Non believers in astrology exist and are present in each and every society. They just don't have a category, because ontological position on the effects of stars on human lives or not, they don't make enough noise about it to be recognised as such.
I apologize. For some reason I read "astronomy" rather than "astrology".
Believing stars affect our lives is not an ontological position. One can be a believer in astrology and still not add anything unique to their ontology. As such, it is not an individual and unique ontological position, nor is it well defined.
Do you understand ontology? An "ontological position" (preferably one unique and well-defined) is one that holds a certain number of things exist and a certain other number do not.
You may have confused this with the epistemological positions, which posit whether or not knowledge can be attained on a given subject. "Agnosticism" is an epistemological position - it states that one can never answer (or, at least at this time in history we cannot answer) the question of whether or not a god or multiple gods exist.
It is very easy to be both an atheist and an agnostic. In fact, from a realist philosophy point of view, it is the most neutral and undemanding position to take in that it requires no further arguments past the basis of realist philosophy.
Ontologically Atheist means that in the grand scope of all the things Person X believes to exist, god is not in there. They are not part of the Set Of All Things Person X Believes To Exist.
Epistemologically Agnostic means that Person X believes that it is impossible to prove a god or gods do or do not exist.
These two claims sit in perfect harmony with each other. If you change the word "god" to, well, just about anything else, it becomes very clear why this is so.
Ontologically Without-Belief-In-Aliens means that in the grand scope of all the things Person X believes to exist, aliens are not in there. They are not part of the Set Of All Things Person X Believes To Exist.
Epistemologically Without-Knowledge-Of-Aliens means that Person X believes that it is impossible to prove aliens do or do not exist.
I would say this is probably what 90% of the educated world believes towards aliens. We have no proof, have not seen any evidence, and so they are not part of our ontology. Speaking about epistemology, we can easily see that with today's current technology, and with the given massive size of the universe, it is impossible for us to answer the question of alien existence conclusively.
Do you understand now? I know it's difficult. Frankly, most people who identify themselves as "atheists" or "agnostics" do not actually understand this question or the difference between ontology and epistemology.