Eta (letter)

Greek alphabet
Α α Alpha	Β β Beta
Γ γ Gamma	Δ δ Delta
Ε ε Epsilon	Ζ ζ Zeta
Η η Eta	Θ θ Theta
Ι ι Iota	Κ κ Kappa
Λ λ Lambda	Μ μ Mu
Ν ν Nu	Ξ ξ Xi
Ο ο Omicron	Π π Pi
Ρ ρ Rho	Σ σ ς Sigma
Τ τ Tau	Υ υ Upsilon
Φ φ Phi	Χ χ Chi
Ψ ψ Psi	Ω ω Omega
obsolete letters
Digamma	San
Qoppa	Sampi
Ϛ ϛ Stigma	Sho

For other uses, see Eta.

Eta (upper case Η, lower case η) is the seventh letter of the Greek alphabet. In the system of Greek numerals it has a value of 8.

Pronunciation

In Modern Greek letter is pronounced [i]. In the Classical version of the Ancient Greek language, it was pronounced as a long open-mid front unrounded vowel, [ɛː]. The symbol was initially used to denote the rough breathing, [h]. In the Ionic dialect, where this sound had disappeared by the sixth century BC, the letter was used to represent the long [ɛː]. When the Ionic alphabet was adopted in Athens in 403 BC, [ɛː] (previously written as Ε) was also written as Η there too, hence the modern usage.

Symbol

Eta is sometimes used in place of eng (ŋ) when eng is not available, because of their similar appearance.

The upper-case letter Η is used as a symbol for:

• In textual criticism, the Alexandrian text-type (from Hesychius, its once-supposed editor).
• In chemistry, enthalpy.

The lower-case letter η is used as a symbol for:

• In thermodynamics, the efficiency of a Carnot heat engine.
• In chemistry, the number of electrons shared between a metal center and a ligand in a coordination compound. For example, an allyl group can coordinate to palladium in the η¹ mode or the η³ mode.
• In optics, the refractive index of an optical medium (although the letter n is more common).
• In particle physics, to represent the η baryons.
• In statistics, η² is the "partial regression coefficient".
• In astronomy, the seventh brightest (usually) star in a constellation. See Bayer designation.
• In experimental particle physics, η stands for pseudorapidity.
• In mathematics, η-conversion, see lambda calculus
• In fluid dynamics, η stands for viscosity.

The content of this page is retrieved from http://en.wikipedia.org/wiki/Eta_%28letter%29 under GFDL