Theta

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[[Image:Greek_alphabet_{{{letter|alpha-omega}}}.png|center]]

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 Sigma Τ τ Tau Υ υ Upsilon Φ φ Phi Χ χ Chi Ψ ψ Psi Ω ω Omega obsolete letters Ϝ ϝ Digamma Image:Greek alphabet san.png San Ϙ ϙ Qoppa Ϡ ϡ Sampi

For other uses, see Theta (disambiguation).

Theta (upper case Θ, lower case θ) is the eighth letter of the Greek alphabet. In the system of Greek numerals it has a value of 9.

In its archaic form, theta was written as:

• a cross within a circle (Image:Cross within circle.png or Image:Cross within circle 2.png)

• and later, as a line or point within a circle (Image:Line within circle.png or Image:Astrological glyph for sun.png).

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.

The Egyptians also used the symbol of a point within a circle (Image:Astrological glyph for sun.png) to represent the Sun god Ra, which might be a possible origin of its use as the astrological glyph for the Sun. It is worthwhile to note that theta has the same numerical value in isopsephy as Helios (ΘΗΤΑ = 318 = ΗΛΙΟΣ).

In classical Athens, as an abbreviation for the Greek word thanatos (θάνατος: "death"; see Thanatos and Thanatology) 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.

Theta was adopted into the early Cyrillic alphabet as fita (Ѳ, ѳ).

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

• A plane angle in geometry.
• 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 upper-case letter Θ is used as a symbol for:

• Dimensionless temperature in transport phenomena.
• Denoting an asymptotically tight bound in analysis of algorithms. See Big O notation.
• Θ (set theory), a certain ordinal number.
• Representing pentaquarks, exotic baryons in particle physics.
• Earth Day.

References

• The Greek Qabalah, by Kieren Barry, Samuel Weiser, Inc., 1999. ISBN 1578631106
• Jesus Christ, Sun of God, by David Fideler, Quest Books, 1993. ISBN 0835606961als:Θ

ca:Theta de:Theta el:Θήτα es:Θ fr:Thêta ga:Téite nl:Thèta ja:Θ pl:Theta pt:Θ ru:Фита fi:Theeta sv:Theta zh:Θ