"What did one tectonic plate say when it bumped into another?    Sorry, my fault."

THE DAILY ASTRONOMER
Wednesday, June 14, 2023
Polar Twilights

Now that we're merely one week shy of the Summer (June) Solstice, we wanted to devote today's article to answering a question about the extent of polar twilights.  As we know, the midnight sun effect can occur anywhere north of the Arctic Circle and south of the Antarctic Circle.    Yet, how far south and north would one find polar twilights: regions where the sun sets but the sky never becomes completely dark due to the twilight effect?

To what latitude does polar TWILIGHT extend in the northern and southern hemispheres? {Obviously, it's beyond the polar circles)

Yes, such twilight does extend beyond the polar circles. First, we should define the three phases of twilight:

• Civil twilight: when the Sun is between 0 - 6 degrees of the horizon. We refer to this twilight period as “dusk” in the evening and “pre-dawn” in the morning.
• Nautical twilight: when the Sun is between 6 - 12 degrees of the horizon. During this phase, the brightest stars and planets* become visible, but the horizon, itself, has not vanished. (It was during this phase that celestial navigators could measure the angles of stars and planets relative to the horizon so as to establish their position at sea, hence the term “nautical.”
• Astronomical twilight: when the Sun is between 12 - 18 degrees of the horizon. During this phase, the horizon disappears and the faintest stars visible at a given location -determined by the ambient light- become visible. Once the Sun is below 18 degrees, any atmospheric scattering of the solar light is negligible. (In my mind, it becomes almost negligible once the Sun is below 15 - 16 degrees.)

Northern Hemisphere first:

On the summer (June) solstice, the southern boundary of the midnight sun region -where the Sun doesn’t set- is at 66 degrees. Although the Arctic Circle’s latitude is 66.5 degrees north, the actual southern boundary is 66 degrees due to atmospheric refraction. (The atmosphere projects the Sun half a degree above the horizon.) That effect makes the math simple. On the June solstice:

• All Night Civil twilight extends down to 60 degrees North. Helsinki will experience civil twilight throughout the night on the summer solstice
• All Night Nautical twilight extends down to 54 degrees North.
• All Night Astronomical twilight extends down to 48 degrees North. However, I think that the latter stage of astronomical twilight is indistinguishable from “pure” night - sk glow apart. So, any resident north of, say, 52–53 degrees will experience twilight throughout the night on the summer solstice.

So, although the Astronomical twilight boundary technically extends to 48 degrees, the truly visible twilight boundary likely extends down just below the nautical twilight boundary, or around 53 degrees north.

While the lower boundary of “all night astronomical twilight” extends down to 48 degrees, the truly visible boundary of “all night visible twilight” extends down to around 53 degrees or so. [Image credit: Wikimedia commons.]

Southern Hemisphere:

Yes, we’re offering the same numbers, but changing the sign, During the December solstice, the northernmost boundary of the midnight Sun region is 66 degrees North. On the December solstice:

• All Night Civil Twilight: the northernmost boundary of “all night civil twilight” is at 60 degrees S latitude. (I would list a city here, but there are no cities at or south of 60 degrees S latitude.**
• All Night Nautical Twilight: the northernmost boundary of “all night nautical twilight” is at 54 degrees South. Less than a degree North of Punta Arenas, Chile.
• All Night Astronomical Twilight: the northernmost boundary of “all night astronomical twilight” is at 48 degrees South. But, again, the true northern boundary of “all night visible twilight” is likely to be at about 52–53 degrees South.

Side note: More people will see northern “all night twilight” simple because of the unequal land distribution between the two hemispheres, as one can see from the map below:

[Image credit: Look Up!]

I hope this answer proves helpful.

*Yes, sometimes particularly bright planets will become visible during civil twilight.

**Source; Pantheos