The twilight phenomena

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Visually even more exciting than sunrise or sunset, I personally find the twilight phases with their gentle transitions from bright yellow to a strong orange to a faint red in the evening or vice versa in the morning. The spectacular contrast of these color changes to the blue or purple sky above consistently fascinates me. But how and when exactly do these spectacular color plays occur?

Let’s start by defining the term. „Twilight“ means those moments when the sun is below the horizon before it rises or after it sets, but nevertheless already or still contributes to illuminating the sky and, to a small extent, the landscape. From an astronomical point of view, twilight is defined as the period of time that the sun needs to decrease its position from 0° (sunset) to an angle of -18° below the horizon in the evening and to increase it in the morning. However, this time span is likely too lengthy to simply accept. For this reason, people typically divide it into three categories: civil twilight, which occurs between 0° and -6°, nautical twilight, which occurs between -6° and -12°, and astronomical twilight, which occurs from -12° to 18°. We will learn more about the differences between these phases in a moment.

But the definition is only one part. What I consider more important is that the length of twilight varies with both latitude and the time of year. Depending on the position of the observer and the time of year, it can be over in less than an hour or last all night. Only in the tropical regions around the equator does it almost always last the same length of time, a good hour, due to the relatively constant angular relationship between the earth and the sun throughout the year.

The reason for the deviations in the twilight length lies mainly in the position of the apparent solar orbit to the respective latitude circle. Near the equator, the orbit is nearly perpendicular to the horizon plane, and the sun takes a relatively short path with respect to its elevation angle, thus rising and setting steeply. However, as one moves further north or south from the equator, the sun’s orbit deviates from this ideal short path due to its flatter course. And on a flat path, the sun simply needs longer to reach a certain angle above or, as in our case, below the horizon than on a steep one. Figure 23 shows this clearly if we compare the apparent solar orbits of the same date for 0° and 50° northern latitude. For confirmation, we also want to compare some calculated values for the length of the twilight: 30° northern latitude 80 minutes, 40° northern latitude 91 minutes, 50° northern latitude 110 minutes, 60° northern latitude 147 minutes (in each case for 21.03.).

Additionally, the earth’s position relative to the sun changes throughout the year. The northern hemisphere tilts towards the sun in June, while the southern hemisphere does the same in December. This becomes clear in the differently steeply pronounced orbits in figure 24. They show that the twilight at 40° northern latitude lasts longest at the summer solstice on June 21 with 123 minutes and shortest at the equinoxes on March 21 and September 23 with 91 minutes each. Due to the angular relationships, the winter solstice on December 21 with a twilight length of 97 minutes falls between the other two and is not, as perhaps expected, in third place. Another thing: the length of the twilight after sunset is in every described case identical to the one before sunrise!

Figure 25 shows the essential phenomena of twilight.The twilight arc is the area where the morning and evening colors unfold. We can delimit it not only on the basis of the colors but also because of its spatial distribution, because it extends at an angle of 90° to both sides of the sun and, due to the disproportionately powerful air mass there, which is necessary for light scattering, up to 30° high above the horizon.

The following timetable illustrates the most important phenomena that the evening twilight has to offer. By the way, these phenomena are essentially reversed in the morning, but there are two important points to consider. First of all, our eyes fully adapt to the preceding darkness in the morning, making them more sensitive than in the evening. In contrast, evening twilight derives richer, more saturated colors and thus a qualitative advantage from the often higher humidity and the increased dust and particles in the atmosphere associated with the greater turbulence of the air. All minute data are approximate values for the latitudes of Europe and North America, which, as we have seen, can vary depending on position and season.

30 minutes before sunset – elevation angle of the sun +5°
When the sun sinks to an altitude of 5° above the horizon, the twilight arc makes its first appearance, announcing the impending sunset with a clearly visible change in the color of the sky near the western horizon to a warm yellow or red-yellow. At the same time, the eastern horizon and any clouds above it change to a faint pink, a change often overlooked by the greater intensity on the other side.

Sunset – elevation angle of the sun 0° – beginning of civil twilight
Now begins the phase of twilight, also called civil twilight, when there is still enough light for precise work or reading outdoors. At this time, most towns need to turn on streetlights, and our eyes‘ receptors remain sufficiently stimulated to perceive colors.
When the sun sinks below the horizon line, the yellow-red color in the west intensifies to an immense glow. At the same time, in the east, the flat bluish band of the earth’s shadow and the delicate pink counter-dawn arc above it rise above the boundary between heaven and earth.
The earth’s shadow is nothing more than the projection of the earth’s curvature onto the atmosphere caused by the low sun. The higher we are and the farther our view reaches, the more clearly we perceive it. But normally we cannot follow it with our eyes further than 6° above the horizon. Furthermore, the duration of its visibility depends on the degree of purity of the atmosphere: the more haze particles it contains, the sooner it disappears from our view. If it appears that the earth’s shadow is visible long before sunset, it is usually a reflective haze. The counter-twilight arc is caused by the backscattering of light in the very dense lower atmosphere.

12 minutes after sunset – elevation angle of the sun -2°
With the sun’s elevation at -2°, the earth’s shadow rises higher and surrounds everything in its range with a dull blue-green. The counter twilight arc above now shows a color gradient from violet to orange and yellow to blue from bottom to top. In the western direction, the yellowish twilight arc now darkens a little with increased intensity.

30 minutes after sunset – elevation angle of the sun -5°
A good 30 minutes after its setting, the sun is at -5° and the western horizon has taken on a yellow-orange color. At an altitude of 45° above the sun, the twilight halo spreads out over a diffusely bounded oval part of the sky. Noteworthy from a photographic point of view is the fact that the light here fades vertically (by two exposure values over a short distance) rather than horizontally (rapidly by one exposure value, which then remains constant for a long time).
This area of the sky also displays the so-called purple light oval under favorable conditions. This spectacular display results from the mixing of the direct sunlight, reddened in the atmosphere by Rayleigh scattering, with the indirect, more blue, portion of the stratosphere. Since the amount of haze necessary for the formation of the latter component varies in the stratosphere, which is 20-25 km high, the purple light is sometimes more or less intense and may even remain invisible for years in some places. On average, people see it more frequently in late summer and fall than in spring. Tragically, it is most pronounced a few months after a violent volcanic eruption, when the resulting sulfur dioxide aerosols have dispersed into the high layers of air around the globe.
Photographing the entire sky is challenging due to the contrast between the horizon and the edges that fade into darkness. If you are lucky enough to encounter the purple light somewhere, you will not be able to avoid using a graduated gray filter and narrowing down the area showing valuable details according to accurate contrast measurements.
The eastern sky now sees the increasingly diffuse earth’s shadow and the fading counter-dawn arc because the sun is at an angle to the atmosphere where too little light is scattered to keep it alive.

40 minutes after sunset – elevation angle of the sun -6° – beginning of nautical twilight
At this time the nautical twilight begins with which the horizon „disappears“, i.e., can no longer be clearly distinguished from the sky. Now, we can only see objects on earth dimly, the colors fading to a gray in our perception, but we can see individual bright stars.
As soon as the center of the sun has reached a position of 6° below the western horizon about 40 minutes after sunset, the horizon takes on a striking orange color. The twilight halo and any purple light above it now slowly give way to a sky that is blue again. In the east, the earth’s shadow and the counter-twilight arc have passed, but the first purple and then red alpenglow now lends remarkable accents to the higher elevations located in that direction of the sky.
The alpenglow, a twilight arc that has already set and is only visible to observers standing further west, is captured or reflected by high mountains or even high clouds in opposite the sun. So it is light that has lost a large part of its blue spectrum due to Rayleigh scattering and now appears to us in the remaining strong red tones. Despite the significance of haze particles in enhancing the red hue of the sunset, the intensity of the alpenglow increases with the purity of the air. This is because, due to the long path, the sorting of the wavelengths by scattering alone is sufficient here.
Even with a strong graduated gray filter, the landscape can only appear in the picture as a silhouette due to the stark contrast between the illuminated horizon and the foreground in complete darkness. All more expressive compositions need the illumination of the sun, at least just above the horizon.

50 minutes after sunset – elevation angle of the sun -8°
A good three quarters of an hour after sunset, the visibly fading reddish halo still reaches up to 10° into the sky above the western horizon. In the east, however, the counter-twilight arc has given way to a last faint reflection against the otherwise dark firmament.

Elevation angle of the sun -12° – beginning of astronomical twilight
After 70 minutes, the sky above the western horizon has lost all color and the nautical twilight gives way to the astronomical twilight, which lasts until complete darkness at a sun elevation of -18°. From this moment on, the sun no longer contributes to the sky’s illumination, scattering its light on the upper atmosphere, leaving many stars visible in an unclouded sky.

The following series of numbers may give a manageable impression of the actual twilight lengths depending on latitude and season. I provide the twilight lengths in minutes, where SR stands for sunrise and SS for sunset. The following holds true for the respective latitudes of the southern hemisphere: The dates for 03/21 and 09/23 are identical because of the equinox; 06/21 in the northern hemisphere corresponds to 12/22 in the southern hemisphere, and 12/22 in the north corresponds to 06/21 in the south.

0° N 0° W Equator
21.03.  civil  21  nautical  24  astronomical  24  SR  06:04  SS  18:10
21.06.  civil  23  nautical  24  astronomical  26  SR  05:58  SS  18:06
23.09.  civil  21  nautical  24  astronomical  24  SR  05:49  SS  17:56
22.12.  civil  23  nautical  26  astronomical  26  SR  05:55  SS  18:03

20° N 0° W Mexico City, Mumbai
21.03.  civil  22  nautical  26  astronomical  25  SR  06:03  SS  18:12
21.06.  civil  24  nautical  30  astronomical  30  SR  05:21  SS  18:42
23.09.  civil  22  nautical  25  astronomical  26  SR  05:49  SS  17:55
22.12.  civil  24  nautical  28  astronomical  27  SR  06:31  SS  17:27

40° N 0° W Naples, New York City
21.03.  civil  27  nautical  31  astronomical  33  SR  06:01  SS  18:14
21.06.  civil  33  nautical  42  astronomical  48  SR  04:31  SS  19:33
23.09.  civil  28  nautical  31  astronomical  32  SR  05:49  SS  17:55
22.12.  civil  31  nautical  34  astronomical  33  SR  07:19  SS  16:39

60° N 0° W Bergen, Anchorage (21.06.: Polar summer)
21.03.  civil    42  nautical  50  astronomical  65  SR  05:58  SS  18:18
21.06.  civil   308  nautical  0   astronomical  0   SR  02:36  SS  21:28
23.09.  civil    42  nautical  50  astronomical  54  SR  05:47  SS  17:56
22.12.  civil    58  nautical  57  astronomical  52  SR  09:03  SS  14:55

Next Atmospheric refraction – The stars are lower than we think

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