In 1954, a man named Maruice Allais spent a month studying and measuring the swing of a Foucault pendulum in his Paris laboratory. He dutifully recorded the direction of rotation, which corresponds to the spin of the Earth. Allais experiment happened to coincide with a solar eclipse, and while the moon blotted out the sunlight, something strange happened to the pendulum: It slowed.

Allais repeated the experiment during an eclipse five years later and got the same results. But other scientists havent been so lucky. Allais findings remain unproven and controversial, but with the last solar eclipse of the century to occur on Wednesday, the mystery might finally be resolved.

Researchers at NASAs Marshall Space Flight Center plan to use a sophisticated gravity sensor called a gravimeter to determine if the solar eclipse has any measurable effect on the planets gravity. Since gravity is a global phenomenon, the experiment does not have to take place within areas that experience the total eclipse.

The NASA experiment, however, is being conducted in conjunction with volunteer scientists, universities and museums around the world, including many in Europe that will be in shadow and total darkness during Wednesdays eclipse. The moons shadow will race across the planet at more than 1,500 mph on Wednesday, inking out sunlight in a 69-mile wide swath across the Atlantic Ocean, Europe, the Middle East and India before dying at sunset in the Indian Ocean.

So far, 19 organizations plan to participate in the gravity studies during the eclipse, using Foucault pendulums and gravimeters. With so many participants, the NASA team should be able to rule out local effects on the instruments caused by temperature or seismic changes.

If the Allais effect proves to be real, scientists already have theories about why it occurs. Possible explanations include gravitational waves, solar radiation and something called anisotropy the condition of having different properties in different directions.