per century. The bulk of the advance was accounted by perturbations from other planets. However, a remaining small advance, by 43'' per century, was eventually explained as an effect predicted by Einstein's theory of → general relativity. In the case of close binary stars, the advance of pericenter may additionally be caused by mass transfer and the stars' distorted (elliptical) shapes. Advance of perihelion (or pericenter) is also known as → apsidal motion.

Advance, from O.Fr. avancer "move forward," from V.L. *abantiare, from L.L. abante "from before," from ab- "from" + ante "before," PIE *ant- "front, forehead;" → perihelion.

Pišraft "advance," from piš "forward; in front; before," Mid.Pers. peš + raft "going; walk, travel," from raftan "to go."

A wave that travels backward in time according to Maxwell's electromagnetic theory; it arrives before it is transmitted. → Maxwell's equations have two solutions, the normal solution describes the ordinary waves, called → retarded waves, traveling forward in time. However, no advanced waves have ever shown up in any experiment. The advanced solutions of Maxwell's equations are usually simply discarded as "unphysical."

Advanced, adj. from advance, → advance of perihelion; → wave.

Mowj, → wave; pišras "advanced," from piš "before," Mid.Pers. peš + ras "arriving," from rasidan "to arrive," Mid.Pers. rasitan, O.Pers./Av. rasa- present stem of ar- "to move, go or come toward," cf. Skt. ar-, rcchati.

1) Geology: The process of transport of a quantity by the velocity field due to the movement of a fluid. Advection differs from → convection, which describes thermally driven circulation.
2) Meteorology: The predominantly horizontal, large-scale motions of the atmosphere. In contrast, convection describes the predominantly vertical, locally induced motions.

From L. advecti "act of conveying," from advectus, past participle of advehere "to carry," from ad-, "to" + vehere "to carry, bring;" Skt. vah-, vahati "to carry, conduct, guide," Av. vaz-, vazaiti "to guide, lead"; PIE *wegh- "to go, transport in a vehicle".

Pahnbaz from pahn "flat, wide, → broad," + baz "to carry," → evection.

The first term on the right side in the → induction equation.

→ advection; → term.

Adj. from → advection.

A word that serves to qualify a verb, an adjective, another adverb, or an entire sentence. More specifically, adverbs indicate manner, time, place, cause, or degree, and answer questions such as "how," "when," "where," "how much."

From L.L. adverbium "adverb," literally "that which is added to a verb," from → ad- "to" + verbum "word, → verb," a translation of Gk. epirrhema "adverb," from → epi- "upon, on" + rhema "verb."

Apivâz, from api-, → epi-, + vâz "word," → verb.

1) A person, group, or force that opposes or attacks; opponent; enemy; foe.
2) A person, group, etc., that is an opponent in a contest; contestant (Dictionary.com).

M.E. adversaire; O.Fr. adversaire "opponent, adversary, rival," from L. adversarius "opposite, hostile, contrary," from adversus "turned against," from → ad- + verse, → universe.

Hamestâr, from Mid.Pers. hamestâr "adversary," from Av. hamaēstar-, from ha-, → com-, + maēd- "to bring down, to suppress."

1) In theories of magnetized → accretion disks, the distance from a non-rotating star where the → free fall of a spherical accretion flow is stopped, which occurs where the → ram pressure of the infalling matter equals the → magnetic pressure of the star.
2) More generally, the distance from an accreting or wind-blowing star where the → Alfvén Mach number of the flow (→ inflow or → outflow) is unity.

→ Alfvén wave; → radius.

1) In theories of magnetized → accretion disks, the distance from a non-rotating star where the → free fall of a spherical accretion flow is stopped, which occurs where the → ram pressure of the infalling matter equals the → magnetic pressure of the star.
2) The distance from an accreting or wind-blowing star where the → Alfvén Mach number of the flow (→ inflow or → outflow) is unity.

→ Alfvén wave; → radius.

1) In a → planispheric astrolabe, the small revolving rod fixed to the center of the goniometric scale plotted on the → front or → back of the instrument. Unlike the → rule, the alidade has little vanes with holes or slots at each end, called → pinnules, which are used as sights, through which the observer can aim at a particular object. An index, often consisting of the edge of the alidade itself, shows on the goniometric scales the angle between the line of sight of the targeted object and a predetermined axis, i.e. the vertical of the observation locality (online museo galileo, VirtualMuseum).
2) A rule having a sight at each end, used in surveying.
3) A topographic surveying and mapping instrument used for determining directions, consisting of a telescope and attached parts.

M.E. allidatha, alhidade, from L. alhidada, from Ar. al-'izâda (العضاده), from al- "the" + 'izâda "an armlet, a bracelet; a sickle-like piece of metal, used by camel drivers, to pull down tree branches to camels."

Ezâdé, from Ar. al-'izâda.

→ alpha process.

→ alpha; → ladder.

1) By this or that time; previously; prior to or at some specified or implied time.
2) Now; so soon; so early (Dictionary.com).

From M.E. al redy, literally "fully ready," → all, + M.E. redy "ready," from rædig, from O.E. ræde "prompt" + -ig "-y."

Pišnun, literally "prior to now," from piš-, → pre-, + Mid./Mod.Pers. nun "now, at present" (variants aknun, konun, ultimately from Proto-Ir. *hak-nun); Av. nū- "now," nūrəm "now;" O.Pers. nūram "now;" cf. Skt. nú- "now, just, but," nūnám "now, at present, indeed;" Gk. nun "now;" L. nu- "now" (in nu-dis "the day after tomorrow"); Goth. nu "now;" O.E. nu; E. now; PIE base *nu- "now."

A number that represents a → charge-coupled device (CCD)'s output and is proportional to the → electron charge created by the → photons, plus the constant → bias offset. The relationship between the ADUs generated and the number of electrons acquired on the CCD is defined by the → CCD gain. Intensities given in ADUs provide a convenient method for comparing images and data generated by different cameras. Also referred to as → count and digital number. In most cases, the analog signal is digitalized by an analog-to-digital (A/D) converter and fed into a computer where further manipulation and analysis are done on what the detector originally produced from the star's photons (Howell, S.B., Handbook of CCD Astronomy, Cambridge Univ. Press, 2000).

→ analogue; → digital; → unit.

A high-contrast imaging technique that reduces minute temporal and spatial → seeing fluctuations and facilitates the detection of faint point sources, in close separation from their stars. It consists of the acquisition of a sequence of images with an → altazimuth mounting telescope while the instrument field derotator is switched off. This keeps the instrument and telescope optics aligned and allows the field of view to rotate with respect to the instrument. For each image, a reference → point spread function (PSF) is constructed from other appropriately selected images of the same sequence and subtracted to remove quasistatic PSF structure (Marois et al. 2006, ApJ 641, 556).

→ angular; → differential; → imaging.

The number of units in one → mole of any → substance (defined as its → molecular weight in grams), equal to 6.022 140 857 × 10²³. The units may be electrons, atoms, ions, or molecules, depending on the nature of the substance.

Named after Amedeo Avogadro (1776-1856), whose law allowed other physicists to calculate Avogadro's number; → number.

A statement according to which equal volumes of different gases contain an equal number of molecules under the same conditions of temperature and pressure.

After Amedeo Avogadro (1776-1856), Italian chemist and physicist, who advanced the hypothesis in 1811.

→ Avogadro constant.

Named after Amedeo Avogadro (1776-1856), whose law allowed other physicists to calculate Avogadro's number; → constant.

An area of the sky with relatively low amounts of  → interstellar dust along the → line of sight, occurring toward the  → constellation of  → Sagittarius, near the → globular cluster NGC 6522. The window, through which stars in the → Galactic bulge are visible, lies 3.9 degrees south of the → Galactic center, corresponding to a line of sight that passes within 1,800 → light-years of the → Milky Way's core. It is named after Walter Baade, who used it to observe → RR Lyrae stars  in the → Galactic bulge region. 

→ Baade-Wesselink method; → window.

A method used to determine the size of certain types of pulsating stars, including Cepheids, from their magnitude variations (photometry) and the corresponding radial velocities (spectroscopy).

Baade, from Walter (Wilhelm Heinrich) Baade (1893-1960), German/American astronomer, who made important contributions to the research on variable stars; Wesselink, from Adriaan J. Wesselink (1909-1995), Dutch/American astronomer, the originator of the method. → method.

The isotropic residual microwave radiation in space left from the primordial → Big Bang. Same as → cosmic microwave background (CMB) radiation.

→ background; → radiation.