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Greek alphabet | |||||
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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 | |||||
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Other characters | |||||
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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:
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]
The lower-case letter θ is used as a symbol for:
The upper-case letter Θ is used as a symbol for:
The common usage of theta in mathematical problems began with the Greeks. They logically chose this Greek symbol since it was the next in their alphabet not yet used and was easily legible when recreating for multiple uses. Archaic crossed forms of theta are seen in the wheel letters of Linear A, Linear B, and Egyptian hieroglyphics, the mathematical tensor product, exclusive disjunction, and direct sum operators, the astrological symbol for the Earth (as already mentioned), and the Celtic cross. The circumpunct, or circled dot, is seen in a wide range of places.