Then I was like: oh my god, the aliens have arrived, and their massive mother ship is descending on us at this moment! And then I realized that was insane.
Finally, I realized that, of course, it was just clouds. But it was still really awesome. There wasn't enough cloud cover close to the surface to partially obscure anything, the clouds that did block out the moon were dense enough to do so, but not compact enough so that they could allow repeated blocking and unveiling of the moon, and the clouds were at the best distance to both allow all of this and still be moving fast enough that you got a total "cloudclipse" every twenty/thirty seconds or so.
Now, I've obviously seen clouds obscure the moon before. But I've never before seen a relatively clear night where clouds could be seen repeatedly and completely (progressively) darkening the moon and then moving on. I definitely smoked more cigarettes today than I normally do sitting and watching it: it was pretty sweet.