In alphabets, Theta, or Θῆτα (Greek), symbol ‘Θ’ (capital letter), or theta, symbol ‘θ’ (lowercase letter), is the eight letter of the Greek alphabet, and representative, in the Greek numerical system, of the number 9.

Overview
The following shows the basic etymology of the symbol theta:


Egyptian hieroglyphs
(many symbols)
(2500BC)
Phoenician alphabet
(22 letters)
[1050BC]
#Greek alphabet
(24 letters)
[800BC]
Etruscan
(# letters)
[300BC]
Roman
(26 letters)
[800AD]
Derived terms
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Sun gif
Sun
(Ra)
Alphabet letter Q sunAlphabet letter Q sun
ouroboros 2
Ouroboros
ʘ⊗, teth
teth / tet
9Theta (Greek pot, c.300BC), Θ
Theta
θ
theta
,A E T
Th
Theos, theology, Theodorus, Theophrastus, Thanatos (see Mor). therm, thermal, temperature, thermo-, ΘΔ, think, thought

In c.2800BC, Heliopolis, main hieroglyph "Heliopolis (heiroglyph) 2", originally called An, Anu, Junu, Iunu, in Egyptian, remained Heliopolis (Greek), from Helio- meaning "sun" + -polis meaning "city", aka "sun metropolis" or “city of the sun”, later named On (Hebrew), was a prominent religious capital city of ancient Egypt, which arose to power in c.2800BC, being one of the foremost religious centers, along with Memphis and Thebes, wherein the so-called Heliopolis creation myth was promulgated, centered around the Heliopolis ennead or paut (group) of nine gods, with Atum and later Atum-Ra as the supreme god of this paut.

The following, from Wallis Budge’s Egyptian Hieroglyphic Dictionary (1920), shows the various hieroglyphs for the Egyptian city of An (or Anu, Junu, Iunu), renamed later by the Greeks as “Heliopolis”: [5]

Heliopolis (hieroglyphs)


The numerical sum of the letters in the word θῆτα (theta), in what is called ‘isopsephy’ (Ѻ), a learning technique where early Greeks used pebbles arranged in patters to learn arithmetic and geometry, has the same numerical value as Ηλιος (Helios), as follows:

Θῆτα (Theta) = 9 + 8 + 300 + 1 = 318
Ηλιος (Helios) = 8 + 30 + 10 + 70 + 200 = 318

In 800BC, Hesiod penned his Theogony , or Θεο-γονία (Theo-gony), a term meaning “generation of the gods”, wherein, after studying the creation myths of Egypt (see: Egyptian pantheon), penned a more anthropomorphized, god reduced, version for the Greeks to read (see: Greek pantheon), basically a rescript of the former; this would seem to indicate that the Greek term "Θεο" (god), rooted in the letter symbol "Θ", representative of the sun, sun god, or one of the supreme gods, was a commonly accepted language term in Hesiod's time.

In c.110, Philo of Byblos (Ѻ), said that Greek Theta owed its form to the Egyptian habit of designating the deity by a ringed serpent, with its head turned inward [see: Ouroboros], the dot representing the eye of god in the world. [3]

In c.280, Porphyry, in some publication, stated that the Greek theta corresponds firstly to the ‘soul’ of the world (see: world soul), and also, more importantly, that the significance of number “nine”, in respect to its use in as “theta”, was symbolic or representative of the famous “Ennead” [9 gods], or paut (group) of nine deities, of Heliopolis, as told in their Heliopolis creation myth, which is behind the constructions of the pyramids. [1]

In c.555, John Lydus, supposedly, noted that the Egyptians also used a symbol in the form of a theta for the cosmos, with an airy fiery circle representing the world, and a snake, spanning the middle, representing the agathos daimon or ‘good spirit’; which would seem to correspond to what we now call the "ouroboros". [2]

The following, according to the Philo-Porphyry-Lydus view, then would be the presumed origin of the Greek theta:

Theta (origin)

(add)

Helios | Theta
Sun reborn on southern crux
In 2007, Peter Joseph, in his Zeitgeist, via citation to Dorothy Murdock, Gerald Massey, among others, conjectured that the sun perceptually “dies” on the location of the Southern crux for three days, Dec 22-24, before being “reborn” in its new yearly solar cycle; this may be one possible origin for the "+" or "x" symbol in the sun circle "O", prior to it becoming the Theta Θ symbol?

Quotes
The following are related quotes:

“What relation serpents had to the earliest letters it is now impossible to say. Philo [c.110] says that the Greek Theta [Θ] owed its form to the Egyptian habit of designating the deity by a ringed serpent, with its head turned inward [see: Ouroboros], the dot representing the eye of god in the world. But the serpent was the name and symbol of the Phoenician letter Tet [], which preceded the Greek Theta, and that Greek letter still represents the deity in abbreviated writing.”
— Caroline Dall (1867), “Review of Charles Bunsen’s 1848 Egypt’s Place in Universal History [1]

“In Old Hebrew and Phoenician as the letter tet ‘⊕’, also known as the ‘compass’, is part of all early alphabets, including Mayan, Chinese, Linear A & B, Etruscan, and the Indus Valley scripts. It also appears on early rock paintings all over the world. The tet became the Greek theta, originally written ⊕ (now θ). Theta is the first letter of god: theos [see: theology], and his throne or thronos.”
— Esther Stein (2018), The Visible Kingdom of God: the Song of Noah [4]

See also
Theos | Dios
Theology

References
1. (a) Anon. (c.2019). “Theta Symbol and Its Meaning” (Ѻ), Mythologian.net.
(b) Gardiner’s sign list – Wikipedia.
2. Barry, Kieren. (1999). The Greek Qabalah: Alphabetical and Mysticism and Numerology in the Ancient World (pg. 73). Publisher.
3. (a) Bunsen, Charles. (1848). Egypt’s Place in Universal History: An Historical Investigation, Volumes 1-5 (translator: Charles Cottrell; notes: Samuel Birch). Publisher.
(b) Dall, Caroline H. (1867), “Egypt’s Place in Universal History” (quote, pg. 39; table, pg. 40), Review of Charles Bunsen’s 1848 Egypt’s Place in Universal History: An Historical Investigation, The Friend, 3(26):33-, Oct 5.
(c) Dall, Caroline. (1868). Egypt’s Place in History: a Presentation (Philo, 5+ pgs; quote, pg. 87). Lee and Shepard.
4. Stein, Esther. (2018). The Visible Kingdom of God: the Song of Noah (pg. #). Balboa Press.
5. Budge, Wallis. (1920). An Egyptian Hieroglyphic Dictionary, Volume Two: With an Index of English Words, King List and Geographical List with Indexes, List of Hieroglyphic Characters, Coptic and Semitic Alphabets (pg. 958). Cosimo, 2013.
6. Higgins, Godfrey. (1833). Anacalypsis: an Attempt to Draw Aside the Veil of the Saitic Isis: Or an Inquiry Into the Origin of Languages, Nations and Religions, Volume 1 (cross, pg. 500). Longman, 1836.

External links
Theta – Wikipedia.

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