Space Shuttle's Sonic Boom

Factual Questions
1

I had the opportunity once to watch a Space Shuttle launch from about 150 miles away. The plume was quite noticeable, but so was something else…

At the point of launch on the horizon, and stretching directly overhead all the way to the opposite horizon, was a dark line (roughly as wide as a hands’ width with your arm extended).

This was neither cloud nor smoke, but more like a shadow, and the shuttle rose directly up the middle of it (before veering off slightly after two minutes or so, at which point I assumed it was in space).

Others I spoke to who had been to launches or had seen them at similar distances never noticed this phenomenon. Any clues as to what this might have been, guys?

2

Just a guess, but I’d say it was the shadow of the exhaust trail.

When I lived in the Mojave Desert (the first landing site for the Shuttle), mountains beyond the eastern horizon cast their shadows in the sky before sunrise.

3

I’d bet it wasn’t a sonic boom for two reasons. 1) The boom would most likely be transparent, and 2) if it was visible, it would appear as a conical section with the point of the cone at the nose of the shuttle. I really can’t hazard a guess as to what it may be without a more detailed description.