In general, the length of a normal Earth day is about 86,400 seconds; over the course of a year, the length varies by 1 millisecond, or 1,000 microseconds, because of a redistribution in the planet’s mass. This is because Earth’s rotation is not always consistent from year to year or even season to season. Scientists know there are other factors in the rotational equation, including the differences caused by widespread climatic conditions or natural events—and all of them are determined using geometry and mathematics.

For example, during El Niño years (the periodic upwelling of warmer waters around the equator in the Pacific Ocean off South America), the “drag” of the ocean water welling up can slow down the Earth’s rotation. It happened between El Niño years 1982 and 1983, when Earth’s rotation slowed by 1/5,000th of a second, prolonging the day.

Even a giant earthquake can speed up the planet’s rotation by redistributing the Earth’s mass. For example, on March 11, 2011, the 8.9-magnitude earthquake that struck northeast Japan shortened the length of Earth’s day by 1.6 microseconds; the 2010 quake measuring magnitude 8.8 in Chile shortened it by 1.26 microseconds; and a 9.1-magnitude quake in 2004 that struck off Sumatra sped up the planet’s rotation, shortening the day by 6.8 microseconds.