June 30, 1972. The ‘leap second’ is introduced for the first time simultaneously across the world, effectively stopping the world’s clocks for one second. With the length of the mean solar day – by which we base our concept of time – constantly increasing, leap seconds are required to bring clocks back into sync. The International Earth Rotation and Reference Systems Service decides when the world needs a leap second, either on June 30 or December 31. To date there have been a total of 24 leap seconds, with the most recent occurring on the last day of 2008.
Also on June 30: Charles Blondin crosses over the Niagara Falls on a tightrope (1859); publication of Gone With the Wind (1936); Congo gains its independence from Belgium (1960).
Born on June 30: Assia Djebar (1936), Mike Tyson (1966), Sanath Jayasuriya (1969), Michael Phelps (1985).