Huge Elongated skulls, aliens. Out of place artifacts, aliens. TPTB, aliens. Ritual mutilation/sacrifice, aliens. Mysterious human origin, aliens. Complex ancient structures, aliens. Religious/spiritual entities, aliens. Atmospheric phenomena, aliens
the list goes on, feel free to add.
they used the stars to track the precession of the equinoxes. it was there calendar and navigation tool. Sky gods where from heaven to them, not space.