Yeah, you got that right.
Halley’s Comet!
Hey, I know! Krakatoa! It threw a tremendous pile of debris and dust into the atmosphere. Everybody in the world probably saw that for the next generation or so.
Or maybe … yeah … the Ice Age. We look at the aftermath of that every day! 
And that’s just the earlier risers on the West Coast. The second plane hit at ~7:00 a.m. Pacific, and both towers had collapsed by ~8:30 a.m. Pacific, so I’d say a pretty large chunk of the Western U.S. was asleep or otherwise occupied while the events were actually happening.
Still, I’d guess that the entire East Coast and Midwest was pretty well awake and mostly tuned in to TV news coverage by the time of the collapse, if not the second plane, not to mention the Pentagon strike and the Pennsylvania crash.
What that really means as far as who witnessed what live as it happened, I can’t really say.
I think that the difficulty in determining just what represents a distinct and separate event and just what it means to “witness” it means that you can’t really say that any one of these many choices is “clearly” or “hands down” the one, correct answer.
I don’t see how this is possible. How many people do you think were looking in just the right direction at that exact time? A few thousand? Even that sounds like a generous number. It’s not like all of Manhattan was staring at the towers *before *anything had already happened.
I’m thinking a complete solar eclipse would hardly have been expected in the days before modern astronomy, and it would be very hard to miss for anyone living in the hemisphere facing the sun, so that would potentially make the number of direct witnesses about a quarter billion people or so.
Total solar eclipses are quite narrow. Make it partial and you’ve got a better case. Make it a total Lunar eclipse, and I’ll accept that.
I saw the tops of the towers leaning early on and knew they were coming down which is why I turned off the TV. I didn’t want to see that. (Earlier, I had just turned on the TV when the local AM show threw to the network. I had it on in the background when I heard someone say there was another plane and regrettably turned just in time…)
I also remember hearing a family member say “Come in here, they’re bringing out the guy that show JFK.” And I went in just in time. In terms of % of US viewers, that has to be up there.
Does the unexpected event have to be real? Because everybody knows somebody who knows somebody who put a alligator sewer baby in a microwave hitchhiker ghost in the rain with bathtub full of ice…
How is that unexpected?
I came in to say the '89 San Francisco earthquake during the World Series. I very clearly remember that one.
And “the Spanish Inquisition” is the funniest thing I’ve read all week.
It was to me, but not to Ivylad. I was pooh-poohing him, saying the pilot of the first plane must have had engine trouble, because who in the world would deliberately fly a plane into a skyscraper?
Then I saw the second plane hit. That whole day was a series of unexpected events. 