Disk-shaped saucer seen over Covinton, WA, 1978

I have decided to record my sighting, the date is approximate I was very young at the time and did not even realize what I had seen until years later.  My father worked for Boeings, so I was very familiar with aircrafts.
We were living in a rural community called Covington in Washington State.  My younger brother and me routinely snuck out and ran the cow pastures at night.  It was something to do.  He had a friend named ((male name deleted)) who would normally join us on these adventures.  
One night in May or June of 1978, we snuck out.  We went to ((name of friend deleted)) house to get him.  I would hang in a grove of fir trees and wait for my brother to get his friend.
Anyway, this night I was waiting for them to return.  I had been waiting about 5 mins or so when I caught a glimpse of a red light.  I was scared I had been caught.  I looked up and directly above me about 15 feet above the fir trees was a Craft.  
I remember looking at it and thinking it looked like a horseshoe crab.  It was about 100 feet in diameter with red lights ribbing the concaved bottom.  I stared at it for maybe a minute and then ran for my brother.  
I have always regretted not looking more closely and I find it hard to believe what I saw.
Witness indicates that the date of the sighting is approximate.  PD