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.
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 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.
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.
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
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.
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.