Gyroscope measurements of the precession and nutation of Earth's axis.

High-precision, Sagnac interferometry has long been proposed as a route to test fundamental questions in physics such as the magnitude of relativistic precessions (e.g., the Lense-Thirring effect). Although many elaborate experiments have been performed using, for example, matter wave interferometry or even quantum entanglement, none are within six orders of magnitude of the sensitivity and stability required to achieve such a measurement. We report on the operation of a free space ring laser gyroscope over a period of 250 days under an ambient pressure stabilizing vessel continuously in an unperturbed underground laboratory. Because we measure relative to local inertial space, the precession and nutation motion of Earth's axis are intrinsically contained in the observations. It is demonstrated that optical interferometry, using an ultrastable cavity, yields an accuracy limit for rotation sensing of 48 parts per billion (i.e., picoradians per second), less than an order of magnitude away from the regime in which relativistic effects can be measured.