I was at Foothills Nature Preserve in Palo Alto, CA taking photos of the sunset. My camera was pointed roughly north towards Sutro Tower in San Francisco. What caught my attention was a bright object in the sky. On the back of the camera, it appeared gaseous in nature with a tail. It was not a fast moving object in that it took about 6 minutes to descend from the cloud layer to behind Mt. Tamalpais. Any ideas what I might have seen?

Nikon D750 at 600mm, ISO 200, f/8 and 1/10s.

Lightroom