Light and Vision at Sea
This blog had originally been an Appendix to Chapter 8 - Sailing Vision. It remains self-contained and deals with the ways that light is altered by the atmosphere - playing tricks on the sailor's vision.
LIGHT AND THE UPPER ATMOSPHERE
Now let us consider how the atmosphere itself affects light. The atmosphere is responsible for such diverse phenomena as blue skies, red sunsets, the “green flash,” and marine mirages.
If we pause to consider the propagation of light, it seems paradoxical that we routinely perceive light from stars billions of miles away, yet, at the same time, strain to see a buoy light at a scant distance of 1 mile. The reason is that interstellar space is so rarefied that there is almost nothing to impede the progress of light rays until they enter our atmosphere. Once within the atmosphere, they run an increasing risk of colliding with one or more of the atmospheric molecules.
Imagine a shaft of light piercing a dusty room. As light strikes a dust particle, it is radiated in all directions—a so-called light scatter. We see the shaft of light because of the sideways scatter from each dust particle. If the room were perfectly free of dust, the shaft of light would be invisible. A similar scattering occurs when light from the sun strikes the air molecules of the atmosphere. This is known as Rayleigh scatter, after Lord Rayleigh, who showed that the shorter blue wavelengths are scattered much more than the longer green and red wavelengths. The blue appearance of the sky is due to the fact that blue is scattered sixteen times as much as red! At noon, when the sun is high in the sky, the sunlight strikes the atmosphere in an almost perpendicular fashion and blue light is scattered over the sky (Figure 1).
These rays are eventually scattered down to earth. The scattering removes some of the blue color from the appearance of the sun, leaving the red and the green, which together produce its yellowish hue.[1] As the sun continues to approach the horizon, the light rays strike the atmosphere at an increasingly larger angle and must travel a greater distance through the atmosphere. As the distance increases, the longer wavelengths of green now begin to be scattered. The additional scatter of green means that the light rays that reach the earth are a mixture of oranges and reds. These colors are prominent low in the western sky at sunset, while the blues and purples predominate higher. The play of these colors upon the clouds and atmospheric contaminants creates the spectacle of sunset. The sun itself, progressively stripped of its longer wavelengths as it nears the horizon, undergoes its own variegated metamorphosis from yellow to orange to red.
Figure 1 The appearance of the sun and sky at both noon and sunset. At noon, the blue wavelengths are scattered throughout the sky, while the sun appears yellow from the mixture of red and green wavelengths. At sunset, progressively longer wavelengths become scattered, producing a red sun and a multicolored sunset.
If conditions are favorable, and if the sailor is attentive, he or she may be treated to an unusual, albeit brief, phenomenon—the green flash. As light passes through the atmosphere, it is refracted as well as scattered. The amount of refraction is slightly different for each wavelength. As a result, multiple images of the sun occur, each of a different color—a kind of prismatic spectrum of images (Figure 2). The difference in register between the images is usually imperceptible except near the horizon, where the prismatic dispersion between the blue and red images may amount to ten seconds of arc. The red light refracts the least so that the red disk is the first to set. The atmosphere scatters most of the blue light. Thus, as the sun sets, there may be a brief period when the green image is the only one visible. It is called the green flash because not only does the upper rim appear green, but green rays seem to emanate from this upper rim. The green flash usually lasts only one to two seconds, and conditions must be favorable to see it at all. There must be a sharp horizon, good visibility,
Figure 2: The green flash
REFRACTION AND NAVIGATION
Consider this: with rare exception, every object that we see in the sky, whether it be sun, moon, planet, or star, is not where it appears to be! Refraction displaces the image of these objects in the celestial vault except when they are precisely overhead at the zenith. Even at the zenith, some refractive bending may occur if the various layers of the atmosphere are tilted slightly.
If it is true that no celestial object is where it appears to be, how are our celestial measurements ever accurate? The answer is that scientists have conducted a considerable amount of research to determine the precise amount of refraction at each altitude. The values are contained in a nautical almanac. If you intend to do any celestial navigation, you will need to purchase an up-to-date almanac. Without it you cannot compensate for the refraction, and your sextant values will be invalid.
TERRESTRIAL REFRACTION AND “FLYAWAY ISLANDS”
In 1906, during one of his attempts to reach the North Pole, Robert E. Peary stood at the summit of Cape Thomas Hubbard. About 120 miles (194 kilometers) to the northwest, he saw “snow-clad summits above the ice horizon.” Later, he again saw the spectacular land in the distance and christened it Crocker Land. He wrote, “My heart leaped the intervening miles of ice as I looked longingly at this land and, in fancy, I trod its shores and climbed its summits, even though I knew that that pleasure could be only for another in another season.”[2]
Donald B. MacMillan was destined to lead the expedition to Crocker Land. His party set forth in 1913, but it was not until the following year that they were able to reach Cape Thomas Hubbard. Despite temperatures as low as -25 degrees Fahrenheit (-32 degrees Celsius) and dangerously thin ice, the expedition traveled northwest for several days. MacMillan wrote in his diary, “This morning . . . Crocker Land was in sight. We all rushed out and up to the top of a berg. Sure enough! There it was as plain as day—hills, valleys, and ice cap, a tremendous land extending through 150 degrees of the horizon.”[3] The expedition party then traveled more than 130 miles (210 kilometers) over the polar sea far beyond the place where Crocker Land was seen. Yet there was nothing. Crocker Land was a mirage!
In addition to tantalizing polar explorers, these visual illusions may have indirectly acted as an impetus to the discovery of the New World. But these illusions have more than historic interest for the modern sailor. They have the potential to influence visual piloting and navigation, in either a positive or a negative fashion, and like the green flash, these wonderful phenomena are particularly frequent at sea.
All of the visual illusions now under consideration are due to terrestrial refraction. Astronomic refraction, as we have seen, produces its effects at the higher reaches of our atmosphere. Since the upper atmosphere is relatively unchanging, the visual illusions due to astronomic refraction are quite predictable; witness Figures 3 and 4. Terrestrial refraction is a phenomenon of the lower atmosphere—from the surface to perhaps 1,000 feet (300 meters) at most. As we are aware, this portion of the atmosphere is in a rapid state of flux and accounts for the unpredictability and evanescence of all of these spectacles.
To understand what creates the illusions, we need to consider a few basic concepts of surface meteorology:
(1) The index of refraction of air (the average value for which is 1.0003) varies with air density. The density in turn is dependent on three factors: temperature, pressure, and humidity. In reality, neither pressure nor humidity exerts an appreciable effect, so we can pretend that the air density, and hence the index of refraction, varies with temperature. A high temperature produces a low density and a low index of refraction. Low temperature, conversely, corresponds to high density and a high index of refraction. The greater the density gradient (the change of density with height), the greater the index of refraction gradient and the greater the refractive bending of light rays.
(2) Air generally becomes less dense with increasing height above the surface of the earth.
(3) Temperature normally decreases with increasing height above the surface; air is colder aloft. In meteorological parlance, the temperature-height profile is known as the lapse rate (Figure 3A). An inverted profile with warmer air above cooler air is called a temperature inversion.
English sailors have recognized the phenomenon of looming at least since the nineteenth century, when the word officially entered the English language. These sailors realized that under certain atmospheric conditions, a light or a rock or any other object might loom on the horizon before it was expected to be visible.
Looming is apt to occur in middle to high latitudes when the underlying water is cool, although it may occur anytime that warmer air settles over cooler water; that is, whenever there is a temperature inversion near the surface (Figure 3B). The temperature inversion accentuates the decrease in density that normally occurs with an increase in height (see item 2 in the list above) and also produces less dense air aloft than near the surface. The combination produces an abnormally large density gradient near the surface, which means that there is an abnormally large index of refraction gradient. The greater the density gradient, the greater the amount of refraction.
Figure 3 Normal vision at sea (A), looming with towering and stooping (B), sinking (C)
This index of refraction gradient causes light rays that enter it to be bent down toward the surface of the earth, in turn producing an apparent elevation of the object (object displacement). The optics of terrestrial refraction obey the general rules of refraction; thus, as light travels from a less dense medium into a denser medium, it is always bent toward the perpendicular to the surface. Light traveling from air to water is bent more abruptly than light traveling through varying densities of air (Figure 3B). This is due to the transition in the index of refraction between air (1.0003) and water (1.333). Terrestrial refraction occurs gradually over a gradient of changing layers; the light rays are actually bent at each of these thin layers. However, the direction of the final refraction nearest the observer determines the apparent position of the object in space.
When the decrease in density is relatively uniform, the upper portion of the object is refracted about the same as the lower portion, and the size of the object is unaltered. If a nonlinear change of density with height occurs so that the uppermost rays have a greater curvature, towering is said to exist (Figure 3B). Conversely, if the curvature at the top is less, stooping occurs.
Sinking can be thought of as the opposite of looming. The image now appears displaced below the object (Figure 3C). The conditions that favor sinking are quite different from those that promote looming. As Figure 3C illustrates, there is a marked increase in the temperature gradient near the surface. The warmer surface produces a layer of heated super adjacent air that is less dense than the air aloft (the opposite of normal). In this instance, light rays are refracted up to the observer, producing an apparent depression of the object. Depending on the linearity or nonlinearity of the density gradient, towering or stooping may accompany sinking. Since the necessary prerequisite for sinking is a layer of warm air immediately above the surface, this phenomenon is not uncommon over warm open water in the wintertime. The layer of cooler winter air in contact with the warmer water is heated from below. This temperature profile is also quite common over any enclosed body of water in the early morning. Water retains its heat through the night much better than does the adjacent land, which cools off. Cool air from the land may then flow out over the warmer water and be heated from below.
The clarity of the image is extremely variable. At times, the image is reproduced with remarkable fidelity, whereas at others it is indistinct. Generally speaking, the image which sinking produces (and its first cousin the inferior mirage) is less well defined. The atmospheric conditions responsible tend to be inherently unstable, in contrast to the temperature inversion, which is meteorologically stable. The surface instability produces rapid fluctuations in the image known as optical shimmer. An everyday example of optical shimmer is the distortion of distant objects when viewed through the hot exhaust gases of a jet airplane.
Obviously, both looming and sinking may respectively assist or hinder visual piloting by making the image of the object appear earlier or later than expected. Frequently, the loom of an object is sufficiently defined to obtain a bearing despite the fact that the object cannot be identified.
With both looming and sinking, the refractive effect of the atmosphere has been likened to a giant atmospheric lens. Certainly, it is unlike the lens of a camera or that of the eye in that its huge size defies precise measurement and its refractive properties are somewhat capricious. But the analogy is nonetheless valuable. One unusual feature of the atmospheric lens is that both the observer and the object are inside of it!
This phenomenon does not occur with the illusions known as superior and inferior mirages. These mirages are produced under atmospheric conditions that promote strong stratification of the air, unlike the more gradual stratification in the previous situations. Not only is the stratification well localized, but the observer is now outside of the lens. Figure 4A illustrates the optics of a superior mirage. Just as in looming, there is a temperature inversion, but in this case the base of the temperature inversion is above the height of the observer—the lens is suspended in the sky. This allows light rays from the same point on the object (point A, for example) to travel two different pathways through the atmosphere to the observer’s eye, and the observer perceives the image of this point at two widely disparate positions in space. The image nearest to the observer is nearly always the inverted one. The conditions favorable to a superior mirage are frequently found in lakes, bays, and sounds in the early afternoon. Air over the adjacent land that the sun has vigorously heated may waft over the water above the height of the observer.
An inferior mirage is illustrated in Figure 4B. The atmospheric conditions are similar to those that promote sinking (i.e., warm air near the surface), except that there is greater stratification and the lens, or superstratified layer of warm air, is immediately above the surface of the water. (In flat desert regions, the inferior mirage produces the well-known “oasis” illusion. A portion of the blue sky is “brought down” to earth by the tremendous refractive power of the warm air just above the desert surface.) Again, light rays may reach the observer by two different routes.
The visual illusions we have considered to this point have been distortions of a real distant object. But the most wondrous illusions, such as Crocker Land, result from visual distortion of the surface of the water itself. In a superior or inferior mirage, not only is the distant object distorted, but so is the horizon. In a superior mirage, the horizon is elevated (Figure 4A), creating the impression that the observer is inside of a large, flat, shallow bowl. The use of binoculars heightens this impression. Sir Francis Chichester has described this phenomenon: “Sometimes I saw strange things. Just before crossing the Line the boat appeared to be sailing up a gently sloping sea surface, in other words, uphill. At the time I was a little worried, but when I was 240 miles north of the Line, I noticed the same thing again. This time the sea appeared uphill in every direction, as if I were sailing in a shallow saucer.”[4] Conversely, an inferior mirage (Figure 4B) creates the sensation of being on top of a similar, but now inverted, bowl—one way of feeling “on top of the world.” Needless to say, if either of these conditions prevails, the dip correction for celestial observations may be grossly in error, overestimated with superior mirages and underestimated with inferior mirages.
Figure 4 Nautical mirages: superior (A) and inferior (B)
If conditions are right, the sailor may experience the most enchanting illusion of all, the so-called Fata Morgana [Fairy Morgan]. In many of the Arthurian legends, Arthur’s half-sister Morgan le Fay is credited with the ability to create magical castles in the air. Robert Peary’s Crocker Land is a typical example of a Fata Morgana. An Italian priest who witnessed a mirage over the Straits of Messina in 1643 provides an early description. While looking across the water, “the ocean which washes the coast of Sicily rose up and looked like a dark mountain range . . . there quickly appeared a series of more than 10,000 pilasters which were a whitish-gray color . . . [then the] pilasters shrank to half their height and built arches like those of Roman aqueducts.”[5] The finale consisted of castles with the individual towers and windows visible.
The prerequisites for this mirage are complex, but similar to those for the superior mirage. The water at the horizon, instead of appearing to rise up slightly (producing the bowl illusion), now is markedly distorted to form a visual wall. The key to the illusion is that the brightness of the face of the wall is not uniform. Rather, it is composed of bright and dark patches irregularly elongated (towering) and shrunk (stooping), a kind of atmospheric astigmatism. The light rays from the surface of the water, which undergo vertical elongation (towering), spread their light over a greater distance and appear darker. The rays that undergo stooping are concentrated and thus look bright. The patches vary in size and shape as shimmering and gravity waves affect the atmosphere.
All that the eye perceives are these irregular and distorted bright and dark patches. The rest the brain “imaginatively” interprets. Just as the brain constructs a Picasso nude from a single curved black line on a white background, so it formulates the most fantastic patterns from these patches of light and darkness above the horizon.
Undoubtedly, humans have been treated to mirages since prehistoric time. Yet there is a curious silence regarding them in historic documents. There is no reason to believe that atmospheric conditions have changed drastically in the last few thousand years. Then why are mirages not mentioned in Egyptian, biblical, or Roman sources? Probably the first mirage to be documented was the original Fata Morgana, which appears periodically off the Straits of Messina, and this was not until after the Crusades.
One plausible explanation is that although these mirages were witnessed, their insubstantiality was not appreciated; in other words, they were probably considered real! The original Fata Morgana was the first to be recognized as a mirage because the geography of the region was well known to Mediterranean sailors. They knew that there was no island there. In other regions, subjected to less maritime scrutiny than the Mediterranean, it would be difficult to disprove the ineluctability of the visual. Is there any evidence for this hypothesis?
S. E. Morison describes many “mythical” islands in The European Discovery of America: The Northern Voyages. These islands located off the west coasts of Europe and Africa were periodically sighted, but whenever they were searched for, they seemed to disappear. Sailors had a term for these illusive islands: Flyaway Islands. Two of the most prominent were Antilia and Hy-Brasil (or O’Brazil). Hy-Brasil is Gaelic for “isle of the blessed” and has nothing to do with the South American country. This island was regularly glimpsed from the Aran Islands and the west coast of Ireland. Its appearance on nautical charts dates to 1325. Amazingly, it continued to appear on charts of the British Admiralty until 1873, despite the fact that shipping had traversed the area for centuries. Admiral Morison relates that “fishermen of the Aran Islands told Professor Westropp of the Royal Irish Academy that it appeared every seven years; he saw it himself in 1872! ‘Just as the sun went down, a dark island suddenly appeared far out to sea, but not on the horizon. It had two hills, one wooded; between them from a low plane, rose towers and curls of smoke.’”[6]
Another mythical island that rivaled Hy-Brasil in longevity was Antilia (which means “island opposite,” since it was opposite Portugal). Usually the island was charted with smaller daughter islands. Connected with this island was the recurring myth of the Seven Cities of Gold. Antilia was not “disembodied” until the nineteenth century. Columbus was well aware of the Antilia–Seven Cities legend: “His son states that the Discoverer wished to find ‘some island’ en route, as a convenient staging point for the ocean route to the Indies. And his sea journal from 25 September 1492 proves that he expected to find it about where [his chart] placed Antilia.”[7] Thus, the mythical island of Antilia undoubtedly contributed to Columbus’s belief that a westward route to the Indies was feasible. Ironically, once news of his discovery reached Europe, skeptics disparaged the achievement by proclaiming that he had discovered nothing more than Antilia! This misconception is forever enshrined in the Portuguese and French names for the islands: Antillas and Les Antilles.
However, not everything that floats above the water is a Fata Morgana. As has been pointed out on the website “Metabunk.org” [iv]a fog bank may obscure the horizon—creating a false horizon. Figure 5 is from the website (originally from the Science channel) purporting to show a sailboat in the air. One give-away is that with a real Fata Morgana there is often an inverted image and invariably there is much more distortion of the image.
Figure 5 Illusion of sailboat in the air, resembling a Fata Morgana
Terrestrial refraction may have hastened the discovery of the New World in yet another way. It has been speculated that the Celtic and early Norse explorers were tempted not by islands that did not exist, but by images of real islands on the horizon to the west!
Outside of polar regions, variations from standard refraction values seldom approach two to three minutes, occasionally more. In polar regions, refraction variations of a couple of minutes are commonplace and on occasion extreme values of 5 degrees have been reported, which would produce an error of over 300 miles (484 kilometers) in a line of position. It is less than 250 miles (403 kilometers) from the Faroe Islands to Iceland and only another 180 miles (290 kilometers) to Greenland. Could it be that intermittently over the years these islands loomed above the horizon tempting the Norse explorers with “real” mirages? Just possibly this unfair natural advantage explains why the Nordic explorers beat their southern nautical colleagues to the New World by five hundred years.
Notes
[1] The primary colors of wavelengths of light are blue, green, and red. Yellow is composed of a mixture of green and red light. If this seems odd, that is because many of us have been raised to conceptualize color only in terms of pigment mixture, a somewhat different system.
[2] Cited in A. B. Fraser and W. H. Mach, “Mirages,” Scientific American 234 (1976): 102-111.
[3] Cited in Fraser and Mach, “Mirages.”
[4] Sir Francis Chichester, Gipsy Moth Circles the World (Sevenoaks, England: Hodder and Stoughton, 1967), 206.
[5] Cited in Fraser and Mach, “Mirages.”
[6] S. E. Morrison, The European Discovery of America: The Northern Voyages (New York: Oxford University Press, 1971), 103.
[7] Morrison, The European Discovery of America, 101.
[8] www.metabunk.org/debunked-fata-morgana-hovering-boat-mirages.t9112/