Twilight takes up a significant part of every 24-hour period during the summer months. Paired with the latest sunsets of the year the sky doesn’t get dark until around 10:30 p.m. in the central U.S. and an hour later than that in the northern states. Where I live, twilight is like honey pouring from a jar. The sky remains light for 2½ hours after sunset with darkness setting in only 25 minutes before midnight.
Twilight has three ranges of brightness — civil, nautical and astronomical. Civil twilight begins at sunset and ends when the center of the Sun is 6° below the horizon. There’s still enough sunlight reflecting off the atmosphere to see your way around and recognize faces and landmarks. Nautical twilight spans the time when the center of the sun is between 6° and 12° below the horizon, about 45-60 minutes after sunset. The horizon is indistinct, but sailors can still make out the horizon at sea.
Astronomical twilight occurs when the center of the sun is between 12° and 18° below the horizon. Skyglow is barely perceptible but still present until –18° when the sun is too far below the horizon to illuminate the atmosphere anymore. Only then does true night begin.
I don’t begrudge the slow road to nightfall but instead use the time to explore phenomena best seen (or smelled) during the semi-light. Do you find dusk relaxing? I love the soft light especially after a full day of intense sunlight. Summer dusk is also a paradise for the nose. Like baked bread pulled from the oven the heat of the day “cooks” the ground, plants and trees, making them release all manner of aerosols and odors that linger in the cooling air from dusk until dawn. Where I live it always smells like Christmas around the summer solstice because the air is thick with the perfume of balsam fir sap.
Fireflies become active in late twilight and often continue flashing throughout the night during June and July in the eastern half of the U.S. and Canada. Aside from the quiet pleasure of watching the male fireflies’ courtship dances I enjoy identifying all the different species. You can tell them apart by their individual flash patterns. Probably the most commonly seen firefly are the “Big Dippers” (Photinus pyralis) with their yellow, J-shaped flashes that last just under a second and repeat every 5-7 seconds. These fellows dive down and then pull up hoping to attract the attention of females stationed on the ground.
To date I’ve seen at least 10 different species though I’m still trying to figure out which is which. I use Fireflies, Glow-worms and Lightning Bugs by Lynn Frierson Faust. Watch the flashes carefully — firefly lanterns flash more than just yellow-green. Some are orange or amber while others flicker very quickly. I’ve seen double, triple and even five closely-spaced pulses.
I’ve written previously about noctilucent clouds, one of the most intriguing twilight phenomena. Seek these blue, “meteor smoke” clouds low in the northern sky 1-2 hours after sunset and before sunrise. Then there’s the moon, currently at first quarter phase (a quarter of the way around its orbit of the Earth since new moon) and swelling towards full in the coming week. If you make a drive to the country not only will you far more fireflies and firefly species than near the city, but the softness of the moonlight in summer must be experienced without the savagery of artificial lighting.
Last night as the moon sunk lower the western sky it encountered some high clouds. To my surprise the clouds’ ice crystals draped it in a rare double moon pillar. Both moon and sun pillars form when moonlight (sunlight) reflects from the bottom sides of hexagonal-shaped ice crystals floating with the their flat faces parallel to the ground. It was midnight at the time with mosquitos pinging my face, but I was the happiest guy on the planet.