Words that are close in pronunciation and deal is similar subject matter are always going to be erroneously transposed.
colostomy/colonoscopy is another pair.( and one sure to ellicit an ooof! from some people
)
What is grating to those here is the fact that in the case of astronomy/astrology one is a science backed by centuries of careful study and calculation while the other is not, and confusing(or conflating) the two is seen to bring disreput to the former.