Wishing You A Good Astronomical Twilight

I loathe not being a morning person. Many a time, have I thought of getting up an hour or two before sunrise or staying up all night to see the Sun rise. On one or more occasions, when I had to catch a train, I got out of bed at 3:30 in the morning —and that's when I had my fair share of, how do I put it, romanticizing the Blue Hour. 

And I equally love sunsets, if not more. Sunrise and sunset are more or less the same stuff. The Sun never actually rises, nor does it set. Only from our vantage point on the spinning Earth do we see the apparent rise and set of the Sun as it arches across the celestial dome, the sky.   

Never say day follows night or night follows day. Day follows twilight, then night. As the Sun sets or before it rises, there follows a period of indirect illumination, indirect in the sense that although the Sun is not above the horizon to light up the neighborhood, the sky remains partly illuminated from atmospheric scattering. If you are a morning person, you must have noticed that close to an hour before sunrise, the darkness of the night gradually lightens to a shade of deep blue. Just like that, during sunset, as the red-orange-crimson twilight colors begin to darken on the west, the sky turns a shade of deep blue on the east, and in an hour following the actual sunset, the sky takes on deeper and deeper shades of blue. This particular period sandwiched between the transition of night into day is the blue hour or l'heure blueue, often the favorite time for photographers.  

Somewhere 
Image Credits: Photo by form PxHere

L'heure bleue (French), the Blue Hour, the Twilight Hour, or whatever you fancy, is the time, the transitory moment between day and night, and vice versa when the sky is not totally lit, nor is it dark but bleue. 

In the language of astronomy, twilight begins only when the uppermost limb of the Sun fully sets below the horizon. It ends right away at "first light" when the first rays of the rising Sun spill across the horizon. Twilight is defined in terms of what is known as solar elevation angle.

Solar elevation angle is the measure of the Sun's elevation (how high it is in the sky) with respect to the horizon. On this basis, twilight hours are as follows: 

• Civil Twilight: Civil twilight is when the Sun lies 6° below the horizon. Civil dawn refers to the beginning of civil twilight, while civil dusk means the opposite, the end of civil twilight. During civil twilight, the surroundings remain illuminated enough to perform our (human) activities without artificial lights. The western or the eastern sky after sunset and before sunrise is clearly marked by the refracted rays of the setting and the rising Sun, respectively. Venus is described as the evening/morning star because it brightens up or dims down in tandem with the progress of the civil twilight into its next phase.   
• Nautical Twilight: Nautical twilight begins when the setting Sun descends below 6° and ends with nautical dusk when it is about 12° below the horizon. During sunrise, nautical dawn starts when the Sun is 12° below the horizon and ends with civil dawn. The use of "Nautical" comes from the fact that during this time, the sky darkens progressively to the point where sailors find it hard to distinguish the visible horizon. Thus, they shift to the stars for navigational purposes. At this phase, the outdoors is insufficiently illuminated for (human) activities, and we need artificial lights. With each passing minute, one by one, the stars proliferate across the darkening night sky.  
• Astronomical Twilight: Astronomical twilight is the final phase before nightfall or the first installment towards daybreak when the Sun sits 18° below the horizon. During astronomical twilight, every form of scattered light from the Sun disappears completely, and the sky becomes pitch dark for performing astronomical observations of faint stars. The faintest stars detectable by the naked eye, down to what is known as sixth magnitude, appear by this time and disappear around the start of astronomical dawn.  

A schematic to show the three twilights. Drawn not to scale. 

From my location, and taking the help of timeanddate.com, the Sun sets today (as of drafting this article on 07-06-2023) at 06:22 p.m., local time, and marks the start of civil dusk. It ends around 06:44 p.m., where it begins nautical twilight. Nautical twilight ends at about 07:14 p.m. Alas, day follows night with the Sun 18° below the horizon at 07:45 p.m., ending the astronomical twilight. 

Night follows day with the start of the astronomical twilight at 03:24 a.m. the next morning (08-06-2023). Nautical twilight starts at 04:01 a.m., with the eastern sky slightly lighting up again because of atmospheric refraction. Civil twilight kicks around 04:28 a.m., and here comes the Sun at 04:51 a.m.  

Twilight hours vary by latitude and also by the time of the year. Close to the equator, the twilight times generally do not last more than half an hour while, at higher latitudes, beyond the Tropic of Cancer (23° N) and Tropic of Capricorn (23° S), scattered sunlight continues to illuminate the sky for more than an hour after sunset. For still higher latitudes, in the land of the midnight sun, and at certain times of the year, twilight continues for hours to days. In any case, you could go to the website I have just mentioned, timeanddate.com, and see for yourself. 

Throughout this article, I would have loved to include some images from where I live. Living out my days in the city, almost suffocating under all sorts of pollution, including light pollution, I have never seen a picture-worthy twilight. With too many lights around, the blue hour's marred for me. It is not that you can't experience the blue hour amid city-lights. A purist at heart, I want it raw, without artificial lights, and hopefully from some remote island. Living in a large city exacts a cost - in my case, twilight and the starry night. 

Anyhow, wishing someone a good day is becoming overrated. Have a good astronomical dawn!