A very large, dirigible shaped craft gliding slowly over the field next door.

It was very early on a warm summer day. My brother and I were sleeping outside in my parents back yard on a couple of hospital beds. My dog woke me up whining and jumping trying to get up onto the bed.  The dog then looked up at the sky, yelped and ran for her dog box on the porch and jumped inside. I looked up to see what had bothered the dog and there was a huge craft that at first I thought was a dirigible. The sun was just coming up to the east over the Sierra Nevada's and I was facing west where the object was. The first rays of the sun were being reflected by the shiny metal skin. The object was about very low---probably no more than 1,000 feet high. It looked like an old dirigible without the basket under it. The craft appeared to have five tail 'fins', again no basket under it and no markings. It made a low, grumbling noise like a jet plane sounds at some distance away--but steady, no undulations or variations in the sounds.  I would estimate the length at about 400 feet, and the diameter about 100 feet.
Anyway, my brother and I are just gaping at this huge object that is gliding ever so slowly over the next door neighbors pasture. I decide to run in to wake up my parents-- who refuse to believe what I'm telling them is outside. I run back outside and just get out the back door and my brother is telling me 'hurry up---it's starting to really move!'.  I run out in time to see the thing accelerate at an incredible rate (without any change in the sound) going west, and just wink out of sight.
I thought then, and I still do, that the object behaved as if it were lost.  It acted as though it popped into existence to check its' bearings and then took off again. It's the only UFO I've ever seen...…
Date is approximate.  PD