# Omicron

**Omicron**^{} (uppercase **Ο**, lowercase **ο**, literally 'small o':όμικρον <ὂ μικρόν - *ò mikrón*, micron meaning 'small' in contrast to *omega*) is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. This letter is derived from the Phoenician letter ayin . In classical Greek, omicron represented the sound[o] in contrast to *omega*[ɔː] and *ου*[oː]. In modern Greek, omicron represents the mid back rounded vowel/o̞/, the same sound as omega. Letters that arose from omicron include Roman O and Cyrillic O.

## Contents

In addition to its use as an alphabetic letter, omicron is occasionally used in technical notation, but its use is limited since both upper case and lower case (Ο ο) are indistinguishable from the Latin letter "oh" (O o) and difficult to distinguish from the Hindu-Arabic numeral "zero" (0).

### Mathematics

The upper-case letter of omicron (O) was originally used in mathematics as a symbol for Big O notation (representing a function's asymptotic growth rate), but has fallen out of favor because omicron is indistinguishable from the Latin letter O and easily confused with the digit zero (0). Where "Big O" notation is still used, Omicron is generally replaced with a script- or calligraphic-form Latin letter "Oh" (${\mathcal {O}}$).

### Greek numerals

There were several systems for writing numbers in Greek; the most common form used in late classical era used omicron (either upper or lower case) to represent the value 70.

More generally, the letter omicron is used to mark the fifteenth ordinal position in any Greek-alphabet marked list. So for example, in Euclid's *Elements*, when various points in a geometric diagram are marked with letters, it is effectively the same as marking them with numbers, each letter representing the number of its place in the standard alphabet.^{}^{}

### Astronomy

Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position.^{}^{} Such stars include Omicron Andromedae, Omicron Ceti, and Omicron Persei.

In Claudius Ptolemy's (c. 100–170) *Almagest*, tables of sexagesimal numbers 1 ... 59 are represented in the conventional manner^{} for Greek numbers:′α ...′νθ . Since the letter omicron [which represents 70 (′ο) in the standard system] is not used in sexagesimal, it is re-purposed to represent an empty number cell. In some renditions the cell was just left blank (nothing there = value is zero), but to avoid copying errors, positively marking a zero cell with omicron was preferred, in the same way that blank cells in modern tables are filled with a dash (—). Both an omicron and a dash imply that *"this isn't a mistake, the cell is actually supposed to be empty"*. By coincidence, the ancient zero-value omicron (ο) resembles a modern Hindu-Arabic zero (0).

**Greek Omicron / Coptic O**^{}

Preview | Ο | ο | Ⲟ | ⲟ | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | GREEK CAPITAL LETTER OMICRON | GREEK SMALL LETTER OMICRON | COPTIC CAPITAL LETTER O | COPTIC SMALL LETTER O | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 927 | U+039F | 959 | U+03BF | 11422 | U+2C9E | 11423 | U+2C9F |

UTF-8 | 206 159 | CE 9F | 206 191 | CE BF | 226 178 158 | E2 B2 9E | 226 178 159 | E2 B2 9F |

Numeric character reference | Ο | Ο | ο | ο | Ⲟ | Ⲟ | ⲟ | ⲟ |

Named character reference | Ο | ο | ||||||

DOS Greek | 142 | 8E | 166 | A6 | ||||

DOS Greek-2 | 190 | BE | 233 | E9 | ||||

Windows 1253 | 207 | CF | 239 | EF |

**Mathematical Omicron**^{}

These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

Preview | 𝚶 | 𝛐 | 𝛰 | 𝜊 | 𝜪 | 𝝄 | ||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL BOLD CAPITAL OMICRON | MATHEMATICAL BOLD SMALL OMICRON | MATHEMATICAL ITALIC CAPITAL OMICRON | MATHEMATICAL ITALIC SMALL OMICRON | MATHEMATICAL BOLD ITALIC CAPITAL OMICRON | MATHEMATICAL BOLD ITALIC SMALL OMICRON | ||||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120502 | U+1D6B6 | 120528 | U+1D6D0 | 120560 | U+1D6F0 | 120586 | U+1D70A | 120618 | U+1D72A | 120644 | U+1D744 |

UTF-8 | 240 157 154 182 | F0 9D 9A B6 | 240 157 155 144 | F0 9D 9B 90 | 240 157 155 176 | F0 9D 9B B0 | 240 157 156 138 | F0 9D 9C 8A | 240 157 156 170 | F0 9D 9C AA | 240 157 157 132 | F0 9D 9D 84 |

UTF-16 | 55349 57014 | D835 DEB6 | 55349 57040 | D835 DED0 | 55349 57072 | D835 DEF0 | 55349 57098 | D835 DF0A | 55349 57130 | D835 DF2A | 55349 57156 | D835 DF44 |

Numeric character reference | 𝚶 | 𝚶 | 𝛐 | 𝛐 | 𝛰 | 𝛰 | 𝜊 | 𝜊 | 𝜪 | 𝜪 | 𝝄 | 𝝄 |

Preview | 𝝤 | 𝝾 | 𝞞 | 𝞸 | ||||
---|---|---|---|---|---|---|---|---|

Unicode name | MATHEMATICAL SANS-SERIF BOLD CAPITAL OMICRON | MATHEMATICAL SANS-SERIF BOLD SMALL OMICRON | MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL OMICRON | MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL OMICRON | ||||

Encodings | decimal | hex | decimal | hex | decimal | hex | decimal | hex |

Unicode | 120676 | U+1D764 | 120702 | U+1D77E | 120734 | U+1D79E | 120760 | U+1D7B8 |

UTF-8 | 240 157 157 164 | F0 9D 9D A4 | 240 157 157 190 | F0 9D 9D BE | 240 157 158 158 | F0 9D 9E 9E | 240 157 158 184 | F0 9D 9E B8 |

UTF-16 | 55349 57188 | D835 DF64 | 55349 57214 | D835 DF7E | 55349 57246 | D835 DF9E | 55349 57272 | D835 DFB8 |

Numeric character reference | 𝝤 | 𝝤 | 𝝾 | 𝝾 | 𝞞 | 𝞞 | 𝞸 | 𝞸 |

- Greek letters-as-numbers used an older Greek alphabet with three more otherwise unused letters, two of them re‑instated in their old locations, early in the alphabet. So positions higher than 5th place (ε) were shifted from the standard alphabet; 5th place was marked with normal fifth letter epsilon (ε). The 6th letter in the conventional alphabet, that normally followsε isζ (zeta) but the
*number*6 was represented a revived ancient letter′ϝ (digamma), followed by′ζ which was pushed up from 6th to its ancient position (7th) to represent the number 7. All of the letters afterζ were likewise shifted up one place, until the second ancient letter koppa, (ϙ), was reached; it fell betweenπ andρ. Ever letter from ρ to ω was shifted*two*places past its conventional ordinal position. Last place coming right after omega (ω, 800) was sampi (ϡ) which represented 900. (From that point, the system restarted, with a new tick-mark, at**͵**α. The tick-mark was put in a different place (**͵**α rather than′α) to show that the letter represented a multiple of 1,000 rather than 1.)^{[citation needed]} - From Euclid up to the 19th century, mathematical and technical diagrams were habitually marked sequentially with letters (or numbers),
^{[citation needed]}whereas in modern mathematical and scientific diagrams, it is much more common to choose for markers letters that might remind readers of the*word*used to describe the item in question.^{[citation needed]}For example, Feynman diagrams in particle physics label the positions of particles with the first letter of their name, either in the Latin or Greek alphabet. Sop,n, ande , represent the position on a diagram of a__p__roton,__n__eutron, and__e__lectron, respectively. The__n__eutrino is represented byν (Greek "nu"), since the Latin letter "n" is reserved for the__n__eutron.^{[citation needed]} - Sexagesimal Greek numbers in the
*Almagest*are conventional: 1 2 3 4 5 6 7 8 9 = ′α ′β ′γ ′δ ′ε ′*ϝ*′ζ ′η ′θ . Notice that ancient digamma (ϝ) is used for 6 instead of zeta (ζ, which is used for 7) ; 10 20 30 40 50 = ′ι ′κ ′λ ′μ ′ν . Adjacent number-letters add, so all the other numbers are made by letter pairs, such as 29 30 31 = ′κθ ′λ ′λα . The number 59 (′νθ) is the largest value used in a single cell in sexagesimal. That leaves xi (ξ) and the letters following it ξ ο π ϙ ρ σ τ υ φ χ ψ ω ϡ free for other use: Ptolemy picked ο, which normally was used for 70, to mark empty (zero) cells, perhaps because the word for "nothing",οὐδέν starts with an omicron.

- "omicron".
*Oxford English Dictionary*(Online ed.). Oxford University Press.(Subscription or participating institution membership required.) - Martin, Martha Evans (1907).
*The Friendly Stars*(1st ed.). New York: Harper & Brothers Publishers. p. 135. Retrieved8 February 2016. - Wilk, Stephen R. (2007).
*Medusa: Solving the Mystery of the Gorgon*(1st ed.). New York, NY; London, UK: Oxford University Press. p. 201. ISBN 9780199887736. Retrieved8 February 2016. - "Greek and Coptic (Range: 0370–03FF)"(PDF).
*The Unicode Standard, Ver. 8.0*. Unicode, Inc. 2015. Retrieved8 February 2016. - "Mathematical Alphanumeric Symbols (Range: 1D400–1D7FF)"(PDF).
*The Unicode Standard, Ver. 8.0*. Unicode, Inc. 2015. Retrieved8 February 2016.