Do you ever feel like some days drag on longer than others? That feeling may be psychological, but actual day length really does fluctuate--by a fraction of a millisecond. (A millisecond is one thousandth of a second)

The length of a day, which is measured by the time it takes Earth to rotate once on its axis, can be measured to an accuracy of about 10 microseconds, or 10 millionths of a second. Earth's rotational rate depends on the distribution of mass across its surface. This includes the roiling aggregation of gases that comprise the atmosphere, the solid earth itself, its fluid core, and the sloshing ocean. For example, when a major earthquake shifts the planet's mass, it can slow or speed the day by as much as a few thousandths of a second.

In fact, the Indonesian Sumatra earthquake in December 2004 that spawned a deadly tsunami moved so much water that it slightly changed our planet's shape and sped its rotation by 2.68 microseconds, or nearly three millionths of a second.

This change in rotational speed, though it is minimal, has been observed for centuries. In 1695 English astronomer Sir Edmund Halley (who also discovered the eponymous periodic comet) hypothesized that the moon was accelerating in its orbit. In reality, Earth's rotation was slowing down, making it appear that the moon was gathering speed.

Since then, scientists have used various methods to measure our planet's rotation, including astronomical devices such as the sundial as well as satellites and lunar observations. And these days scientists have placed thousands of Global Positioning System (GPS) receivers around the world that can track Earth's orientation to within a few millimeters, says geophysicist Richard Gross of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. JPL keeps an in-house database of Earth's rotation dating back to 1962.

Gross says that the most important processes affecting day length are changes in the weather, especially unusual variations in the strength and direction of the winds, which bring on alterations in the global circulation of the atmosphere and ocean. In particular, the vast, high-altitude wind currents known as jet streams, which arise from the differences in temperature between the warm tropics and cooler high latitudes, are responsible for shortening or speeding the day.

Perhaps it's not surprising, then, that global warming may actually speed the day, a fact noted by the U.N. Intergovernmental Panel on Climate Change. In one study published in Geophysical Research Letters in 2007, scientists at the Max Planck Institute for Meteorology in Hamburg, Germany, estimated the mass redistribution resulting from ocean warming would shorten the day by 120 microseconds, or nearly one tenth of a millisecond, over the next two centuries.

Such changes—whether caused by global warming or earthquakes—remain too small to be reliably detected at present, Gross says. After all, there are 86,400 seconds in a 24-hour day and billions of microseconds. Even with GPS, predicting changes in day length remains as difficult as forecasting the weather.

On April 17, 2008, for instance, the day lasted 1.1686 milliseconds longer than the norm According to Gross, the excess varies: Just a few years ago, days were about three milliseconds longer. And all those milliseconds add up: Over the course of a year, scientists estimate that the fluctuations add about a second.

But don't worry, scientists are on top of the phenomenon. The National Institute of Standards and Technology in Boulder, Colo., occasionally adds a "leap second" to the atomic clocks used to standardize time. The last such update took place on January 1, 2006. There's plenty of time to adjust your calendars: "If the excess length of day continues to be about 1.2 milliseconds, another leap second won't be needed for about three years," Gross wrote in an e-mail.