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| 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 |
| Obsolete letters | |
 Digamma |
 Qoppa |
 San |
 Sampi |
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| Greek diacritics | |
Theta (uppercase Θ, lowercase θ or ϑ; Greek: Θήτα) 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. In Classical Greek θ represented an aspirated voiceless dental plosive (/t̪ʰ/), but in Koiné and later dialects it became a voiceless dental fricative /θ/. In American English the name of the letter is commonly pronounced /ˈθeɪtʌ/. In British English, the name is pronounced /ˈθiːtə/.
Letters such as the early Cyrillic letter fita (Ѳ, ѳ) developed from θ.
In its archaic form, θ was written as:
- a cross within a circle (
or 
)
- and later, as a line or point within a circle (
or 
).
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).[1]
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.
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.[1] 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.[2]
Symbol
The lower-case letter θ is used as a symbol for:
- A plane angle in geometry.
- A special function of several complex variables.
- The voiceless dental fricative in the International Phonetic Alphabet, such as in the words thick or thin (but not as in the, which is the voiced dental fricative).
- 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.
- Theta has also gained a new significance in the north-eastern region of the United States as a symbol for affection, in a similar capacity to the “heart” symbol.citation needed
- The parameter in the likelihood function
The upper-case letter Θ is used as a symbol for:
- Dimensionless temperature in transport phenomena.
- Big O notation. Denoting an asymptotically tight bound in analysis of algorithms.
- Θ (set theory), a certain ordinal number.
- Representing pentaquarks, exotic baryons in particle physics.
- Earth Day.
- Brain Signal Frequency (Beta, Alpha, Theta, Delta) ranging from 4–8 Hz
- One of the variables known as Greeks (finance), representing the lost-value rate of an option over time.
Usage in names
- Kappa Alpha Theta is a sorority whose members are designated “Thetas”.
- Theta refers to a member of Sigma Phi Theta, a local fraternity at Texas Lutheran University.
- Theta is also the name of a melodic death metal band based in Plattsburgh, New York.
- Theta is the name of a biological weapon and boss featured in the Atari video game Area 51.
- Thetamen is a nickname also used by the fraternity Iota Phi Theta.
- Theta Phi Alpha is a sorority.
- Theta refers to a member of the Alpha Pi Theta Brotherhood of Emerson College
- Theta healing is a name for healing procedure where practitioner and client enter theta brainwawe frequency.
- Theta Delta Beta is a fraternity whose members are referred as "Thetas" in Southern California.
References
- ^ a b Barry, Kieren (1999), The Greek Qabalah: Alphabetic Mysticism and Numerology in the Ancient World, Samuel Weiser, p. 73, ISBN 1-57863-110-6.
- ^ Kaplan, Robert (2000), The Nothing that Is: A Natural History of Zero, Oxford University Press, p. 66, ISBN 0-1951-4237-3.
