← 54 56 → 55
← 50 51 52 53 54 55 56 57 58 59 → List of numbers — Integers 0 10 20 30 40 50 60 70 80 90 →
Cardinal	fifty-five
Ordinal	55th (fifty-fifth)
Factorization
Divisors	1, 5, 11, 55
Roman numeral	LV
Binary	1101112
Octal	678
Duodecimal	4712
Hexadecimal	3716

55 (fifty-five) is the natural number following 54 and preceding 56.

In mathematics

Fifty-five is a heptagonal number, a centered nonagonal number, and a triangular number (the sum of the numbers 1 to 10) and a square pyramidal number (the sum of the squares of the integers 1 to 5). It is also a Fibonacci number (the largest Fibonacci number to also be a triangular number) and a Kaprekar number. The number 55, like 89 and all numbers in sequence A028387 of the OEIS, displays an interesting property of the Fibonacci Sequence.

55 is a semiprime, being the product of 5 and 11 and it is the 2nd member of the (5.q) semiprime family. 55 is one of only two integers with an aliquot sum of 17 (the other being 39). 55 has an aliquot sequence of 4 members: (55, 17, 1, 0).

In science

• The atomic number of caesium.

Astronomy

• Messier object M55, a magnitude 7.0 globular cluster in the constellation Sagittarius
• The New General Catalogue object NGC 55, a magnitude 7.9 barred spiral galaxy in the constellation Sculptor
• The Saros number of the solar eclipse series which began on -1255 July 6 and ended on 43 August 23. The duration of Saros series 55 was 1298.1 years, and it contained 73 solar eclipses. The Saros number of the lunar eclipse series which began on -935 April 25 and ended on 345 June 1. The duration of Saros series 55 was 1280.1 years, and it contained 72 lunar eclipses.

In music

• A song produced by Group X was called "Schfifty Five". With the addition of a flash animation, "Schfifty Five" became a common internet meme
• The name of a song by British Indie Rock Band Kasabian. The song was released as a B side to Club Foot and was recorded live when the band performed at London's Brixton Academy.
• Sammy Hagar sang "I Can't Drive 55"
• Tom Waits (and later Sarah McLachlan) sang "Ol' 55"
• Cristian Vogel released an album in 2005 with the title "Station 55"
• Primer 55 an American band

In other fields

55 US gallon drum

Fifty-five is:

• The number of delegates who attended the United States Constitutional Convention in 1787
• In US gallons, a common size of drum containing liquids; see 55 US gallon drum
• In miles per hour,
  • the highest speed limit allowed in the United States between 1974 and 1986 per the National Maximum Speed Law
  • In many states of the United States, a very common speed limit for rural secondary roads and many urban freeways.
• In years of marriage, the emerald wedding anniversary
• The code for international direct dial phone calls to Brazil
• The number of European route E55 from Helsingborg, Sweden to Greece
• The designation of I-55, a freeway that runs from Louisiana to Illinois
• Restaurant slang for root beer
• The number of Sacha Baron Cohen's character, Jean Girard, in the 2006 film Talladega Nights: The Ballad of Ricky Bobby. In real life, however, it is the number of Michael Waltrip
• In 1973, Spiro Agnew, resigned the Vice Presidency at age 55
• In sports, the jersey number, "55" ,is also know as "Double Nickels" or "Double Nicks".
• In the dice game, craps, if you roll two 5's or a "55" this is commonly known as a hard 10 or double nickels.
• Pablo Picasso completed his "Guernica" in 1937, at age 55

• Pro Blend 55 Mocha Capuccino mix

• 55 Days at Peking starred Charlton Heston and David Niven
• Britvic 55 , A soft drinks brand in the UK
• There are 55 languages recorded on the Voyager Golden Record
• Adolf Hitler was the fifty-fifth member of the National Socialist German Workers Party

Historical years

55 A.D., 55 B.C., 1955, 2055, etc.