The act of making educated predictions about whether or not the girl has an ass. Using deductive reasoning to predict quality of ass.
The girl sitting next to us looks like she has a nice butt. I can't for sure tell though. Asstrology
The practice of pulling horoscopes and other as(s)trological information out of one's rear.
Don't bother with your horoscope, it's just asstrology.
My friend checks the asstrology column in the newspaper every day.