Specific word for the point of the terrestrial surface "under" a star or other object in the sky The title says it : I am looking for the word to use for a point, located on Earth surface (maybe an hypothetical perfectly spherical Earth), that has the same coordinates as a specific star, and therefore can be considered to be "under" the star in layman's terms.
Positions are usually given relative to the observer, not relative to the object. So instead of saying "I am under the star X", you would say "star X is overhead". The word for an object directly overhead is zenith: "the star X is at the zenith".