Greek alphabet
Αα	Alpha	Νν	Nu
Ββ	Beta	Ξξ	Xi
Γγ	Gamma	Οο	Omicron
Δδ	Delta	Ππ	Pi
Εε	Epsilon	Ρρ	Rho
Ζζ	Zeta	Σσς	Sigma
Ηη	Eta	Ττ	Tau
Θθ	Theta	Υυ	Upsilon
Ιι	Iota	Φφ	Phi
Κκ	Kappa	Χχ	Chi
Λλ	Lambda	Ψψ	Psi
Μμ	Mu	Ωω	Omega
History
Archaic local variants
Ligatures (ϛ, ȣ, ϗ) Diacritics
Numerals (6) (90) (900)
In other languages
Bactrian Coptic Albanian
Scientific symbols
Book Category Commons

Eta (uppercase Η, lowercase η) Greek: ἦτα) is the seventh letter of the Greek alphabet. Originally denoting a consonant /h/, its sound value in the classical Attic dialect of Ancient Greek was a long vowel [ɛː], raised to [i] in medieval Greek, a process known as iotacism.

In the system of Greek numerals it has a value of 8. It was derived from the Phoenician letter heth . Letters that arose from Eta include the Latin H and the Cyrillic letter И.

  History

  Consonant h

  Eta (Heta) in the function of /h/ on the ostrakon of Megacles, son of Hippocrates, 487 BC. Inscription: ΜΕΓΑΚLES HIΠΠΟΚRATOS. On display in the Ancient Agora Museum in Athens, housed in the Stoa of Attalus.

  Eta in the function of /h/ on an Attic red-figured calyx-krater, 515 BC. Amongst the depicted figures are Hermes and Hypnos. Inscriptions: HERMES - HYPNOS.

The letter shape H was originally used in most Greek dialects to represent the sound /h/, a voiceless glottal fricative. In this function, it was borrowed in the 8th century BC by the Etruscan and other Old Italic alphabets, which were based on the Euboean form of the Greek alphabet. This ultimately gave rise to the Latin alphabet with its letter H.

  Long e

In the East Ionic dialect, however, the sound /h/ disappeared by the sixth century BC, and the letter was re-used initially to represent a development of a long vowel /aː/, which later merged in East Ionic with /ɛː/ instead.^[1] In 403 BC, Athens took over the Ionian spelling system and with it the vocalic use of H (even though it still also had the /h/ sound itself at that time). This later became the standard orthography in all of Greece.

Other regional variants of the Greek alphabet (epichoric alphabets), in dialects that still preserved the sound /h/, employed various glyph shapes for consonantal Heta side by side with the new vocalic Eta for some time. One of them was a tack-like shape, looking like the left half of an H. This system was first used in the southern Italian colonies of Heracleia and Tarentum. When Greek orthography was codified by grammarians in the Hellenistic era, they used a diacritic symbol derived from this half-H shape to signal the presence of /h/, and added as its counterpart a reverse-shaped diacritic to denote absence of /h/. These symbols were the origin of the rough breathing and smooth breathing diacritics that became part of classical Greek orthography.^[2] The tack symbol has been reintroduced into modern scholarly representation of archaic Greek writing under the name of Heta.

  Iotacism

During the time of post-classical Koiné Greek, the /ɛː/ sound represented by eta was raised and merged with several other formerly distinct vowels (iotacism). Thus in Modern Greek, Eta is pronounced [ˈita] and represents the sound /i/ (a close front unrounded vowel). It shares this function with several other letters (ι, υ) and digraphs (ει, οι), which are all pronounced alike (see iotacism).

  Cyrillic script

Eta was also borrowed with the sound value of [i] into the Cyrillic script, where it gave rise to the Cyrillic letter И.

  Uses

  Letter

In Modern Greek the letter, pronounced [ˈita], represents a close front unrounded vowel, /i/. In Classical Greek, it represented a long open-mid front unrounded vowel, /ɛː/.

  Symbol

  Upper case

The upper-case letter Η is used as a symbol in textual criticism for the Alexandrian text-type (from Hesychius, its once-supposed editor).

In chemistry, the letter H as symbol of enthalpy sometimes is said to be a Greek eta, but since enthalpy comes from ἐνθάλπος, which begins in a smooth breathing and epsilon, it is more likely a Latin H for 'heat'.

  Lower case

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

• Thermodynamics, the efficiency of a Carnot heat engine.
• Chemistry, the hapticity, or the number of atoms of a ligand attached to one coordination site of the metal in a coordination compound. For example, an allyl group can coordinate to palladium in the η¹ mode (only one atom of an allyl group attached to palladium) or the η³ mode (3 atoms attached to palladium).
• Optics, the electromagnetic impedance of a medium, or the quantum efficiency of detectors.
• Particle physics, to represent the η mesons.
• Experimental particle physics, η stands for pseudorapidity.
• Cosmology, η represents conformal time; dt = adη.
• Relativity and Quantum field theory (physics), η (with two subscripts) represents the metric tensor of Minkowski space (flat spacetime).
• Statistics, η² is the "partial regression coefficient". η is the symbol for the linear predictor of a generalized linear model, and can also be used to denote the median of a population.
• Economics, η is the elasticity.
• Astronomy, the seventh brightest (usually) star in a constellation. See Bayer designation.
• Mathematics, η-conversion, see lambda calculus
• Mathematics, the Dirichlet eta function, Dedekind eta function, and Weierstrass eta function.
• Biology, a DNA polymerase found in higher eukaryotes and implicated in Translesion Synthesis.
• Neural network backpropagation, η stands for the learning rate.
• Telecommunications, Eta stands for efficiency
• Electronics, η stands for the ideality factor of a bipolar transistor, and has a value close to 1.000. It appears in contexts where the transistor is used as a temperature sensing device, e.g. the thermal "diode" transistor that is embedded within a computer's microprocessor.
• Power electronics, η stands for the efficiency of a power supply, defined as the output power divided by the input power.
• Atmospheric science, η represents absolute atmospheric vorticity.
• Rheology, η represents viscosity.
• Oceanography, η is the measurement (usually in metres) of sea-level height above or below the mean sea-level at that same location.

  References