1978 Covington, WA Disc-shaped Object

Disc was cobalt blue in color, roughly 100 ft wide and 6 feet thick. It was first seen in Souther sky as a light with same luminescence as Venus. It approached and to the North and began descending at a 45 degree angle when it was approximately a mile away. We stood up when the disc was roughly 300 years away. The disc hovered over our home, directly over head, and we got out from under it in order to see it. After a minute of looking at it a white strobe light flashed at us for 5-10 seconds and then stopped. Before it took off, the ship hummed like a bench grinder, the oscillations building to an apex and when the oscillations 'fell into phase' the entire ship visably 'clicked' into gear, moving less than an inch, and then headed West at a slow speed (5 mph) for a few seconds and then flew straight across the sky so fast it was a blur until it was so small on the horizon I couldn't see it. Then, the object seemed to 'draw a line of light' from its point on the horizon to the stars, at a 45 degree angle; the line of light caught up with the point of origin and it looked like a star in the Western sky, and then the light went out.
Date is approximate.  PD