55 (number)

55 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, made up of the numbers 1 to 10 added up, and it is also 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.

In 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 other fields

Fifty-five is also:

• The atomic number of caesium
• The number of delegates who attended the Constitutional Convention.
• In US gallons, a common size of drum containing liquids, see 55 US gallon drum.
• In miles per hour in many states of the United States, a common speed limit; it was the national speed limit for a time during the energy crisis of the late 1970s.
• 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 United States Interstate 55, a freeway that runs from Louisiana to Illinois.
• The year AD 55, 55 BC, or 1955.