90 ft saucer, grey with white spinning lights underneath

In August of 1972 at 11:00 pm, I was at Northwestern Univ. parked 60 ft from the observatory with my boyfriend sitting in a car talking. Suddenly we saw a tiny bright light which I thought was a low star. It was very far away. Within 5 seconds it was upon us. It was a classic "flying saucer' looking object. approx 90 ft long. It was a grey silver color. It had a bottom stand that was circular and had holes all around the base. It looked as if there was a white ball of energy swirling around it very fast. There were no doors or windows on the side that we could see. It hovered over the observatory and then made a dead stop in mid-air about 12 ft above the observatory building. I wanted to get out and get a closer look as we were approx 60 ft away but my boyfriend was terrified and refused to allow me to get out of the car. It remained there for about 6-7 minutes. Then suddenly it lifted up about ten ft, moved slightly to the right where there was a clearing, a road, and then took off at meteoric speed. It was as if it went from zero to 10,000 mph in a nanosecond It was completely out of sight in less than 2 seconds. It seemed to go as fast as a bullet. I know that doesn't seem possible but it's true. I thought it was strange that it was going so fast and yet there was no sonic boom sound. It made no sounds at all.