What do you mean with explain the pictures.
Pretty simple, no rocket science.
There was a full/half moon and the airplane was crossing it.
No seriously, this can happen anywhere in the world.
I used to live in Boston and have seen it happen a couple of times. The equipment is certainly a factor, then the weather has to be perfect (clear day), but timing is eveything. You have to be at the right place at the right time.
There are also some high altitude airways......(most internationals departing or arriving the US East Coast will overfly Boston)
Even though my equipment is not as good as Mark's I was able to capture one myself.
Click for large version
Photo © Vasco Garcia
Just imagine how this pic would have looked like, if I had taken it with 1200mm instead of 500mm.
Anyway I am sure Mark will read and answer accordingly.