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Chinese Scientists Gravitate Towards African Solar Eclipse

Two farmers in southern Madagascar watch solar eclipse June 21. AFP Photo by Alexander Joe
by Wei Long
Beijing - June 26, 2001
Chinese scientists were able to obtain high quality data for an unusual physics experiment during the total solar eclipse on Thursday in Zambia. As the shadow of the Moon swept across the African continent to the cheers of millions of locals, dignitaries, eclipse chasers and scientists, a group of Chinese scientists were busy collecting data on the Earth's gravity field during the fleeting moments of totality.

The total solar eclipse was the first one of the new millennium. As the Moon inserted itself exactly in between the Sun and the Earth in its orbital motion, the shadow of the Moon traversed rapidly across the Zambian capital Lusaka where the Chinese scientific team was based. Totality at Lusaka started at 3:09:19 p.m. local time (13:09:19 UTC) and lasted 3 min. 15 sec.

The science experiment is a follow-up investigation to a similar study four years ago during a total solar eclipse in China.

Chinese scientists theorize that if particles were the carrier of gravitational interaction between two bodies, a third mass in between these two bodies could possibly give a partial shielding to each of them from the gravity of the other.

Measuring gravity accurately is difficult in ordinary circumstances due to the weakness of the gravitational force. Only very massive objects, such as planets and stars, give rise to significant forces that make relatively accurate measurements possible.

The Chinese scientists reason then that if the gravitational shielding exists, its magnitude would be extremely small. They speculate that the effect may be measurable only through a careful experiment set up so that during a solar eclipse, the Moon slightly shields the gravity of the Sun and the Earth's gravity fluctuates accordingly.

In the early morning total solar eclipse of March 9, 1997, a team of Chinese scientists used a very high-accuracy gravimeter to measure the vertical gravitational acceleration from Moho in the northeastern Heilongjiang Province.

From a stable, isolated and temperature-controlled room the team began recording data from four days before the total eclipse until three days after.

The team published the result in the paper "Precise measurement of gravity variations during a total solar eclipse" in the August 15, 2000 issue of the prestigous international journal Physical Review D of the American Physical Society. The data showed two extremely small anomalous drops in gravity near the time of totality.

After taking into account of the tidal force from the Sun and the Moon, and instrumental errors due to effects of air temperature and pressure changes -- which were kept very small during the eclipse -- the team saw in their data two significant gravity dips just before and after the Moon made contact with the solar disk.

Even though the reduction in gravity was extremely minute, it was noticeable in the data with values of 5.3 +/- 1.4 x 10**-8 m s-2 and 6.8 +/- 1.4 x 10**-8 m s-2 respectively. This amounts to a variation of 0.5-millionth and 0.7-millionth of a percent in the standard value of gravitational acceleration on Earth.

Although the Chinese scientists did not claim that the curious and still unexplained drops in gravity as a proof of the existence of gravitational shielding, the finding calls for further experiments to check the extraordinary result.

For the African solar eclipse expedition the team of six scientists left China for Zambia on June 14, according to a report in Beijing Morning Post. Team members include two astronomers from the National Astronomical Observatories (NAO) and four scientists from the Institute of Geophysics of the China Seismological Bureau (IGCSB).

The sponsors of the eclipse expedition are National Natural Science Foundation of China (NSFC), Ministry of Science and Technology (MOST), Chinese Academy of Sciences (CAS) and IGCSB. The team established its observing base in Lusaka.

Beijing Youth Daily reported that on the eclipse day, which the Zambian government declared as a national holiday, many Chinese residents in the capital area gathered to join other locals, government officials and their guests, and tourists to watch the natural spectacle.

When the Moon completely obscured the Sun and the solar corona appeared amidst exciting cries from eclipse watchers, the Chinese science team members had to keep their eyes on the scientific equipment in their observing base in a basement room.

They had to ensure that the instruments were functioning nominally and collecting valuable data. The newspaper did not say whether any team member even had a chance to take a glimpse of the totality.

Tang Keyun, the expedition team manager and a researcher from IGCSB said afterward, "During the entire eclipse period, the gravimeter and other instruments functioned nominally and collected reliable data. Scientists will continue the measurement for another two days after the eclipse."

On Saturday (June 23) after a week of hard work, the team packed up the equipment and prepared to return to China. Before the departure Tang told Xinhua News Agency: "From the point of view of data collection, the quality of the data obtained this time is better than the gravity data collected during the total solar eclipse in Moho.

"Of course the scientific conclusion will have to wait until the completion of detailed analysis of all data."

According to a report in Beijing Youth Daily on Friday (June 22), the latest eclipse experiment to search for gravity anomaly represented a continuation of a 21-year effort which began with the total solar eclipse in 1980.

On Feb. 16, 1980 when the Chinese population celebrated the first day of the Lunar New Year of the Monkey, the CAS eclipse observing team took up position in a cave in Kunming in the southwestern Yunnan Province. This was the first attempt CAS made to collect data which might show gravity anomaly during a solar eclipse.

In order to obtain accurate data, the expedition team chose to base in a cave because of a quiet environment with a lesser degree of external perturbation.

The team carried two domestically built but low-accuracy gravimeters on the expedition. Team members spent more than 20 days in the cave to collect data.

Wang Qianshen recalled that he and his fellow researchers didn't have the luxury to see the total eclipse for an extended period. They could only crawl to the cave opening to take a quick glance and said "Oh, the Sun disappears!"

Since the gravimeters had only a low accuracy, the data failed to show any gravity anomaly.

CAS made another attempt on May 20, 1985 during a partial solar eclipse. CAS scientists entered a cave in a quarry in Hebi, in the east-central Henan Province, to gather data for more than 10 days. Again the data showed no anomaly.

The breakthrough finally came with the March 1997 total solar eclipse. This time scientists imported an advanced foreign-made gravimeter with a digital data recording system.

Despite the frigid temperature far below freezing, the eclipse experiment team obtained excellent data which showed the two mysterious gravity anomalies.

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