Math lovers and numerologists take note: Today, March 3, 2009, is square root day.
The unofficial holiday comes around but nine times a century, when the numbers of the calendar align so that the month and day are each equal to the square root of the year as expressed in two-digit form. (In today's case, 3/3/09, 3 is the square root of 9—in other words, 32 = 3 x 3 = 9.) Square root days arise in every year whose last two digits are a perfect square, or a number whose square root is a whole number: The last square root day was on February 2, 2004 (2/2/04), and the next occurrence will be on April 4, 2016 (4/4/16).
A square root day group on Facebook, boasting more than 2,400 members, suggests celebrating the minor mathematical oddity by eating radishes or other roots—cut into spot-on squares, of course.
Because of the nature of perfect squares, the wait time between square root days increases by two years each time as the century unfolds—five years separated the previous square root day from today's, seven years will pass before the next square root day in 2016, and nine years will elapse before the following one in 2025. But after the final square root of this century, September 9, 2081 (9/9/81), there will be a slightly prolonged layover before the 22nd century starts its own run of square root days on January 1, 2101 (1/1/01).
UPDATES (6:30 P.M.): Commenter tubegeek points out that today could just as easily be called "square day"—and, by the same token, "square root day" could be applied to 9/3/03 and 4/2/02 rather than 3/3/09 and 2/2/04. It's all a question of semantics: the originators of square root day apparently preferred the "3 is the square root of 9" (3/3/09) interpretation to the alternate but equally valid "the square root of 9 is 3" (9/3/03) interpretation.
firesignth on Twitter notes that this month boasts another notable math date, 3/14, known as pi day. Even better, he points out, is pi second, which falls at 1:59:26 on that date, bringing one numerical representation of the date and time, 3.1415926, into line with pi, out to seven decimal places. firesignth also goes so far as to note that March 16 at 2:27 (which, by the pi second notation, becomes 3.16227) in the year 2010 is very nearly a square root time, because the square root of 10 is 3.16227...
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