Hecho matemático: 639 is a number n whose 5^{th} root has a decimal part that begins with the digits of n.
Trivia: 639 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fecha histórica: September 30th is the day in 1980 that Ethernet specifications are published by Xerox working with Intel and Digital Equipment Corporation.