Gravity Probe B circled Earth from pole to pole for 17 months starting 20 April 2004 and used gyroscopes to measure two aspects of general relativity. One, the "geodetic effect," arises because Earth's mass creates a kind of dimple in spacetime that messes up the usual rules of geometry. As a result, the circumference of a circle around Earth should be slightly shorter than Euclid's value of 2π times the circle's radius. Gravity Probe B measured the predicted 2.8-centimeter decrement in its 40,000-kilometer orbit to 0.25% precision.

The satellite also confirmed the frame-dragging effect, in which the rotating Earth twists the surrounding spacetime. It's as if the spinning Earth were immersed in honey, Everitt explained. "When it spins, the Earth will drag the honey with it," he said. "In the same way, the Earth drags spacetime with it." Gravity Probe B confirmed the frame dragging effect, which is less than 1/10 times as pronounced as the geodetic effect, to 19% precision...