3
Sunrises and Sunsets
Undeniable feelings of renewal and hope accompany the sunrise; and comfort and peace with the sunset, Figure 3.1. It is no surprise that so much prize-winning landscape photography is created during these special times. This chapter explains the science behind the phenomena of sunrises and sunsets, and why they appear the way they do.
Consider Earth’s appearance from space, Figure 3.2(a). The half of the earth facing the sun is brightly illuminated and experiences daylight, whereas the half of the earth facing away from the sun is in the earth’s shadow, and experiences night. Sunset or sunrise occurs in the day-to-night transition zones. Clouds permitting, bright blue skies will cover the regions experiencing day owing to preferential Rayleigh scattering1 of the blue wavelengths of sunlight by the oxygen and nitrogen gas molecules in the earth’s atmosphere. Dark, clear skies will cover the regions experiencing night as the result of the intrinsic transparency of our atmospheric gases coupled with the complete absence of sunlight-induced Rayleigh scattering.
3.1
Sunrises and sunsets provide reliable opportunities to create beautiful images, in nature and in the city. Here, a new mid-summer’s day dawns in California’s High Sierra, as alpenglow paints the tips of Mt. Banner, left, and the Sierra crest, right, over Thousand Island Lake in the Ansel Adams Wilderness.
During the day, the entire sky dome becomes a diffuse source of bright, blue light in addition to the predominantly yellow light of direct sunlight. When we’re outdoors on a clear, sunny day, it’s like we’re in a giant amphitheater with a glowing blue ceiling! The diffuse nature of the blue light
3.2
(a) The earth viewed from space and the corresponding sky color observed on Earth for different locations. (b) Schematic showing how sunlight loses its blue components by scattering, resulting in its gradual reddening as the length of its path through the atmosphere increases during sunsets and sunrises. (c, d) Comparison of the same location along the Big Sur coast in California during (c) the harsh light of midday and (d) the warmer, redder light of sunset.
Source: NASA/NSSDCA
from the sky allows it to illuminate observers who are shaded and thus not directly in line with the sun. To observe this phenomenon, carefully examine a white sheet of paper outdoors in the shade of a tree or a building on a sunny afternoon. It will exhibit a distinctly bluish tint. If our sky had no atmosphere, shadows would be completely black, as they are on the moon, except for any light reflecting off nearby objects.
So if the daytime sky is blue and the nighttime sky is clear, what is the source of the vivid oranges and reds of sunsets and sunrises? These colors appear because the light from the sun and adjacent sky that reaches observers during sunset and sunrise has traveled through a much greater distance of the earth’s atmosphere than during the day, Figure 3.2(b). As light travels through this greater portion of the earth’s atmosphere during sunset and sunrise, most of its blue light gets scattered away and lost, thousands of miles away, before it reaches the observer. All that’s left by the time the sunlight reaches the observer is predominantly red and orange light, with scarcely any blue light remaining. The greater the distance of travel, the more of its blue light is scattered and lost, leading to an increasingly red sun and sky as the sun approaches the horizon during sunset and sunrise, Figure 3.2(b).
These fiery, final red rays of sunset and first rays of sunrise that strike high altitude mountains, and clouds are the source of the gorgeous phenomenon of alpenglow, Figure 3.3. These beautiful orange, pink, and red colors only last for a few minutes before the sun slips too far out of position for these intensely colored rays to reach terrestrial and atmospheric objects. Nonetheless, these elusive colors are well worth pursuing—you won’t be disappointed!
3.3
The delicate pink of alpenglow graces the clouds surrounding the peak of Mt. Assiniboine in the Canadian Rockies.
Source: Steve Hallmark/SteveHallmark.smugmug.com
3.4
Comparison of images created during (a) the golden hour and (b) the blue hour: (a) Santa Ynez Valley, California; (b) Lamarck Lake, California. Note the emerging stars in the upper left in (b).
The distinctly warm colors of direct sunlight during the hour or so immediately before sunset and immediately after sunrise lead to these periods being known to photographers as the “golden hour.” They create wonderful opportunities for award-winning images, Figure 3.4(a). You can see this effect by comparing the two photographs shown in Figure 3.2(c) and (d); both created at the same location, but one during the harsh light of midday and the other during the warm light of the golden hour. Conversely, the approximately hour-long twilight periods just after sunset and just before sunrise are known as the “blue hour,” Figure 3.4(b). The distinctly blue light from the sky during this period is the source of this name, since there is a total lack of any direct light from the sun. The blue hour provides many opportunities to create images with a very serene and calm mood.
Civil/Nautical/Astronomical Twilight
The moment when the sun crosses the horizon is the key event for landscape astrophotographers, Figure 3.5(a). The precise moment of sunset or sunrise, i.e. when the last vestige of the sun disappears/appears over the horizon, conveniently represents the point in time against which all preceding and subsequent events can be measured. In case you’re interested, Appendix VI has a more complete description of what actually is represented by the term, “horizon.”
We start with the period of twilight between full darkness and either sunset or sunrise, earlier dubbed the “blue hour.” Twilight is actually divided into three separate periods, each with its own name: civil, nautical, and astronomical twilight, Figure 3.5. Civil twilight is often relatively bright, Figure 3.5(b); it is often legal, although ill advised, to drive without headlights during civil twilight. Nautical twilight is characterized by darkening skies but few if any visible stars, Figure 3.5(c); stars really appear during astronomical twilight, Figure 3.5(d). Following astronomical twilight at night and before it in the morning is the period of full darkness, or what we know as night, during which the sky is as dark as it will become.
The transition between each twilight period occurs when the sun reaches a specific angular position below the horizon, as illustrated in Figure 3.5. The transition between civil and nautical twilight occurs when the sun is 5° below the horizon; between nautical and astronomical twilight when the sun is 10° below the horizon, and between astronomical twilight and full darkness when it is 15° below the horizon.
Astronomical twilight is especially important for landscape astrophotographers since it is the time when stars, constellations, and the Milky Way all attain their full brightness, Figure 3.6. Exquisitely colored skies along with visible stars can be observed during astronomical twilight; see, for example, the first Case Study in Chapter 23. Knowing the beginning and ending times of the different periods of twilight is thus extremely valuable as you prepare for your nightscape astrophotography sessions.
3.5
The three periods of twilight, civil, nautical, and astronomical, and their changing sky colors, are defined by the angular position of the sun below the horizon of the observer (yellow arrow). The sun’s angular position relative to the observer changes as the earth rotates, here from left to right. (a) Sunset and the beginning of civil twilight occurs when the last visible part of the sun slips below the horizon; (b) end of civil and beginning of nautical twilight occurs when the sun is 5° below the horizon; (c) end of nautical and beginning of astronomical twilight occurs when the sun is 10° below the horizon; (d) end of astronomical twilight and onset of full darkness occurs when the sun is 15° below the horizon.
3.6
The brightness of the sky during astronomical twilight strongly affects the number of stars that are visible. As the brightness of the sky decreases, the number of stars that become visible correspondingly increase. These images were created after sunset, and (a) at the beginning; (b) one-third of the way through; (c) two-thirds of the way through and (d) at end of astronomical twilight and the beginning of full darkness.
3.7
The dark blue band of the earth’s shadow, just above the eastern horizon, and the pink Belt of Venus directly above it, develop and progressively rise over Lake Superior in Minnesota, during civil twilight immediately after sunset.
The moment of sunrise and sunset also determines the times during which the earth’s shadow becomes visible, along with the so-called Belt of Venus. The earth’s shadow is that distinctly blue band that slowly rises or sets just above the horizon in the sky opposite the sun, Figure 3.7. Sandwiched between the earth’s shadow and the slowly darkening blue sky overhead is a diffuse, pinkish band dubbed the Belt of Venus, Figure 3.7. The planet Venus is often visible on the opposite side of the horizon during this time; hence the name. The earth’s shadow and the Belt of Venus both appear beginning immediately after sunset and before sunrise and present wonderful opportunities for landscape astrophotography images by themselves, or with a concurrent rising, or setting full moon, Figure 5.11.
3.8
Two sunrise images taken seconds apart facing: (a) east (into the rising sun) and (b) west (away from the rising sun) during the final moments of civil twilight. Note the opposite color transitions in the sky just above the horizon: the sky changes from orange to blue in the vertical direction facing east, but the reverse facing west: blue to pink/red/orange!
Azimuth Effects of Sky Color
The observer’s azimuth, or compass direction, strongly affects the colors of the sky during the different periods of twilight, as is shown for civil twilight in Figure 3.8. This phenomenon provides endless opportunities for nightscape astrophotography images. You can choreograph nearly any sky color combination you wish simply by selecting the appropriate period of twilight and azimuth!
Latitude Effects on Twilight Duration
The appearance of the sun’s east-to-west trajectory across the sky each day depends on the latitude of the observer for precisely the same reasons we discussed in the previous chapter. The daytime sky’s motion is no different! Consequently, just as we saw for stars, for those near either of the earth’s poles, the sun rises at a very shallow angle, Figure 3.9(a). In contrast, the sun rises nearly vertically from the horizon for observers on the equator, passes directly overhead at noon, and then sinks nearly vertically into the western horizon at sunset, Figure 3.9(c). For observers at mid-latitudes, the sun rises in the east at an angle to the normal roughly corresponding to the observer’s latitude, rises to its maximum height and then sinks in the west with the same angular orientation, Figure 3.9(b).
3.9
Illustration of how the latitude of the observer affects the orientation of the ecliptic, or path of the sun, relative to their local horizon at sunrise. In turn, the different ecliptic inclination results in substantially different durations of civil, nautical, and astronomical twilights for observers at different latitudes. This is simply because the sun’s angular position below the horizon changes at different rates owing to its oblique trajectory. For example, in (a), astronomical twilight ends almost 4 hours before sunrise; whereas in (c), it ends only 45 minutes before sunrise, even though the sun is at the same position, 15°, below the horizon for both cases!
These latitudinal differences in the sun’s trajectory during sunrise and sunset also strongly affect the twilight durations, and hence, your nightscape shooting schedule. The night sky becomes darker much faster for equatorial observers than for those at higher latitudes during sunset, and the reverse during sunrise. As the sun drops vertically below the horizon near the equator, its illumination recedes at its greatest rate. In contrast, for observers at higher latitudes, the sky can remain light for hours after sunset or before sunrise, and even stay illuminated for days on end! The reason is simple: the durations of twilight are greater for high-latitude observers simply because the sun’s angular position below the horizon changes relatively more slowly owing to its oblique trajectory.
Bibliography
Dickinson, Terence & Alan Dyer, The Backyard Astronomer’s Guide, 2010, Third Edition, Firefly Books, Limited, Buffalo, New York
Knight, Randall D., Physics for Scientists and Engineers, 2013, Third Edition, Pearson, Glenview, Illinois
Schneider, Stephen E. & Thomas T. Arny, Pathways to Astronomy, 2015, Fourth Edition, McGraw Hill Education, New York
Note
1 Rayleigh scattering is one type of scattering where certain wavelengths of light, in this case blue, are absorbed and re-emitted by gas molecules of a certain size. It isn’t a chemical interaction but one simply based on the physical sizes of the molecules. Since oxygen and nitrogen gas molecules are approximately the same size, they both scatter the same, mostly blue wavelengths of light.