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. They have emerged from the clearest-ever view of how long a day is.
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 regular cycles, though, sudden unexpected jumps in day length emerged from his calculations. Three times in recent years - in 2003, 2004 and 2007 - our planet's spin has stuttered. The jumps interrupted the longer-term changes by a fraction of a millisecond, and they lasted several months before going back to normal.
Satellite readings of the planet's magnetic field over the past 20 years show that the field also undergoes sudden jerks, and Holme found that they coincide with the jumps in the Earth's spin. He says the sudden changes probably occur when a patch of molten outer core temporarily sticks to the mantle, causing a steep change in angular velocity.