A skeptic would likely assume it's not true. In my mind, a scientific-minded person comes up with a hypothesis, models with explaining power, and then thinks "

A skeptic would likely assume it's not true. In my mind, a scientific-minded person comes up with a hypothesis, models with explaining power, and then thinks "how could I devise an (ideally repeatable) experiment that could help me distinguish between this hypothesis and another (likely null) hypothesis?" The fundamental mindset isn't "accept only proven claims", it's formulating hypotheses and then evaluating them via testing. I'd guess many astrological hypotheses aren't especially likely to fare well, and to the extent that would-be scientists have the same intuition we probably won't get a lot of tested astrology. But I think it's also an interesting question where that intuition comes from; generating & exploring hypotheses are pretty fuzzy parts of the process. And how you formulate them matters. If the hypothesis is that Neptune's position is causal, well, there's not a lot of mechanisms we're aware of to build around. OTOH, if the hypothesis is that Neptune's position is correlated with some other variable, still could be a waste of time but there's a lot more mechanisms one could look into. "SAD is a problem with vitamin D" is an acknowledgement that there are these kind of correlations in play. How likely are they to bear fruit? My gut says not super likely, but at some level that's back in that field of intuition. In some ways the ultimate measure of whether something is "scientific" and included/excluded from scientific inquiry is basically a declaration about how interesting or uninteresting it is to scientists.