This is my second forty minute exposure of stars circling over the Point Reyes the other night. The original version was horizontal. As this exposure progressed, low-flying clouds were sweeping across the sky, softening and darkening the scene. At the extreme left of the photo, a working boat on Tomales Bay flooded the scene with light. Time passed.
On a technical note, it’s pretty tough to accurately gauge exposures from the LCD at night because the display compensates. It might look decent on the screen, and still be four stops underexposed. So the exposure histogram is a better way to tell if your exposure is in the ball park. It’s unrealistic at night to expect nice, bell-shaped histograms in the middle of the range. But if your histogram is totally clumped on the left, you have a problem.
I glanced at the histogram for the previous exposure, and lightened things up a bit, moving from f/13 to f/10. Had I known the scene would darken as much as it did (because of the clouds), I would have opened it up at least another full f-stop.
[Nikon D300, 12-24mm zoom lens at 12mm (18mm in 35mm terms), 2,407 seconds (about 40 minutes) at f/10 and ISO 100, tripod mounted.]