What celestial motion is tracked via Earth observation to mathematically pinpoint the Moon's rotational axis intersection?

Libration

Determining the fixed reference point of the lunar South Pole relies on tracking subtle movements the Moon exhibits as seen from Earth, a phenomenon known as libration. Libration encompasses the slight nodding and weaving motions the Moon performs relative to an observer on Earth. By precisely tracking these movements, scientists can mathematically model and pinpoint the exact location where the Moon's rotational axis intersects its surface. This mathematical establishment of the intersection point creates the fixed reference frame necessary for all subsequent lunar mapping, coordinate systems, and critical mission planning activities, ensuring spacecraft maintain consistent orientation relative to the Moon's spin.