The moon’ tidal forces are pulling on Earth enough to slow down her rotation, by about 1.4 milliseconds every 100 years. At the same time Earth’s gravitational forces are pushing the moon away by 1.6 inches annually.
Because of what Space.com calls “gravitational jostling,” a leap second has been added to today. On the evening of June 30 the worlds official clocks will add one second to factor in the Earth’s slightly slower rotation, Space.com reported.
“At the time of the dinosaurs, Earth completed one rotation in about 23 hours,” Daniel MacMillan, of NASA’s Goddard Space Flight Center in Greenbelt, Md., said in a statement. “In the year 1820, a rotation took exactly 24 hours, or 86,400 standard seconds. Since 1820, the mean solar day has increased by about 2.5 milliseconds.”
The way it’s done is that the master clock at the U.S. Naval Observatory will move to 7:59:60 p.m. EDT, or 23:59:60 UTC, before turning over to the new hour, Space.com said. Many systems’ clocks will actually be turned off for one second, according to NASA researchers.
The practice began in 1972, with a leap second added 25 times total, the last time on New Year’s Eve 2008, Space.com said.