23
23 is a prime, odd, a calendar year.
Twenty-three is the ninth prime, the smallest odd prime that is not a twin prime, and the answer to the birthday problem: in a group of 23 people there is roughly a 50% chance two share a birthday.
Historical context — 23 AD
Calendar year
AD 23 (XXIII) was a common year starting on Friday of the Julian calendar.
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Historical context — 23 BC
Calendar year
Year 23 BC was either a common year starting on Saturday or Sunday or a leap year starting on Friday, Saturday or Sunday of the Julian calendar and a common year starting on Friday of the Proleptic Julian calendar.
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Year facts
- Year type
-
Common year
Standard 365-day year; not divisible by 4 (or divisible by 100 but not 400).
- Days in year
- 365
- ISO weeks
- 52
- Started on
-
Sunday
January 1, 23
- Ended on
-
Sunday
December 31, 23
- Friday the 13ths
-
2
2 Friday the 13ths this year.
- Decade
-
20s
20–29
- Century
-
1st century
1–100
- Millennium
-
1st millennium
1–1000
- Years ago
-
2,003
2003 years before 2026.
In other calendars
- Hebrew
-
3783 / 3784 AM
Rosh Hashanah falls in September/October.
- Chinese
-
Year of the zodiac:Water zodiac:Goat
Sexagenary cycle position 20 of 60. Lunar new year falls in late January / mid-February.
- Buddhist Era
-
566 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Ethiopian
-
15 / 16 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
-55 / -56 Saka
Indian national calendar; year starts in March.
Cultural significance
The "23 enigma" — Discordian/Illuminati-flavored coincidence-spotting.
Popularized by Robert Anton Wilson in his Illuminatus! trilogy.
Sourced from Wikipedia (Numerology, Chinese numerology, Gematria, and per-culture articles).
Properties
Primality
23 is prime. It has exactly two divisors: 1 and itself.
Divisors & multiples
Sums & aliquot sequence
Continued fraction of √n
√23 = [4; (1, 3, 1, 8)]
Period length 4 — the block in parentheses repeats forever.
Representations
- In words
- twenty-three
- Ordinal
- 23rd
- Roman numeral
- XXIII
- Binary
- 10111
- Octal
- 27
- Hexadecimal
- 0x17
- Base64
- Fw==
- One's complement
- 232 (8-bit)
- Scientific notation
- 2.3 × 10¹
Historical numeral systems
- Babylonian (base 60)
- 𒌋𒌋𒁹𒁹𒁹
- Egyptian hieroglyphic
- 𓎆𓎆𓏺𓏺𓏺
- Greek (Milesian)
- κγʹ
- Mayan (base 20)
- 𝋡·𝋣
- Chinese
- 二十三
- Chinese (financial)
- 貳拾參
Digit at this position in famous constants
- π — Pi (π)
- Digit 23 = 6
- e — Euler's number (e)
- Digit 23 = 2
- φ — Golden ratio (φ)
- Digit 23 = 5
- √2 — Pythagoras's (√2)
- Digit 23 = 6
- ln 2 — Natural log of 2
- Digit 23 = 2
- γ — Euler-Mascheroni (γ)
- Digit 23 = 5
Also seen as
As an ASCII codepoint, 23 is control character (0x17). ASCII control character.
TCP/UDP port 23 is the well-known port for Telnet — Plaintext remote login (deprecated).
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.23.
- Address
- 0.0.0.23
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.0.23
Unspecified address (0.0.0.0/8) — "this network" placeholder.
As a geographic coordinate in degrees, this matches:
- Tropic of Cancer / Capricorn (latitude) — Approximately 23.4° latitude — the Sun is directly overhead at noon on the June (Cancer, north) or December (Capricorn, south) solstice.
Heard as a frequency, 23 Hz is closest to:
- Concert pitch (A4 = 440 Hz): F♯0 (-9¢)
- Scientific pitch (C4 = 256 Hz): F♯0 (+28¢)
On the periodic table, atomic number 23 is Vanadium (V) — period 4.