Fr.: suppression de zéro
The removal of non-significant zeroes from a number. For example, replacing 531.2300 by 531.23.
zeroth law of thermodynamics
qânun-e sefrom-e garmâtavânik
Fr.: loi zéro de la thermodynamique
Two objects that are in → thermal equilibrium with a third object will be in thermal equilibrium with each other.
Fr.: ζ Ophiuchi
A blue star, also called HD 149757 and HR 6175, which is the nearest, and probably the most widely studied, → massive star. It is variable in several wavelength bands and has a mean visual magnitude of V = 2.58, B - V = 0.01. It lies ~ 222 pc away and has a formal spectral type of O9.5 Ve and a luminosity of 105 Lsun. ζ Ophiuchi is a very rapid rotator with a v sin i ~ 400 km s-1, i.e. ~ 85% of the → break-up velocity. It is one of the earliest prototypes of the → Be phenomenon. Moreover, it shows episodes of Hα emission variability, a common feature of Be/Oe stars. It also shows periodic non-radial pulsations and UV → P Cygni profile variability, as is evident in the periodic behavior of → discrete absorption components (DACs). It has a → mass loss rate of 10-7 Msun yr-1 and a → terminal velocity of wind v∞ = 1550 km s-1. ζ Ophiuchi is a well-known → runaway star with a velocity of 30 km s-1. The interstellar → CH molecule and → CN molecule were first detected toward ζ Ophiuchi. It has been recognized for some time that this star lies close to the blue edge of the → beta Cephei instability strip.
A prefix of the → SI units, denoting 1021. Symbol: Z.
Fr.: zone de Zhevakin
Named after the Russian astronomer Sergei A. Zhevakin (1916-2001), who identified these zone in the 1950s; → mechanism.
The generic name applied to books in Arabic and Persian that tabulate parameters used for astronomical calculations of positions of the Sun, the Moon, and the five planets of antiquity. Some examples: Zij al-Sindhind, by Khwarizmi (c. 780-850), Az-Zij as-Sabi by al-Battâni (Albatenius) (853-929), Zij-i Ilkhâni by Nassireddin Tusi (1201-1274), Zij-i Sultâni by Ulugh Brg (1437), → Toledan Tables, → Alfonsine Tables.
The word is derived from Mid.Pers. zig, variant zih
"cord, string" (Mod.Pers. zeh "cord, string"); Av. jiiā-
"bow-string," cognate with Skt. jiyā- "bow-string,"
PIE base *gwhi- "thread, tendon" (from which derive
also Gk. bios "bow", L. filum "thread", Russ. žca "thread").
A metallic, lustrous, bluish-white → chemical element; symbol Zn. → Atomic number 30; → atomic weight 65.38; → melting point 419.58 °C; → boiling point 907 °C; → specific gravity 7.133 at 25 °C. It was recognized as a → metal as early as 1374.
From the Ger. Zink of unknown origin. Zinc compounds were known in prehistoric times, where they were used for healing wounds and for making brass.
Ruy, from Mid.Pers. rôy, rôd "copper, brass;" Av. raoiδita- "red, reddish;" cf. Skt. rohita- "red, reddish," lohá- "red, reddish, reddish metal."
A common mineral, zirconium silicate, ZrSiO4, occurring in small tetragonal crystals. The color is variable, usually brown to reddish brown, but also colorless, pale yellowish, green, or blue. A red variety, used as a gem, is called hyacinth. Zircon contains trace amounts of uranium and thorium and therefore can be used for radiometric datings. Also called jargon.
From Ger. Zirkon, from Ar. zarqun "cinnabar, bright red," from Pers. zargun "gold-colored," from zar→ gold + -gun "resembling; manner, fashion; color" (Mid.Pers. gônak "kind, species;" Av. gaona- "color").
A metallic chemical element; symbol Zr. Atomic number 40; atomic weight 91.22; melting point about 1,852°C; boiling point 4,377°C; specific gravity 6.5 at 20°C. Zirconium was discovered in the mineral zirconia by the German chemist Martin-Heinrich Klaproth in 1789. It was first isolated by the Swedish chemist Jons Jacob Berzelius in 1824 in an impure state and finally by the chemists D. Lely Jr. and L. Hamburger in a pure state in 1914.
From → zircon.
zirconium oxide, ZrO
Fr.: oxide de zirconium
An imaginary belt around the heavens extending about 9° on either side of the → ecliptic. The orbits of the Moon and of the principal planets also lie entirely within the zodiac. The zodiac was created during the first half of the first millennium B.C. by Babylonian astronomers, who divided it into 12 constellations (→ sign of the zodiac) each considered to occupy 1/12, or 30°, of its great circle. They were named after living creatures, with the exception of Libra: → Aries, → Taurus, → Gemini, → Cancer, → Leo, → Virgo, → Libra, → Scorpius, → Sagittarius, → Capricornus, → Aquarius, → Pisces. However, as a result of → precession, these signs no longer correspond to the astronomical constellations in which the Sun actually appears. The constellations are irregular in size and shape, and the Sun regularly passes through 13 constellations as it moves along the ecliptic. The additional 13th constellation is → Ophiuchus, situated between → Scorpius and → Sagittarius.
From M.E. zodiaque, from O.Fr. zodiaque, from L. zodiacus, from Gk. zodiakos (kyklos) "zodiac (circle)," literally "circle of little animals," from zodiaion, diminutive of zoion "animal," literally "a living being," from PIE base *gwei- "to live, life;" cognate with Pers. zist, → bio-.
Borjgân, from borj, → sign of the zodiac, + -gân suffix denoting multiplicity, order, organization.
Fr.: lumière zodiacale
A cone-shaped faint glow along the → ecliptic, visible to the naked eye in the west after sunset or in the east before sunrise. Zodiacal light results from sunlight reflected by interplanetary dust concentrated in the plane of the ecliptic.
The portion of the surface of a sphere included between two parallel planes.
From L. zona "geographical belt, celestial zone," from Gk. zone "a belt," related to zonnynai "to gird," from PIE base *yes- "to gird, girdle;" cognate with Pers. parhun "circle," as below.
Zonâr most probably loan from from Gk., as above; it is related to Pers. parhun "circle," ultimately from Proto-Iranian *pari-iâhana- "girdle, belt," from pari-, variant pirâ-, → circum-, + iâhana- "to girdle," cf. Av. yâh- "to girdle." The Pers. pirâhan "shirt" is a variant of parhun.
zone of avoidance
Fr.: zone d'évitement, zone vide
Zonâr, → zone; parhiz "avoidance, abstinence," from parhizidan, parhixtan "to restrain onself, to abstain;" Mid.Pers. pahrêc "preservation, protection, guard," pahrêxtan "to protect, to take care of."
Fr.: bande ZrO
Any of the three → absorption bands due to the molecule → zirconium oxide present in the blue and visual spectral regions → S-type stars. The bands are centered on the wavelengths 4614, 5551, and 6468 Å.
ZZ Ceti star
setâre-ye ZZ Ketus
Fr.: étoile de type ZZ Ceti
A member of a class of non-radially pulsating stas that change their brightnesses with periods from 30 seconds to 25 minutes and amplitudes from 0.001 to 0.2 mag in V. Their location on the → Hertzsprung-Russell diagram identifies them as → white dwarf stars that have entered the → instability strip as they evolve along the white dwarf track.