Editing Omicron (section)

===Astronomy===
Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position.<ref name="martin">
{{cite book
 |last=Martin |first=Martha Evans
 |year=1907
 |title=The Friendly Stars
 |edition=1st |page=[https://archive.org/details/friendlystars01martgoog/page/n154 135]
 |publisher=[[Harper (publisher)|Harper & Brothers Publishers]]
 |location=New York
 |url=https://archive.org/details/friendlystars01martgoog
 |access-date=8 February 2016
}}
</ref><ref name="wilk">
{{cite book
 |last=Wilk |first=Stephen R.
 |year=2007
 |title=Medusa: Solving the Mystery of the Gorgon
 |edition=1st |page=201
 |publisher=[[Oxford University Press]]
 |location=New York; London
 |isbn=9780199887736
 |url=https://books.google.com/books?id=OnHO4orvz18C&pg=PT201
 |access-date=8 February 2016
}}
</ref> Such stars include [[Omicron Andromedae]], [[Omicron Ceti]], and [[Omicron Persei]].

In [[Ptolemy|Claudius Ptolemy]]'s ({{circa|100–170}}) ''[[Almagest]]'', tables of [[sexagesimal]] numbers &nbsp;{{math|1 ... 59}}&nbsp; are represented in the conventional manner for [[Greek numerals|Greek numbers]]:{{efn
|name=why_omicron_is_available| 
[[Sexagesimal]] [[Greek numerals|Greek numbers]] in the ''[[Almagest]]'' are [[Greek numerals|conventional]]: &nbsp; {{math| 1 2 3 4 5 6 7 8 9 &nbsp;{{=}}&nbsp; ′α  ′β  ′γ  ′δ  ′ε  ′''ϝ''  ′ζ  ′η  ′θ}} &nbsp; and &nbsp; {{math| 10  20  30  40  50 &nbsp;{{=}}&nbsp; ′ι  {{prime}}κ  ′λ  ′μ  ′ν}}&nbsp;. Notice that ancient [[digamma]] ({{mvar|ϝ}}) is used for &nbsp;6&nbsp; instead of [[zeta]] ({{math|ζ}}, which is used for 7)&nbsp;. Adjacent number-letters add, so all the other numbers are made by letter pairs, such as {{math|&nbsp;29  30  31 &nbsp;{{=}}&nbsp; ′κθ  ′λ  ′λα}}&nbsp;. The number 59 ({{math|′νθ}}) is the largest value used in any single number cell in [[sexagesimal]]. That leaves [[Xi (letter)|xi]] ({{math|ξ}}) and the letters following it ({{math|&nbsp;ξ ο π ϙ ρ σ τ υ φ χ ψ ω ϡ&nbsp;}}) free for other use: [[Ptolemy]] picked {{math|&nbsp;′ο&nbsp;}}, which normally was used for {{math|70}}, to mark empty (zero) cells, perhaps because the word for "nothing", {{math|οὐδέν}} starts with an omicron.
}}{{math| ′α  ′β ... ′νη ′νθ }}. Since the letter omicron [which represents {{math|70}} ({{math|′ο}}) in the standard system] is not used in [[sexagesimal]], it is repurposed to represent an empty number cell. In some copies, zero cells were just left blank (nothing there, value is zero), but to avoid copying errors, positively marking a zero cell with omicron was preferred, for the same reason that blank cells in modern tables are sometimes filled-in with a long dash (—). Both an omicron and a dash imply that ''"this is not a mistake, the cell is actually supposed to be empty."'' By coincidence, the ancient zero-value omicron ({{math|′ο}}) resembles a modern [[Arabic numerals|Hindu-Arabic]] zero&nbsp;({{math|0}}).