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

A small starlike symbol (*), used in printing or writing as a reference mark, as an indication of the omission of letters or words, to denote a hypothetical linguistic form, or for various arbitrary meanings.

M.E. astarisc, from L.L. asteriscus, from Gk. asteriskos "small star," from aster-, → astro- + -ikos "diminutive suffix."

Axtarak, from axtar "star" → astro- + -ak "diminutive suffix."

A group of stars in the sky which are traditionally imagined to present a pattern within a → constellation. Examples include the → Big Dipper, the → Northern Cross, the → Square of Pegasus, and → Orion's Belt.

Gk. asterismos "a marking with stars, constellation," from aster, → astro- + → -ism.

Axtargân, from axtar "star" → astro- + -gân suffix denoting collective nature.

1a ) A distinguishing feature or quality.
1b) The integer part of a → common logarithm. For example, log₁₀ (23) = 1.362, where the characteristic is 1 and the → mantissa is 0.362.
2) Pertaining to, constituting, or indicating the character or peculiar quality of a person or thing; typical; distinctive.

→ character; → -istic.

Of a pulsar, a normalized period of rotation assumed to be a good approximation to pulsar's true age.

→ characteristic; → age.

Graph representing an optical film's response to the amount of light falling on it.

→ characteristic; → curve.

Physics: An analytical relationship between a set of physical variables that determines the state of a physical system.
Math.: The equation which is solved to find a matrix's eigenvalues, also called the characteristic polynomial.

→ characteristic; → equation.

A typical or most likely mass for the formation of an astronomical object. In current star formation models, it is of order of a few tenths of a → solar mass.

→ characteristic; → mass.

The quantity kT in the → Maxwell-Boltzmann distribution law, where k is → Boltzmann's constant and T the gas temperature. See also → thermal energy.

→ characteristic; → thermal; → energy

A → satellite of the → dwarf planet → Eris.

Dysnomia in Gk. mythology is the daughter of Eris and the goddess of lawlessness.

A table of computed positions occupied by a celestial body over successive intervals of time such as daily; plural ephemerides.

From L. ephemeris "day book, diary," from Gk. ephemeris "diary, account book," from ephemeros "short-lived, lasting but a day," from → epi "on, upon" + hemerai, dative of hemera "day."

Ruzij, from ruz, → day + zij "astronomical table," from Mid.Pers. zig "astronomical table," originally "string," since the lines of a table were compared to strings used on a weaver's instrument, variant zih, meaning "cord, string" (Modern Persian zeh "cord, string"); Av. jiiā- "bow-string;" cf. Skt. jiyā- "bow-string;" PIE base *g^whi- "thread, tendon" (from which derive also Gk. bios "bow;" L. filum "thread;" Russ. žca "thread").

86,400 → ephemeris seconds.

→ ephemeris; → day.

A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by → Terrestrial Dynamical Time (TDT).

→ ephemeris; → meridian.

The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 → ephemeris time.

→ ephemeris; → second.

The uniform time-scale used as the independent variable to calculate the orbits in the solar system prior to 1984. Ephemeris Time was adopted in 1960 to deal with irregularities in the → Earth's rotation that had been found to affect the course of mean solar time. The definition of Ephemeris Time is based on Newcomb's analytical theory of the Earth's motion around the Sun (Newcomb 1898), according to which the geometric mean longitude of the Sun with respect to the Earth-Moon barycenter is expressed by:
L = 279° 41' 48".04 + 129 602 768".13 T + 1''.089 T²,
where L refers to the → mean equinox of date while T measures time from noon 1900 January 0 GMT in Julian centuries of 36525 days. Ephemeris Time is therefore defined as the instant near the beginning of the calendar year A.D. 1900 when the mean longitude of the Sun was 279° 41' 48''.04, at which instant the measure of ET was 1900 January 0, 12h precisely. In this system the fundamental unit was the → ephemeris second, which was defined so that the → tropical year at the epoch 1900.0 should be exactly 31 556 925,9747 seconds of ephemerides. Ephemeris Time was inconvenient in many ways and was supeseded with the → Terrestrial Dynamical Time (TDT), whose fundamental unit is the SI second.

→ ephemeris; → time.

The passage of a celestial body or point across the → ephemeris meridian.

→ ephemeris; → transit.

A → dwarf planet which is a → trans-Neptunian object (TNO) with an orbital → eccentricity of 0.44, an → inclination of 44 degrees and a surface composition very similar to that of → Pluto. It orbits the Sun as far as twice Pluto's distance from the Sun. → Occultation observations carried out in 2010 were used to measure the size of Eris accurately. Eris's newly determined diameter is 2326±12 km. The observations show that Eris is an almost perfect twin of Pluto in size. They also reveal a very reflective surface, with an → albedo of 0.96, suggesting that it is uniformly covered in a thin layer of ice, probably a frozen atmosphere (Sicardy et al. 2011, Nature 478, 493). Like Pluto, Eris has a moon, which has been officially named by the → International Astronomical Union as (136199) Eris I (→ Dysnomia). The informal names of Eris were Xena and 2003 UB313.

Named after Eris the Gk. goddess of chaos and strife. She created a quarrel among goddesses that led to the Trojan War.

An optical instrument for viewing objects which are above the eye-level of the observer, or are placed so that direct vision is blocked.

→ peri-; + → -scope.

A system of canyons located just south of the Martian equator. The system is about 4000 km long. The central individual troughs, generally 50 to 100 km wide, merge into a depression as much as 600 km wide. In places the canyon floor reaches a depth of 10 km, 6 to 7 times deeper than the Grand Canyon on Earth.

L. Valles Marineris "Mariner's Valleys," named after the Mars orbiter Mariner 9, which discovered the Martian canyon in 1971-72. → valley.