German scientists have made use of a phenomenon of light called the Sagnac effect to directly measure the rotation of the Earth. In so doing, they have observed not only the simple fact that the Earth rotates, but they have also been able to measure the slight wobbles that occur in the Earth’s rotation due to precession and the gravitational effects of the Sun and the Moon.
The Sagnac effect occurs when two beams of light are sent in opposite directions around a rotating path. (This set-up is usually created with a laser arranged with mirrors which split the laser beam such that the split beams travel in opposite directions around a square-shaped circuit with a detector. This apparatus is called a ring laser.) The light beams travel around the circuit and intersect, at which point they cause interference with each other which can be measured. Because the light beams themselves are traveling at a fixed velocity, if you continuously rotate the whole circuit, the point at which the beams intersect is continually accelerating towards one beam of light and away from the other, which means that each beam of light has traveled a different distance when they finally intersect. Since each beam has traveled a different distance, the beams are no longer in phase when they intersect, which changes the interference pattern. Measuring these changes gives information about the rotation of the whole apparatus.
The Earth itself is rotating, of course, so a sensitive enough detector will detect this effect even in a ring laser that appears to be sitting still relative to the Earth. The scientists in the German experiment have constructed such a ring laser anchored to a concrete pillar, buried underground, and isolated from every factor they could think of that would affect the precise measurement of interference. Their measurements have shown that the Earth not only rotates (which wasn’t really in question) but, more importantly, have also allowed more accurate observations about the slight wobbles in the Earth’s rotation that are normally so small as to be unnoticeable. Refining these measurements of the Earth’s motion will not only provide a greater understanding of the Earth’s motion through space, allow more precise astronomical calculations, and reveal information about the Earth’s structure, they will also allow timing and navigational systems to be more accurately calibrated in the future.