Millimeter deviations from the expected wobble of the Earth’s axis are giving geophysicists clues to what happens 1,800 miles underground, at the boundary between the Earth’s mantle and its iron core.

A: Astronomers call the wobble of Earth’s axis precession. While most of us are familiar with our planet’s primary motions — rotation (spinning once a day) and revolution (orbiting the Sun once a year ...

Earth’s spin is not as steady as it looks from the ground. As ice sheets melt and aquifers are drained, scientists now say the planet’s axis has shifted by more than 30 inches, a subtle but measurable ...