Jefferson Park, Cambridge Mass, 2 children sight 50ft craft-speak about it 30+years later

Between 1973 and 1978 on a summer evening my brother and I (ages 3and4 to 8 and 9) saw a 50ft craft, 100ft in the air moving slowly and quietly above Jefferson Park.  Numerous colored lights lighting in a rotating pattern.  The lights themselves DID NOT rotate.  There was no "cockpit" or area on the bottom to act as one like a blimp would have.  It was round and somewhat flat unlike a blimp.  
I told my mother and aunt who where outside and they dismissed us.  It was grey and don't remember windows.  It was light out and very easy to see but nobody else saw it or said anything.
Witness indicates that the date of the sighting is approximate.  PD