How Chile's Earthquake Shortened Earth's Day
by Jeffrey Kluger
If you want to make sure you get enough sleep on Tuesday night, you might have to get to bed earlier. You don't have to adjust your schedule by much: about 1.26 millionths of a second ought to do it. According to a NASA scientist's computer modeling, that's how much an Earth day should have been shortened by the subterranean upheaval that triggered the Feb. 27 earthquake in Chile. Some basic physics explains why.
Every point on the planet takes the same 24 hours or so to complete a single rotation around Earth's north-south axis, but some points have to move faster than others to spin the full 360° by the one-day deadline. That's because some parts of the planet are much bigger than others, at least in circumference. The Earth's equator is 24,901 miles around. The perimeter of the Arctic Circle, by contrast, is just 9,945 miles, and if you stand five feet from the North Pole, the circumference you inscribe as the Earth rotates is a scant 31.4 feet. Yet in all of those places, it still takes 24 hours to complete a single rotation. (The fact that points along the equator move faster than others is the reason NASA and the European Space Agency put their launch pads in Florida and French Guiana, respectively; fire off your rockets in the direction the Earth spins, and you get a free 1,000 miles per hour).
Earthquakes alter planetary speed in two ways. Shifting plates rearrange the distribution of the Earth's mass, causing it to bulge imperceptibly in spots it didn't bulge before and contract in others. That rearrangement should further shift the Earth's inclination, or figure axis (the axis around which the Earth's mass is balanced, which is slightly different from the north-south axis around which the Earth rotates) — in the case of the Chile earthquake, by about three inches. The law of conservation of angular momentum, however, requires that even under these exigent circumstances, the Earth's angular momentum stays constant, which means the planet must step on the gas (or the brake) to accommodate shifting mass. The same thing happened in 2004 with the 9.1 Sumatran earthquake that triggered the tsunami. That earthquake should have shifted the Earth's figure axis by 2.76 inches and shortened its day by 6.8 millionths of a second, according to computer models.
If the physics seems a bit arcane, consider that you probably spent much of the past two weeks seeing the angular-momentum principle in action — at least if you watched the Olympics. Earthquakes change the Earth's rotation the same way a twirling figure skater changes hers — by extending or tucking her arms in, for instance, to slow down or speed up accordingly. The only difference is that the skater does so decidedly more elegantly.