Large L-shaped or triangle object, no lights,passed low overhead, no sound. Structure underneath.

I am an amature  astronomer. It was after Halley's Comet was a good viewing object but was still in the sky or possibly when Halley's was behind the sun. I was in my  backyard but not looking through my scope when from the southwest moving toward the northeast an L-shaped or triangle shaped "craft" passed directly overhead. It moved slowly and with no sound. It was generally flat underneath but had structures underneath that looked like I-beams and large circles. It had a dull, flat metallic look. It was as big as an airliner,if low but it was bigger than that if it was higher in the sky. It had no bright lights or any lights. Later when the Stealth aircraft were shown i thougt it resembled a large one of  those but it was silent and slow moving.