Chinese researchers are poised to find out for sure whether gravity fluctuates during a total eclipse (Image: Sean Mahoney/Rex Features)

From remote observatories on the Tibetan plateau to a cave in a Shanghai suburb, Chinese researchers are poised to conduct an audacious once-in-a-century experiment. The plan is to test a controversial theory: the possibility that gravity drops slightly during a total eclipse.

Geophysicists from the Chinese Academy of Sciences are preparing an unprecedented array of highly sensitive instruments at six sites across the country to take gravity readings during the total eclipse due to pass over southern China on 22 July. The results, which will be analysed in the coming months, could confirm once and for all that anomalous fluctuations observed during past eclipses are real.

“It sounds like what is really necessary to break the uncertainty,” says Chris Duif of Delft University of Technology in The Netherlands. “I’m not really convinced the anomaly exists, but it would be revolutionary if it turned out to be true,” he says.


Pendulum anomaly

The first sign that gravity fluctuates during an eclipse was in 1954. French economist and physicist Maurice Allais noticed erratic behaviour in a swinging pendulum when an eclipse passed over Paris.

Pendulums typically swing back and forth as a result of gravity and the rotation of the Earth. At the start of the eclipse, however, the pendulum’s swing direction shifted violently (see diagram), suggesting a sudden change in gravitational pull.

Fluctuations have since been measured during around 20 total solar eclipses, but the results still remain inconclusive.

Relative doubt

Most physicists doubt the anomaly’s existence, because it would challenge our ideas about how gravity works.

As a result, a number of conventional explanations have been suggested. “There could be different reasons: atmospheric changes in temperature or air pressure, people suddenly moving or not moving, or other sudden changes,” says C. S. Unnikrishnan of Tata Institute of Fundamental Research in Mumbai, India.

However, in 2004, Duif posted a theory online concluding that none of the suggested external factors could account for the magnitude and timing of observed anomalies (see www.arxiv.org/abs/0408023).

Best chance

In the run up to July’s eclipse, Chinese researchers have prepared eight gravimeters and two pendulums spread across six monitoring sites. The team hopes that the vast distance between the sites (roughly 3000 kilometres (1864 miles) between the most easterly and westerly stations), as well as the number and diversity of instruments used, will eliminate the chance of instrument error or local atmospheric disturbances.

“If our equipment operates correctly, I believe we have a chance to say the anomaly is true beyond all doubt,” says Tang Keyun, a geophysicist at the Chinese Academy of Sciences.

The opportunity won’t come again soon. At over five minutes, the event will be the longest total solar eclipse predicted for this century. What’s more, the event will occur when the sun is high in the sky; a time when, according to Tang, any potential gravitational anomaly should be greatest.