Still, the darkening inexorably continues. The next phase is nautical twilight, when the sun is between six and 12 degrees below the horizon. The sky may be a deep purple, and stars begin to appear.

The new moon occurs on Nov. 1, and three days later the moon will make a close pass to Venus in the evening sky.

At the North Pole we have to wait until Oct. 24 for nautical twilight to end. Finally, astronomical twilight — when the sky ...