Theta (uppercase Θ, lowercase θ) 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 (/t^h/), but in Koiné and later dialects it fricativized to a voiceless dental fricative /θ/. In English, the name of the letter is commonly pronounced "thayta". In Modern Greek, the name sounds more like "theeta", rhyming with "cheetah".

Letters such as the early Cyrillic letter fita (Ѳ, ѳ) developed from θ.

In its archaic form, θ was written as: or Cross within circle 2.png)

or 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 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 (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.

Symbol

• 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 Potential temperature in meteorology.

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.

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 0835606961

Greek letters | Θ | Theta | Theta | Theta | Θήτα | Θ | Thita | Thêta | Téite | Θ | תטא | Thèta | Θ | Theta | Theta | Theta | Θ | Тэта (буква) | Theeta | Theta | Θ