Cardinal	thirty-nine
Ordinal	39th (thirty-ninth)
Factorization
Roman numeral	XXXIX
Binary	100111
Hexadecimal	27

39 is the natural number following 38 and preceding 40.

In mathematics

Thirty-nine is the sum of five consecutive primes (3 + 5 + 7 + 11 + 13) and the sum of the first three powers of 3 (3¹ + 3² + 3³). Given 39, the Mertens function returns 0.

In astronomy

• Messier object M39, a magnitude 5.5 open cluster in the constellation Cygnus
• The New General Catalogue object NGC 39 , a spiral galaxy in the constellation Andromeda
• The Saros number of the
  • solar eclipse series which began on -1718 May 26 and ended on -438 July 3. The duration of Saros series 39 was 1280.1 years, and it contained 72 solar eclipses.
  • lunar eclipse series which began on -1380 March 26 and ended on -82 May 14. The duration of Saros series 39 was 1298.1 years, and it contained 73 lunar eclipses.

In other fields

Thirty-nine is also:

• The atomic number of yttrium
• In the title of the John Buchan novel, The Thirty-Nine Steps.
• The code for international direct-dialed phone calls to Italy.
• I-39 is the designation for a US interstate highway from Normal, Illinois to Wausau, Wisconsin.
• The eternal age of comedian Jack Benny, and many other people as they grow older.
• The traditional number of times citizens of Ancient Rome hit their slaves when beating them, referred to as "Forty save one."
• Japanese Internet chat slang for "thank you" when written with numbers.
• The year A.D. 39, 39 BC, or 1939.