Why It Takes Forever To Get Dark In Summer

I took this photo at 11:15 p.m. June 1 with twilight still lingering in the northern sky. All the tiny “grains” are stars. In summer, the farther north you live the later the sun sets and the longer the twilights. Bob King

This morning I hunted for a too-faint comet and got my first good look at Saturn. I had set the alarm for 2:15. When I finished at 3 a.m. I looked up and saw the pale glow of dawn in Andromeda low in the northeastern sky. Wow, that was quick! Sunrise was still more than 2 hours away, but already the first sign of its coming was plain to the eye.

Twilight comes in three varieties: civil, when there’s enough illumination to clearly see your way around a landscape; nautical, when the horizon at sea becomes too hard to see to navigate and astronomical, when the sky is truly dark without any solar glow. Degrees show how far the sun is below the horizon. TWCarlson

A week ago I caught the other “short end of the stick” when I arrived at my observing site at 11:15 p.m. with the northwestern sky still faintly aglow with twilight. Twilight comes in several varieties, the most familiar of which is civil twilight, the interval between sunset and when the center of the sun sinks to 6° below the horizon. During this time you don’t need artificial illumination to find your way around, and the brightest planets and stars make their first appearance in the sky. At dawn, civil twilight starts when the sun is 6° below the horizon and ends at sunrise.

More relevant for you and I is what’s called astronomical twilight. That begins when the center of the sun dips to 18° below the horizon. At the end of astronomical twilight the sky is truly dark with no sign of sunlight illuminating the atmosphere. Where I live it doesn’t get dark till a little past 11:30. The late hour coupled with inevitable mosquitos can make summertime observing a challenge. We do it anyway because we’re drawn by the beauty of the Milky Way and a myriad of fascinating sights to see in our telescopes.

Nights are up to 4 hours longer for observers in the southern U.S. for two reasons: the sun sets earlier and rises later, and twilights are shorter. Duluth and New Orleans share a similar longitude but differ in latitude (north-south distance) by 17°. That difference lops an hour off the sunset time, adds 45 minutes to the time of sunrise and shortens both dusk and dawn a full hour. Seeing that in writing makes me really jealous right now.

On the summer solstice the sun never sets from Prudhoe Bay, Alaska. Even when lowest around 1 a.m. daylight time, it’s stand 3.5° above the northern horizon. Any point from latitude 66.5° to the north pole experiences the midnight sun on the solstice. Stellarium

On the other hand, if you travel to International Falls, Minn. at 48.6° N, evening twilight blends into morning twilight with no true darkness in between. Head north to Fairbanks, Alaska (latitude 65°N) on the summer solstice — the shortest night of the year — and the sun only dips 1.5° below the horizon at local midnight. The sky is so bright no stars are visible around that time. From Prudhoe Bay (latitude 70° N) the sun doesn’t set at all around the solstice, and at the north pole it circles around the horizon never setting for 6 entire months, from March 21 to Sept. 21. Imagine 180 starless nights. Devastating.

Nothing but daylight … North Pole web cam spring – summer 2015

Depending on latitude the sun intersects the horizon at varying angles. From low latitudes like the southern U.S. and Caribbean, it stands high in the sky in summer and intersects the horizon at sunset at a steep angle. That steep angle means it drops to 18° below the horizon — the end of astronomical twilight — relatively quickly, so twilight lasts only an hour and a half.

he sun circles the sky on a path called the ecliptic which cuts through the zodiac. The steep angle it makes to the horizon from San Diego (32.7°) means it moves a shorter distance (and less time) — shown in red — to reach 18° below the horizon for true night to begin. This gives southern California skywatchers earlier sunsets and short twilights. SkyMap with additions by the author

From higher latitudes the sun is lower in the sky and its path is more nearly parallel to the horizon. At sunset it takes much longer to dip to 18° below the horizon and for twilight to end. From 66.5° N and beyond its path is nearly flat to the horizon, so it just goes round and round every 24 hours, never setting.

The sun’s shallow-angled path seen from Duluth, Minn. on and around the summer solstice means it takes much longer (red path) to drop to 18 degrees below the horizon, making twilight nearly3 hours long. The farther north you live, the shallower the sun’s angle, the later the sunset and the longer twilight lasts. SkyMap with additions by the author

In summer, the north pole is tilted toward the sun, the reason the days are incredibly long there and the nights impossibly brief. Locations closer to the equator don’t face the sun nearly as directly, so the normal day-night cycle persists through the summer. If you’re curious how long twilight lasts for your location, find your sunset time and latitude then use the table below to find the length of twilight. Add those times to sunset to find out at what time your sky is truly dark. Times are accurate through the 3rd week of July:

Latitude 25° N to 30° N — 1 hour 32 minutes
”           ” 36° N to 36° N — 1 hour 43 minutes
”           ” 37° N to 42° N —  1 hour 59 minutes
”           ” 43° N to 47° N — 2 hours 27 minutes
”           ” 48° N to 90° N — all-night twilight, midnight sun at high latitudes

In Prudhoe Bay, Alaska the sun circles around the sky nearly parallel to the horizon and never dips below it around the time of the summer solstice. Stellarium

Twilight times decrease starting in late July through March as the angle the sun drops lower in the sky and the angle it makes to the horizon steepens uniformly across the northern hemisphere moving from late summer into winter. Lengths pick up again as winter turns to spring and then summer.

At the bottom of the sun’s changing angle from winter to summer and back again is a simple fact: Earth’s titled axis. If the planet rotated straight up and down, there would be no seasons. Every location would see the sun rise in exactly the same direction every day of the year, reach the same altitude and set at exactly the same point along the horizon. Twilight length would be fixed per location — short for the equator and tropics and longer the further north or south you traveled.

Here’s the same 24 hours on the solstice but seen from New Orleans, La. Stellarium

Dusk is one of my favorite times to walk. I like the slow-down feeling and soft light — qualities that make life feel easy. Despite the loss of the night, long twilights are still one of summer’s charms.

14 Responses

  1. kevan hubbard

    I suspect in Duluth you would have perpetual twilight on the longest day as it’s probably one of the most northerly towns in the USA excluding the exclave of Alaska.i notice a slightly darker evening in Oxford (about 51.5n.)than in Seaton Carew (54.6)one being in southern England the other being in the north east.in Oxford it was sort of dark at about 2330 when I was looking at Jupiter and Antares up north it would not have been.i got m13 in my Zeiss 5×10 mini quick monocular and 1 Moon of Jupiter and added m29 and m3 in my vortex 8×36 monocular.i couldn’t get m27,the dumbell, normally easy,well in the 8×36 but probably too much light.the Moon is back soon and all this talk of transient lunar phenomenon, which I’ve never seen,adds a huge new interest to moongazing! I’ve tried a comparison just how much 3 degrees allows you to see further south based on Oxford and Seaton Carew but it’s not a fair test as there’s a huge amount of chemical works south of Seaton Carew plus a range of hills, the Yorkshire Moors, reaching 420m whilst Oxford’s south is flatish and not much light pollution (yet!).

    1. astrobob

      Hi Kevan,
      On the longest day we get about 3 hours of true darkness, but just 150 miles north of here, it’s twilight all night.

      1. kevan hubbard

        About 2 years,well 2 and a bit now,I was in Aukland, New Zealand for the austral summer,which is about 35 sth.and even at that latitude darkness was at a premium.i remember walking to a park, actually an extinct volcano called the Dominion, and stargazed from a cricket pitch in there but I had a long wait for darkness to set in.i spent much of my childhood in pietermartzburg, South Africa (about 25 sth) and I can’t remember noticing much difference between daylight hours in summer or winter obviously getting too close to the tropics.

  2. caralex

    I’m from Ireland, and when I lived there, the long summer evenings were one of the pleasures of an otherwise mediocre climate. At midsummer in Dublin it never really got fully dark in the northern sky.

    1. kevan hubbard

      Yes Dublin will stay semi light in the northern skies. I think, although I’ve never been there in June, somewhere like Inverness, Scotland,will only ever have a dusk.further up like lerwick, Shetland islands, doesn’t get dark at all

      1. astrobob

        I once visited a friend in Alaska at 62° north in the summer, and the sky stayed deep blue through the night.

        1. kevan hubbard

          About 15 years back I got right up to narvik, Norway,68.45nth.it was in an April and it didn’t get probably dark even so early in the spring.about 8 years ago I got up to torshavn, Faroe islands,62nth.in I think late July and it never got dark however the weather was unpleasant with constant wind and driving rain.as the hotels are so expensive up there I bought a cheap tent which to it’s credit survived the wind and is probably still in a land fill up there!I Montreal is the furthest north I’ve been in North America.

          1. kevan hubbard

            The faroes make Ireland look like a subtropical paradise although I suspect that the faroese winters don’t get too cold, they’re very green and loads of waterfalls.no flights from Dublin to torshavn the nearest place is Edinburgh which is infact the only place outside Scandinavia with a plane service.the flights from London and Paris seem to have stopped.there might be a ferry from Scotland too but the only problem is it goes from lerwick in the Shetland islands which are nearly as remote! don’t knock the weather in Ireland I got sunburnt on innismore once!and an exgirlfriend of mine sunburnt in Bray!

  3. caralex

    Well, it CAN hit 21 or 22C, if you’re lucky! Lol! Never much higher – but you’re right – an Irish summer day that’s warm and calm is a godsend. In fact, it has a name: a pet day – a day so beautiful that it’s remembered for years afterwards. Seriously.

  4. Hindy Mory Hirt

    A NUMBER of years ago we were in Brittany France on the coast. I still remember it being light at almost midnight on the summer solstice. I may have mistaken it for sunset but I think I remember the light still being in the sky that late in the day yet in Paris it had been dark by about 10:30 Is that possible?

    1. astrobob

      Both are at a similar latitude (48° N) where you might notice twilight lasting until past midnight. My guess is that it was more obvious in Brittany because of less light pollution compared to Paris. Did you have dark skies, fewer lights? A bright, lighted streetscape can make appear dark. You only notice a late, midnight twilight when you’re under a dark sky.

    2. kevan hubbard

      Brittany is sort of slightly south west of Paris so should get darker a tiny bit earlier.paris is about 48.5 north about 3 degrees south of Oxford (51.4n) and I know that Oxford gets dark enough to see DSO’s in Sagittarius, Scorpius,etc at this time of year but the northern sky always has a bit of light in it but it’s still dark for about 3 hours I’d add 1 hour for Paris but otherwise it’d be similar to Oxford a hint of light to the north.

Comments are closed.