When an earthquake struck South America last weekend, the ground rumbled in Chile, the sea rose in the Pacific, and a day on Earth got shorter.

LOS ANGELES — When an earthquake struck South America last weekend, the ground rumbled in Chile, the sea rose in the Pacific, and a day on Earth got shorter.

Not by much.

Earthlings ended up losing 1.26 millionth of a second of a day. You can't sense it. Nor can your dog — the one you insist senses approaching earthquakes — feel it.

But while other experts charted the shift of tectonic plates and the swell of ocean waters wrought by the quake, geophysicist Richard Gross mathematically calculated the temblor's disruption of the day. The thrust quake — in which plates underground move vertically — caused mass to move everywhere, according to Gross, and made the planet slightly denser.

"It got a bit more compact," Gross said. With the Earth's mass pulled a bit closer to its center, the planet rotates faster — "just like a spinning skater brings her arms in closer to her body to rotate faster," he said. When the planet rotates faster, the day shortens. Gross, who works at the Jet Propulsion Laboratory in La Canada Flintridge, Calif., studies the Earth's rotation and how it is affected by cataclysmic forces of nature.

While the Chilean quake shortened the day by 1.26 microseconds — the unit of time for millionths of a second — the 2004 Indian Ocean earthquake that triggered the catastrophic Asian tsunami shaved 7 microseconds off the day, according to Gross' calculations.

Of course, losing just 1.26 microseconds a day would take a couple of millennia to add up to one single second of lost time. (2,174 years to be more precise.)

Gross suggests that it's not worth tallying that way. "It takes a lot of these big earthquakes to add up to even a second," he said. "The bigger changes are in the liquid core within the Earth." Those changes can throw off a day by a whopping three or four milliseconds, he said. "Those are the things that cause us to have a leap second every year or so."

Winds and ocean currents are other forces plying the Earth, changing its shape and the length of its day, according to Gross.

Far from evoking that textbook illustration of a smooth round ball of continents and blue oceans, Gross describes Earth as a planet of unevenly distributed mass wobbling as it rotates around its imperfectly balanced axis, its physique woefully pear-shaped. "It's a bit fatter south of the equator," he said.

"The Earth is not completely elastic. It's kind of like putty," he said. "If you have a sudden shock to it, it will continue to deform later in response to that shock."

Gross only mathematically calculated these minute changes in day lengths. He is not sure it can be physically observed, even by sophisticated GPS equipment. "We have a network of receivers located globally," he said. "I don't think those GPS observations are going to be accurate enough. I'm going to look at the measurements, of course, but I would be surprised if I find anything."

So why do the math at all? "For the most part, the Earth's rotation changes all the time and doesn't have much practical consequence except here at JPL," Gross said. "We need to know this to navigate spacecraft to planets like Mars or Saturn." Since JPL's tracking stations for those craft are on Earth, "we need to know how Earth's orientation is changing."