I tried to figure out what species this spider is. Apparently narrowing it down that precisely is usually done by looking at their genitals with a microscope, but I think it's pretty definitely a member of the orb-weaver genus Araneus (http://www.volny.cz/alspiders/main_araneidae.html), probably Araneus diadematus (http://animaldiversity.ummz.umich.edu/site/accounts/information/Araneus_diadematus.html) or a closely related species.