All days are not created equal. Some don't just seem longer than others -- they are.

A classic movie about World War II's D-Day was called "The Longest Day." However, scientifically, that title was not correct. That was not the longest day. Nor is a summer solstice, June 21, when the period between sunrise and sunset is the longest in the northern hemisphere.

The longest day in the past century occurred sometime during 1912, according to JPL geophysicist Dr. Richard Gross. The shortest day in the past 100 years was August 2, 2001, when the length of time that it took Earth to make one complete turn on its axis actually dipped below 24 hours by about one-thousandth of a second.

Gross studies Earth's rotation. As it turns out, Earth doesn't rotate like clockwork. In a recent paper in the journal Physics of the Earth and Planetary Interiors, Gross combined several series of length-of-day measurements into one that spans from 1832 to 1997 and smoothed out some of the error with a sophisticated mathematical formula.

"The length of the day changes about a millisecond over the course of a year," says Gross. "It gradually increases in the winter, when Earth rotates more slowly, and decreases in the summer. There are also longer patterns of changes in the length of day that last decades, even centuries."

Since there are 86,400 seconds in a 24-hour day, a few thousandths of a second might not seem to make much difference, but they do. Knowing exactly when and how much Earth's rotation varies may lead to better models of the atmosphere and oceans, improved weather prediction and a greater understanding of the planet's inner workings.

In addition to his research, Gross works with a group at JPL that uses the global positioning system to measure Earth's rotation very precisely, to about one-hundredth of a millisecond. "JPL is one of the few places in the world that has an application for this kind of work," says Gross, "and that's spacecraft navigation."

"If, say, you want to send a lander to Mars," says Gross, "you can track the spacecraft with respect to Earth. So you need to know exactly how Earth is oriented in order to make the right course maneuvers to target a particular landing site on Mars."

"If Earth rotated uniformly, you would know how it is oriented at any particular time in relation to Mars or any other place," says Gross, "but Earth doesn't rotate uniformly."

Gross tries to understand these changes and how to predict them.

"Variations in the length of day were first noted by Edmond Halley in 1695," says Gross. "He was looking at the Moon's motion and thought he saw it accelerating. What he really saw was the Earth slowing down."

Since Halley's time, scientists have used a variety of techniques to measure Earth's rate of rotation. First they used astronomical methods. Now they use lunar and satellite laser ranging; a technique called very long baseline interferometry; and the global positioning system.

The length of the day--how fast or slow the Earth rotates--depends on how Earth's mass is distributed. Its mass includes the atmosphere, the solid Earth and its fluid core. When the distribution of Earth's mass changes, as during a major earthquake, for example, so does the speed of its rotation. "It's like an ice skater," says Gross, "who spins faster as she brings in her arms. She is changing her mass distribution."

"The annual changes in the length of the day," says Gross, "are caused mostly by the atmosphere -- changes in the strength and direction of the winds, especially the jet stream. The Sun warms the equator more than the poles. That temperature difference is largely responsible for the jet stream. Seasonal changes in that temperature difference cause changes in the winds and, hence, the length of the day."

The longer patterns in changes of the length of the day can last for decades. "These are caused by processes within Earth's core," says Gross. "The core is a fluid. Its motion generates Earth's magnetic field. Changes in its motion can change the rotation of solid Earth. Observing the magnetic field at the surface gives us an idea of how fluid is moving within the core. These changes in the fluid motion inferred from the magnetic field match the longer period changes we see in the length of the day."

We happen to be in the midst of one of these long-term patterns now. Annually, the length of the day has been getting shorter since 1992 . If the trend continues, says Gross, the shortest day may well appear this summer.