Theta

Look up Θ or θ in Wiktionary, the free dictionary.

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 Diacritics Ligatures Numerals ϛ (6) ϟ (90) ϡ (900)
Use in other languages
Bactrian Coptic Albanian
Related topics
Use as scientific symbols
Book Category Commons

Theta (uppercase Θ or ϴ, lowercase θ (which resembles digit 0 with horizontal line) or ϑ; Ancient Greek: θῆτα, thē̂ta, [tʰɛ̂ːta]; Modern: θήτα, thī́ta, [ˈθita]; UK /ˈθiːtə/, US /ˈθeɪtə/) is the eighth letter of the Greek alphabet, derived from the Phoenician letter Teth . In the system of Greek numerals it has a value of 9.

Greek

The alphabet on a black figure vessel, with a point-and-circle theta.

In Ancient Greek, θ represented the aspirated voiceless dental plosive /t̪ʰ/, but in Modern Greek it represents the voiceless dental fricative /θ/.

Forms

In its archaic form, θ was written as a cross within a circle (as in the Etruscan or ), and later, as a line or point within a circle ( or ). Archaic crossed forms of theta are seen in the wheel letters of Linear A and Linear B.

The cursive form ϑ was retained by Unicode as U+03D1 ϑ "GREEK THETA SYMBOL" ("=script theta"), separate from U+03B8 θ "GREEK SMALL LETTER THETA". For the purpose of writing Greek text, the two can be font variants of a single character, but θ and ϑ are also used as distinct symbols in technical and mathematical contexts.

Latin

The name ARAÐOVNA on a Gaulish tombstone.

In the Latin script used for the Gaulish language, theta developed into the tau gallicum, conventionally transliterated as Ð, although the bar extends across the centre of the letter. The phonetic value of the tau gallicum is thought to have been [t͡s].

Cyrillic

The early Cyrillic letter fita (Ѳ, ѳ) developed from θ. This letter existed in the Russian alphabet until the 1918 Russian orthography reform.

International Phonetic Alphabet

In the International Phonetic Alphabet, [θ] represents the voiceless dental fricative, as in thick or thin. It does not represent the consonant in the, which is the voiced dental fricative.

Mathematics and science

Lower case

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

• A plane angle in geometry.
• A Variable in trigonometry
• A special function of several complex variables.
• One of the Chebyshev functions in prime number theory.
• The potential temperature in meteorology.
• The score of a test taker in item response theory.
• Theta Type Replication: a type of bacterial DNA replication specific to circular chromosomes.
• Threshold value of an artificial neuron.
• A Bayer designation letter applied to a star in a constellation; usually the eighth star so labelled but not necessarily the eighth-brightest as viewed from Earth.
• The parameter frequently used in writing the likelihood function (Bayes theorem).
• The Watterson estimator for the population mutation rate in population genetics.
• Indicates a minimum optimum integration level determined by the intersection of GG and LL schedules. The GG-LL schedules are a tool used in analyzing the potential benefits of a country pegging their domestic currency to a foreign currency.
• The reserve ratio of banks in economic models.
• The ordinal collapsing function developed by Solomon Feferman^[1]

Upper case

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

• Quantity or temperature, by SI standard (in italics).
• Dimensionless temperature in transport phenomena.
• An asymptotically tight bound in the analysis of algorithms (big O notation).
• A certain ordinal number in set theory.
• Pentaquarks, exotic baryons in particle physics.
• A brain signal frequency (beta, alpha, theta, delta) ranging from 4–8 Hz.
• One of the variables known as "Greeks" in finance, representing the lost-value rate of an option over time.
• A variable indicating temperature difference in heat transfer.
• Measuring the angle of incident X-ray beam during XRD

Symbolism

θ (θάνατος, death) in a mosaic

• In ancient times, Tau was used as a symbol for life or resurrection, whereas the eighth letter of the Greek alphabet, theta, was considered the symbol of death.

According to Porphyry of Tyros, the Egyptians used an X within a circle as a symbol of the soul; having a value of nine, it was used as a symbol for Ennead. Johannes Lydus says that the Egyptians used a symbol for Kosmos in the form of theta, with a fiery circle representing the world, and a snake spanning the middle representing Agathos Daimon (literally: good spirit).^[2]

The Egyptians also used the symbol of a point within a circle (, the sun disc) to represent the sun, which might be a possible origin of its use as the Sun's astrological glyph. It is worthwhile to note that θῆτα (theta) has the same numerical value in isopsephy as Ηλιος (Helios): 318.

Abbreviation

In classical Athens, it was used as an abbreviation for the Greek θάνατος (thanatos, “death”) and as it vaguely resembles a human skull, theta was used as a warning symbol of death, in the same way that skull and crossbones are used in modern times. It survives on potsherds used by Athenians when voting for the death penalty.^[2] Petrus de Dacia in a document from 1291 relates the idea that theta was used to brand criminals as empty ciphers, and the branding rod was affixed to the crossbar spanning the circle.^[3] For this reason, use of the number theta was sometimes avoided where the connotation was felt to be unlucky—the mint marks of some Late Imperial Roman coins famously have the sum ΔΕ or ΕΔ (delta and epsilon, that is 4 and 5) substituted as a euphemism where a Θ (9) would otherwise be expected.

Character encodings

• Greek Theta

• Coptic Thethe

• Cyrillic Fita

• Mathematical Theta

These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

See also

• Ѳ, ѳ—Fita, a letter of the early Cyrillic alphabet derived from the Greek theta
• ʘ—Bilabial click
• Voiceless dental fricative

Notes