Observation of relative celestial location and orientation necessary for celestial navigation, the timing of those cycles (which can be modeled and “recreated”) can be referenced as a calendar against which we can match human events.

Galactic navigation suggests “off-Earth” navigation needs.

A galactic calendar suggests needs of longer term than several human generations.

Both would have been useful to visitors from another planet with a 3,600 year orbit cycle who reportedly enjoyed living several tens of thousands of Earth years.