An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Of a planetary orbit, the angle measured from the perihelion position, to the center of the circumscribing auxiliary circle, to the projected position of the planet on the circle. → anomaly; → mean anomaly; → true anomaly.

→ eccentric; → anomaly.

The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a the semi-major axis. If e = 0, the orbit is a circle. If e < 1, the orbit is an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The eccentricity is one of the six → orbital elements that define a → Keplerian orbit.

→ eccentric; → -ity.

A diffraction grating in which the groves are relatively widely spaced and serves to provide high resolution and dispersion.

Echelle, from Fr. échelle "ladder," , from O.Fr. eschele, from L. scala "ladder;" → grating.

Turi, → grating; nardé, contraction of nardebân "ladder; échelle."

A spectrograph that uses an echelle grating to disperse the light.

→ echelle grating; → spectrograph.

Acoustics: Effect produced when sound is reflected or thrown back on meeting a solid obstacle.
Radio.: A wave returned to the transmitter with sufficient magnitude and delay to be distinguished from the directly transmitted wave. In radar, the portion of the energy of the transmitted pulse reflected back to the receiver.

From L. echo, from Gk. echo, personified as a mountain nymph, from ekhe "sound."

Pažvâk, literally "return sound," from paž "back, against, opposite," varaint pâd- (Mid.Pers. pât-, from O.Pers. paity "agaist, back, opposite to, toward, face to face, in front of;" Av. paiti, akin to Skt. práti "toward, against, again, back, in return, opposite;" Pali pati-; Gk. proti, pros "face to face with, toward, in addition to, near;" PIE *proti) + vâk "sound," Mid./Mod.Pers. vâng/bâng "sound, clamour;" Av. vacah- "word," from vac- "to speak, say;" cf. Mod.Pers. vâžé "word," âvâz "voice, sound, song" (Skt. vakti "speaks, says," vacas- "word;" Gk. epos "word;" L. vox "voice;" PIE base *wek- "to speak").

The passage of the shadow of a celestial body over the surface of another. The maximum number of solar and lunar visible eclipses occurring annually is seven; the minimum number is two, both being solar. → Solar eclipses take place when the new Moon is close to an → orbital node and on the same longitude with the Sun. At that moment either the → umbra, → antumbra, or the → penumbra touches the Earth's surface. For an observer located in the umbra the eclipse is total, while for one placed in the antumbra it is annular. → Annular eclipses occur around lunar → apogee. An observer situated in the penumbra sees only a → partial eclipse. A total or annular eclipse can be seen from a band with a width of 270 km at the most, around which, the much larger partiality zone extends. The Moon's shadow crosses the Earth from west to east at about 3,200 km/h. During → total eclipses the Sun's disk is entirely covered and the → solar corona can be seen. A solar eclipse can last up to 3 h (between the first and the → fourth contacts). Totality has a theoretical maximum duration of 7m 31s, but it is usually shorter. A → lunar eclipse can be seen from any place on Earth where the Moon is above the horizon; it occurs when the full Moon passes through the central dark shadow of the Earth. The Earth's shadow is much wider than the Moon and this is why the lunar eclipses can last up to four hours (between the first and the fourth contact) (M.S.: SDE).

From O.Fr. éclipse, from L. eclipsis, from Gk. ekleipsis "a leaving out, forsaking, an eclipse," from ekleipein "to forsake a usual place, fail to appear, be eclipsed," from ek "out," → ex-, + leipein "to leave."

Gereft, past stem of gereftan "to obscure, close up; to take, seize, catch; to undergo an eclipse," from Mid.Pers. griftan, Av./O.Pers. grab- "to take, seize," cf. Skt. grah-, grabh- "to seize, take," graha "seizing, holding, perceiving" (see also → concept); cf. M.L.G. grabben "to grab;" E. grab "to take or grasp suddenly;" PIE base *ghrebh- "to seize".

The fraction of the Sun's diameter occulted by the Moon. It is strictly a ratio of diameters and should not be confused with → eclipse obscuration, which is a measure of the Sun's surface area occulted by the Moon. Eclipse magnitude may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50). By convention, its value is given at the instant of → greatest eclipse (F. Espenak, NASA).

→ eclipse; → magnitude.

A method for imaging the continuum light distributions of the → accretion disks of → cataclysmic variable stars. It relies on geometrical information contained in eclipse light curves. An alternative method is → Doppler tomography.

→ eclipse; → mapping.

The fraction of the Sun's area occulted by the Moon. It should not be confused with → eclipse magnitude, which is the fraction of the Sun's diameter occulted by the Moon. Eclipse obscuration may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50) (F. Espenak, NASA).

→ eclipse; obscuration, verbal noun from → obscure.

The period during which the Sun is close enough to one of the → lunar orbit nodes so that an eclipse can take place. This time window lasts for 37 days for → solar eclipses and almost 24 days for → lunar eclipses. These seasons occur every 173.31 days. Two eclipse seasons make up an → eclipse year.

→ eclipse; → season.

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.

A binary star in which one of the two stars passes in front of the other so that the system's total light periodically fades. The most famous eclipsing binary is → Algol.

→ eclipse; → binary.

Same as → eclipsing binary.

→ eclipse; → variable.

The Sun's apparent path in the sky relative to the stars in the course of a year. It is also the projection of the Earth's orbital plane onto the → celestial sphere. Because of the inclination of the → Earth's rotation axis, the ecliptic is tilted by about 23.4° with respect to the → celestial equator, an angle known as the → obliquity of the ecliptic. The ecliptic crosses the celestial equator at the → equinoxes.

From L. ecliptica linea "path of eclipses," so called because eclipses happen only when the Moon is near this path, from eclipsis, → eclipse.

Hurpeh "sun path," from hur "sun," variant xor, cognate with Gk. helios, → Sun, + peh "path, way," from O.Pers. paθi- "path, way;" Av. paθ-, variants paθi-, paθā-, pantay-; Mid/Mod.Pers. pand "path, advice, councel;" Khotanese pande "road, path;" Ossetic fœndœg "path, road;" cf. Skt. pánthā- "road, path, course;" Gk. patos "path, way;" L. pons "bridge, path;" E. find; PIE base *pent- "to go, to tread."

One of the two coordinates in the → ecliptic system; the angle measured from the ecliptic, positive toward the north.

→ ecliptic; → latitude.

One of the two coordinates in the → ecliptic system; the angle measured eastwards along the ecliptic from 0° to 360°, with the origin at the → vernal equinox.

→ ecliptic; → longitude.

The plane of Earth's orbit around the Sun.

→ ecliptic, → plane.

A catalog of fields along the → ecliptic observed by the → K2 mission. The catalog is hosted at the → Mikulski Archive for Space Telescopes (MAST).

→ ecliptic; → plane; → input; → catalogue.

Either of the two points on the celestial sphere that are 90° above and below the plane of ecliptic. The north ecliptic pole lies in → Draco, and the south ecliptic pole in → Dorado. Due to → precession, the → celestial pole moves in a circle around the ecliptic poles once every 25,800 years.

→ ecliptic, → pole.

Coordinate system with the ecliptic as the fundamental plane.

→ ecliptic; → system.