I’ve been watching the skies for 50 years and somehow an easy-to-observe fact eluded me.
Evening twilight is sunlight from the recently set Sun, scattering in the Earth’s upper atmosphere and spreading laterally along the horizon. Twilight continues until the Sun is 18 degrees below the horizon, at which point the sunlight is no longer visible and night officially begins.
I always thought of evening twilight as occurring above the western horizon, spreading into the northwest and southwest (‘twilight’ occurs in the morning, too, but many of us refer to morning twilight as ‘dawn’).
Last evening, I observed that my understanding of evening twilight wasn’t exactly correct. Since we’re now only two weeks from the summer solstice (the northernmost point that the Sun appears to travel — the apparent motion of the Sun is due to the Earth’s rotation and the Earth’s orbit around the Sun), the location of sunset has shifted well to the north of west. Because of this, and because the Sun at this time of year sets at a shallow angle with respect to the horizon, twilight has shifted toward the north, as well.
The photo below was taken last evening (June 8), looking northwest (West is beyond the left side of the frame). As you can see, evening twilight spread far into the northwest — so far, in fact, that it passed true north and could be seen along the north-northeast (NNE) horizon. Evening twilight in the NNE???
Before last evening, if someone told me that they’d seen something in the evening twilight above the NNE horizon, I’d have tried to correct them. Now, I’m the one who stands corrected.
Unbeknownst to me (until now), for several weeks before and after the summer solstice, evening twilight can be seen in the NNE from our vantage point on Earth. The proof is in the pudding — well, it’s in the photo, anyway.