Looked up and saw sphere or disk 30-50 feet in diameter which was white or pale green in color.

I was about 12 years old and I was on a tree lined road fter dark. I looked up and saw what I thought was a sphere, but could have been a disk,passing overhead. 
Could not tell how high it was but seemed to be fairly low but well above the pine trees.  I estimate the size to be about 30-50 feet in diameter. 
I didn't know what it was and never heard of a UFO unil at least 2 years later.
Witness indicates that the date of the incident is approximate.  PD