The undark nights of summer

For most of June, and into July, the Sun is so far north in the sky that it doesn’t get much below our horizon.

“Our” is used in our usual north-hemisphere-centric way, annoying to southerners such as my cousins in New Zealand.  I should say that in June the Sun doesn’t get much below the horizon for those of us who live in northern latitudes – especially far-northern latitudes.

Our picture is of a stage in the approach to the summer solstice.  The Earth is traveling away from the viewpoint, and on June 21 will reach the position where its north pole is tipped most steeply sunward.  As you can see, the north pole and a ring around it already have 24-hour daylight.

For a zone of Arctic regions starting about 15 degrees down from the pole, the Sun does set, but gets no deeper than 6 degrees below the horizon; so this zone get only what is called civil twilight.  For the next zone of Canada and northern Europe and Siberia, the Sun in the middle of the night fails to get to 12° below the horizon, and this is what is called nautical twilight.  And the next zone southward gets no deeper darkness than “astronomical twilight,” with the Sun never as much as 18° down.  Only southward of that is there at least some night-time that is dark enough to be defined as astronomical night.

The Arctic Circle, at latitude 66.56°, is the circle north of which there are at least some nights (centered on the June solstice) in which the Sun never sets.  We could say there is a wider circle at about latitude 61°, north of which there are some nights in which the Sun, though it does set, doesn’t get more than 6° below the horizon; so we might call this the “Civil Twilight Circle.”  And another at about latitude 53°: the “Nautical-Twilight Circle.”  And another at about 47°, the “Astronomical Twilight Circle.”  So only south of this are there are no nights without at least some minutes of true astronomical darkness.

The British Isles, lying between about 50° and 59° of latitude, and most of Canada, being north of the long “49th parallel,” are within that outer circle.  They have a patch of nights – as long as from mid June to mid July – in which there is no really deep darkness.  The Earth is bowing too adoringly toward the Sun.

As a bonus: the red line is the great-circle airline route from New York to London, showing that you should choose a window on the left if you want to watch the Sun as it sets, and then to see it come up over Europe at the end of the shortened night.

And the background stars are those you would see, if viewing the Earth from the direction and distance (9 Earth-radii) as in the picture.  This is a feature I started adding for the large pictures in the new Under-Standing of Eclipses.


3 thoughts on “The undark nights of summer”

  1. Entries about flight remind me of two most memorable ones; the first was a flight after the Grand Eclipse of ’91 from La Paz, Baja CA to LAX. as we travelled norht and west it was one of the longest sunsets I’ve ever seen, and the second was a flight from NYC to Bejing for the next in that same saros series in 2009. As we flew over the arctic circle, the sun never went below the horizon. I thought it well worth mentioning, but the flight attendants didn’t think so and kept trying to hush me up and draw my window shade. I bet Guy could imagine me grabbing the mike from the attendants and announcing the phenomenon to the passengers, LOL. It would be hard to ‘keel-haul’ a pirate passenger at 35,000 feet.

  2. Flying from Montreal to Zurich in June of 1997, I passed a night in just those circumstances, with the added bonus of being accompanied by Comet Hale-Bopp over my left shoulder. Then I got to have dinner at a restaurant out in the country near Gruyeres, and found out why everyone was making such a fuss about the comet’s beautiful blue tail that filled half the sky–in Montreal, there was too much man-made light to see it. Unforgettable.

  3. I remember scribing these twilight circles on my globe after returning home from a summer in Norway (Trondheim area) in 1986. I was just a few degrees south of the sunset circle – and there were mountains to the south, raising the horizon a lot. But one had to be careful to go to bed by the clock, and had to have darkening drapery besides. Later I spent a winter there – to see if I was vulnerable to SAD – and would enjoy the nearly ever-present moon in its fat-gibbous phases. I do not recollect noticing a crescent moon ever, in any season, from that locale. When I was in the inland-highlands on non-moon nights, the darkness was intense when artificial lighting was turned off – rather, the very faint light on the snow was even, from the stars, from the periphery, and it was amazing how the very irregular landscape could (dangerously) look flat as paper. The dark light on the snow had a complementary quality to the light/shadow flatness of solar eclipse – where one feels that shadows ray from yourself and that everything around you is illuminated from the other side.

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