Noctilucent clouds and other encounters with the twilight zone

Noctilucent clouds last night June 11 around 11 p.m. in the northeastern sky against the starry backdrop of the constellation Perseus. Credit: Bob King

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.

Wide-angle view of the northern sky and NLCs about 11 p.m. last night. The bright star at left is Capella in the constellation Auriga. Credit: Bob King

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?

After the noctilucent clouds faded and twilight supposedly ended, a very faint glow (not aurora) still lingered. Since the night was very clear, it was probably the faintest remains of twilight. Credit: Bob King

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.

Denver is closer to the equator than Duluth, with the sun making a steeper angle to the horizon. In an hour’s time, the sun travels farther below the horizon than it does during the same time in Duluth. This causes the sky to get darker sooner in Denver during the summer months. ¬†Angles are exaggerated for clarity. Illustration: Bob King

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.

Twilight lingers as the faint glow near the horizon in this photo taken around midnight last night. Twilight occurs when sunlight from below the horizon still illuminates the atmosphere overhead. Credit: Bob King

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?).

Each type of twilight depends on how far below the horizon the sun is either after sunset or before sunrise. 10 degrees is equal to one fist held out at arm’s length against the sky. Credit: TW Carlson

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.

19 thoughts on “Noctilucent clouds and other encounters with the twilight zone

  1. Bob, do you know why one of the twilights is called ‘nautical’? I assume it had something to do with shipping.

  2. This time of year near the summer solstice if you are away from lights one can follow the suns faint glow (twilight I guess) around the northern horizon west to east thru the night and watch the sky grow in its glow heading to dawn. I have done this a few nights at the cabin. Are there degrees of dawn about your phottoo Bob? May I ask the details about your photos please? Take care. Mike

    • Mike,
      The noctilucent cloud shots had variable times but generally ISO 800, f/2.8 and 15-30 seconds. Focal lengths also varied from 16mm to about 100 mm.

    • Hi Mike,
      Yes. Nautical twilight begins when the sun’s center is 6-12 degrees below the horizon before sunrise and civil twilight when the sun is 6 degrees below before sunrise.

    • Hi Mike,
      No, I don’t use it for night photography, but I do occasionally for daytime work. I try to pick an exposure that captures the scene best at night.

  3. at least we don’t live far enough N/S for there to be perpetual twilight (according to almanacs) or even daytime, cuz that would really put a crimp on stargazing! also, of course, depending on Venus’ location, and other factors, it can be seen during daytime even (along with Jupiter at times, and there are claims of other planets; Saturn at peak brightness should not be too different from Jupiter). not easily tho.

  4. Astro Bob,

    My name is Brian and I am a 23-year-old man from south east England and I’d like to ask you a question about civil/nautical/astronomical twilight lasting all night in certain locations during the summer months.

    I want to stay in the UK (without having to go abroad or waiting for the sky to darken again later on in the year) and I was going to go on a stargazing holiday in the Norfolk Broads next June (about 15 miles west of Norwich), but after looking at sunrise and sunset times for Norwich, I have discovered to my disappointment that Norwich is too far north for it to go dark in mid-summer (ONLY 52 DEGREES NORTH!!!).

    I am now planning on going on my stargazing holiday next June to Stonehenge (which is a bit further south and far away from a major city) and I’m planning to stay at a cottage in Amesbury (about a mile or two east of Stonehenge).

    I know that Devon, Cornwall and even the Channel Islands are further south than where I plan to stay, but I really hope Stonehenge is far enough south for it to get completely dark in the summer, I really hope (to the deepest desire of my heart) that Stonehenge isn’t “too far north”.

    • Hi Brian,
      Nice to meet you and thanks for writing. I hate to be the bearer of bad news but the latitude of Stonehenge is 51.2 degrees north. That means you’ll see twilight throughout the night during June. The minimum amount will be from about 12:30-2 a.m.

  5. Astro Bob,

    Bob,

    Thanks for the comment, I really appreciate it. It is a shame, seeing as I love astronomy and I am passionate about my country, however I have just checked twilight times for Truro (most southern city in England according to timeanddate.com) and we get dark nights through most of May, the last week in July and through all of August – In Jersey we also get a few days of dark nights in June and most of July so I don’t feel too bad now.

    Do we see the same stars in early August as in the middle of June (in the northern hemisphere), as I will provisionally go on my stargazing holiday to Stonehenge in early August next year?

    If we see different stars than in June, I will go to the Channel Islands at the end of July next year to observe the night sky.

    Brian.

    • Brian,
      Most of the stars are the same mid-June and early August. Constellations will have moved westward compared to June but earlier sunsets and shorter twilights partly compensate for their movement. Scorpius will be lower and Leo and Jupiter gone, but Sagittarius, the Summer Triangle and the Milky Way will be magnificent in August. Andromeda (along with the Andromeda Galaxy), Pegasus and Cassiopeia, rich in star clusters will be higher in the east and better placed. I know if I had a choice between mid-June and early August observing, I’d take the latter. Mosquitos aren’t as bad as least here in the states. Do you have to deal with them in England by the way?

  6. Bob,

    Many thanks for your comment. no at least we don’t have mosquitos in England, compared to more tropical destinations.

    Astronomical observations also should be avoided when the moon is close to it’s full phase, since the extra light really hampers deep sky viewing. When the moon is full, the extra light is not noticeable in light polluted cities, but in rural areas the sky always remains blue throughout the night and never really goes dark.

    Brian.

  7. Hi Bob (from another Bob),

    I live here in Chandler, AZ. I live in a housing complex with a small lake (two actually), which makes for nice easterly viewing prior to sunrise. We have a large amount of light polution here, being so close to Phoenix.

    Now for my question, along with observations: When I walk to the lake in the morning, I usually start packing my gear during the astronomical twilight minutes, and arrive just about the beginning of nautical twilight. On many occasions, I’ve witnessed reddening of the bottom side of low-level cloud formations during the nautical twilight period, and have several pictures of this phenomenon. I’ve also recorded it in the evening after the end of civil twilight–both only when clouds are evident. The color, or as I like to call it, an “off color mini sunset/sunrise” lasts only for about 10 minutes, if that long, and exhibits darker shades of red, purple, and blue–sans any orange and yellows. I’ve tried to find out more about this, but keep coming up empty handed. I found about NLCs by searching the Internet, so I don’t feel like all is lost, but have you heard or seen anything about what I describe that would confirm my “mini sunrises?’

    Bob

    • Hi Bob,
      Thanks for writing. What you describe sounds to me like reddened sunlight illuminating the edges of distant clouds. Because clouds are at a relatively high altitude they remain aglow shortly before sunrise and after sunset when the sun has disappeared (or yet to appear) for observers on the ground. A similar thing happens to mountaintops. Do you have a photo link?

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