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Precision

The orbit of Mercury is an elongated ellipse. At the end of each orbit it does not come back to exactly where it was before, but slightly ahead of itself.

The role of precision measurements in discoveries runs through the history of physics. Precision measurements provide exacting confirmation that a proposed law of physics is correct; they exclude wrong guesses; and, most important, they can provide an opening to understanding aspects of the universe that not accessible to direct observation.

Precision measurement played a key role in one of the greatest discoveries in 20th-century physics. Einstein’s relativity theory says that no information travels faster than the speed of light. On the other hand, Newton’s familiar law of gravity says the force of gravity acts instantaneously on distant bodies. To resolve this paradox, Einstein proposed that matter bends and warps space and time, giving rise to gravity.

It was not easy to test Einstein’s new theory of gravity, called the general theory of relativity. A precision measurement was required.

Mercury, the innermost planet in our solar system moves in an elliptical orbit. Astronomers had found that the ellipse of Mercury’s path doesn’t quite come back to the same point; each time Mercury revolves around the sun, it always comes back very slightly ahead of the ellipse. The effect is extremely small. Scientists had noted this effect before Einstein, but could not account for all of it with Newton's theory of gravity. The anomalous 43 arcseconds/century was explained by Einstein's theory.

Einstein's theory of gravity predicts that Mercury's orbit should come back ahead of itself just by the observed amount.