The algorithm, crudely speaking, is based on upvotes and age. Nine upvotes in a short enough period of time will get a story to the #1 position; nine upvotes o

The algorithm, crudely speaking, is based on upvotes and age. Nine upvotes in a short enough period of time will get a story to the #1 position; nine upvotes over many hours might not get a story on the front page at all (depending on the other stories.) If you want to see the details, check out the Arc source code.