"Col" wrote in message
...
A bit of Googling (unfortunately most of it's conspiracy theory
nonsense) suggests that this is the answer:
http://contrailscience.com/contrails...es-chemtrails/
I think the "edge shadow (volumetric shadow)" is what I saw,
the contrail was lined up with the sun.

However contrails must be crossing the sun all the time and I've
never seen that effect before. I wonder if you only see it if the
sun is at exactly the right altitude and the observer is in exactly
the right position wrt the sun & contrail.

Yes, read the part of the linked article starting at "[Math Warning]".