"We have come full circle," O'Brian says. The rotation of the Earth had long been the most accurate measure of time for humanity, but now such technologies as atomic clocks and GPS devices make it possible to measure tiny variations in Earth's rotation. And the scientific reverberations are not just for space junkies. In a July paper in the journal Nature, for instance, researchers in England and France argued that sub-millisecond-scale variations in Earth's rotation that occur on a 5.9-year cycle are probably linked to motions and interactions within the planet's molten core where no one has ever been to take a look.
Amato is a freelance science writer who runs DC Science Cafe.