Twilight Zone Day
"It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of man's fears and the summit of his knowledge. This is the dimension of imagination. It is an area which we call the Twilight Zone."
~ The Twilight Zone (1959 television series opening)
In human history and related folklore, twilight is considered a liminal time, a transitional period in which all kinds of strange occurrences may happen - social hierarchies may be reversed or temporarily dissolved, continuity of tradition may become uncertain, and future outcomes once taken for granted may be thrown into doubt. Many cultures regarded it as a time during which humans were especially prone to interactions and hauntings by witches, spirits, faeries or particularly ghosts -- liminal beings, neither alive nor dead. If you live near the Arctic Circle, the "sustained twilight of May" provides other special opportunities of the astronomical kind. "The Belt of Venus" a pink glow above the horizon is visible shortly in the twilight before sunrise or after sunset!
Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlightscattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark.
There are three established and widely accepted subcategories of twilight: civil twilight (brightest), nautical twilight, and astronomical twilight (darkest).
Atmospheric phenomenon which can be observed during the progression of twilight include the Belt of Venus, and Earth's shadow.
Earth's shadow or Earth shadow is the shadow that Earth itself casts onto its atmosphere and into outer space, toward the antisolar point. During twilight (both early dusk and late dawn), the shadow's visible fringe (sometimes called the dark segment or twilight wedge appears in a clear sky as a dark and diffused band low above the horizon.
For more on the sustained twilight of May (the "bright nights") experienced in Denmark, click the picture of Arctic twilight by Josie Shields.