Three times in the past decade, the Earth's spin has missed a beat as seemingly random blips cause days to temporarily stretch and shrink. These stutters have emerged from the clearest-ever view of how long a day is.
The Earth's rotations fluctuate as the oceans and the atmosphere push and tug on the spinning planet. But these small daily variations hide longer-term patterns.
Richard Holme of the University of Liverpool looked at 50 years of GPS and astronomical data to see how day length varied during that time. His analysis highlighted a well-known cycle of slow changes at the Earth's core, which lengthen days by a few milliseconds over roughly a decade, then shorten them again.
There's also a 5.9-year cycle, due to persistent friction between the Earth's fluid outer core and its surrounding mantle, a wobble that changes day length by fractions of milliseconds a year.
When Holme stripped away both of these cycles, though, sudden unexpected jumps in day length emerged from his calculations. The jumps interrupted the longer-term changes by a fraction of a millisecond, and they lasted several months before going back to normal.