The mosquitos have returned but so have the night-shining clouds. Last night they feathered the late twilight sky until almost 11:30 p.m. Thanks to a tip from a friend, I was able to get up on a hill with a good view to the north to make a few photographs.
I’d just written about noctilucent clouds (NLCs) over the weekend and had hoped they’d materialize in time to illustrate the article with fresh images. Of course they didn’t because that’s not how nature works. But who’s complaining?
I suspected something was ‘amiss’ with the horizon twilight glow at 10:30 p.m. an hour and a half after sunset. Sure enough, they slowly became more distinct very low in the north-northwest as the sky grew darker and darker. Never more than 5 degrees (3 fingers held together at arm’s length against the sky) high, the wispy, layered, phosphorescent strands were hard to miss by 10:45-11 p.m. Noctilucent clouds form 50 miles up – 10 times higher than regular clouds – when water vapor in the Earth’s middle atmosphere condenses on meteor soot.
My last sighting was at 11:30 p.m. at twilight’s end. Though the official twilight length for Duluth, Minn. according to the Farmer’s Almanac is 2 hours 30 minutes, a pallid glow lingered in the north until at least 12:30 a.m.
Amazing how long twilight does last in the summer months. For every degree of latitude north of 40 degrees north, twilight increases by an average of about 12 minutes. Duluth’s latitude is 47 degrees and Denver’s is 40. That makes twilight linger more than an hour later here in my neck of the woods. It also makes summertime observing the ken of the insomniac and masochist (the mosquitos, remember?).
Just like there are degrees of dark chocolate, so too is twilight sliced into varying degrees of darkness. Civil twilight is the interval of time from sunset until the center of the sun is 6 degrees below the horizon. That’s about up to 30-40 minutes after sunset or before sunrise. During this time there’s still enough sunlight reflecting off the atmosphere overhead to see your way around and recognize faces and landmarks. The brightest planet Venus is also visible at this time.
Nautical twilight spans the time when the center of sun is between 6 and 12 degrees below the horizon or about 60-80 minutes after sunset. The horizon is indistinct but still visible and the brighter stars are easily visible. ‘Nautical’ refers to when sailors can still see the horizon while using the stars to determine their position at sea.
True night with no trace of sun-stoked glowing atmosphere begins when the center of the sun is 18 degrees below the horizon. This marks the end of astronomical twilight. Sky watchers must wait anywhere from 90 minutes up to 3 hours after sunset before the guardians of the night finally chase away the dusk. The three twilight flavors also play out in the morning hours (in reverse) before sunrise.
With twilight hours longest in the northern hemisphere summer months, watching the stars come up is a languorous affair perfectly suited to wine-sipping or kicking back on a beach.