Cloud would dissipate and another would form the same spot.After watching this reoccur several times I observed with 50x binoculars and saw that a small moving black dot was the source of each new formation. It moved around a bit but stayed in one general area west of my location and over Delaware county,PA. Planes landing at Philadelphia International were much closer so distance was several miles away. This went on for about twenty minutes until dusk. Total clear sky, then cloud, then clears up, then another starts forming around moving black craft ( note: since I faced the setting sun, both cloud and craft were silhouetted). As clouds formed they drifted south but the new ones always formed in a single area that didn't change. No other cloud formations in entire sky but this one.This cycle repeated itself about a dozen times until darkness fell. I have never observed this sort of aerial activity before and wonder if anyone else has.