Glory (optical phenomenon)
A glory is an optical phenomenon that resembles an iconic saint's halo about the shadow of the observer's head, caused by light of the Sun or (more rarely) the Moon interacting with the tiny water droplets that make up mist or clouds. The glory consists of one or more concentric, successively dimmer rings, each of which is red on the outside and bluish towards the centre. Due to its appearance, the phenomenon is sometimes mistaken for a circular rainbow, but the latter has a much larger diameter and is caused by different physical processes.
The cause of the glory remains a matter of scholarly debate.
Appearance and observation
Depending on circumstances (such as the uniformity of droplet size in the clouds), one or more of the glory's rings can be visible. The angular size of the inner and brightest ring is much smaller than that of a rainbow, about 5° to 20°, depending on the size of the droplets. In the right conditions, a glory and a rainbow can occur simultaneously.
Like a rainbow, a glory is centered on the antisolar (or, in case of the Moon, antilunar) point, which coincides with the shadow of the observer's head. Since this point is by definition diametrically opposed to the Sun's (or Moon's) position in the sky, it always lies below the observer's horizon when the Sun (Moon) is up. In order to see a glory, therefore, the clouds or fog causing it must be located below the observer, in a straight line with the Sun/Moon and the observer's eye. Hence, the glory is commonly observed from a high viewpoint such as a mountain, tall building or from an aircraft. In the latter case, if the plane is flying sufficiently low for its shadow to be visible on the clouds, the glory always surrounds it. This is sometimes called The Glory of the Pilot.
When viewed from a mountain or tall building, glories are often seen in association with a Brocken spectre, also called Mountain Spectre, the apparently enormously magnified shadow of an observer, cast (when the Sun is low) on clouds below the mountain on which the viewer is standing. The name derives from the Brocken, the tallest peak of the Harz mountain range in Germany. Because the peak is above the cloud level and the area frequently misty, conditions conducive to casting a shadow on a cloud layer are common. Giant shadows that seemed to move by themselves due to movement of the cloud layer (this movement is another part of the definition of the Brocken Spectre), and that were surrounded by glories, may have contributed to the reputation the Harz mountains hold as a refuge for witches and evil spirits. In Goethe's Faust, the Brocken is called the Blocksberg and is the site of the Witches' Sabbath on Walpurgis Night.
The scientific explanation is still the subject of debates and research. In 1947, the Dutch astronomer Hendrik van de Hulst suggested that surface waves are involved. He speculated that the colored rings of the glory are caused by two-ray interference between "short" and "long" path surface waves—which are generated by light rays entering the droplets at diametrically opposite points (both rays suffer one internal reflection). A new theory by Brazilian physicist Herch Moysés Nussenzveig, however, suggests that the light energy beamed back by a glory originates mostly from classical wave tunneling, which is when light rays that missed a droplet can still transfer energy into it.
The light source is the sun. The light scatterer is the airplane, and the screen where the diffraction pattern is observed is the blanket of clouds or a flat area in a sand desert, for example. The large distance between the sun and the airplane, which is around 150 million kilometre, and the relatively small size of the airplane, which does not exceed 100 meters in any direction, warrants that the structured shape of the air plane can be approximated by a disc. The distance from this disc-shaped airplane to the screen underneath where the diffraction pattern is observed is typically in the range of 1 kilometre and more. With these relations of size and distance, the circular colourful diffraction pattern can be explained by the theory of the Airy disc.
The first intensity minimum occurs at an angle θ = 1.22 λ/D, where D is the diameter of the airplane. When the cloud blanket has a distance L >> D from the airplane, the intensity minimum is observed at a radial distance r' = 1.22 λL/D from the center of the pattern. Because the sunlight appears as white light composed from all wavelengths λ, the ring pattern is resolved with the rainbow colours.
C. T. R. Wilson saw a glory while working as a temporary observer at the Ben Nevis weather station. Inspired by the impressive sight, he decided to build a device for creating clouds in the laboratory, so that he could make a synthetic, small-scale glory. His work led directly to the cloud chamber, a device for detecting ionizing radiation for which he and Arthur Compton received the Nobel Prize for Physics in 1927.
In China, the phenomenon is called Buddha's light (or halo). It is often observed on cloud-shrouded high mountains, such as Huangshan and Mount Emei. Records of the phenomenon at Mount Emei date back to A.D. 63. The colorful halo always surrounds the observer's own shadow, and thus was often taken to show the observer's personal enlightenment (associated with Buddha or divinity).
Leo Frankowski made glories a key plot element in his Conrad Stargard saga, where the protagonist and title character is sent back in time to the 13th century where he has to establish himself and cope with various crises including planning for the eventual Mongol invasion of Eastern Europe in 1241. In the third book, The Radiant Warrior, Stargard begins building a modern army and uses the reliable glories along one stretch of his boot camp to invoke religious faith-backed esprit de corps and feelings of elite invincibility in his newly forming cadre. The same phenomenon dupes the highly pious heir apparent of the Polish duchy into strongly supporting the new model army's pragmatic departures from the day's chivalristic practices.
This atmospheric effect also makes at least one appearance in Gothic fiction. In James Hogg's The Private Memoirs and Confessions of a Justified Sinner, George Colwan walks to the top of Arthur's Seat on a foggy day, while his half-brother Robert Wringhim secretly follows him with murderous intent. George sees shimmering colored light in front of him. Then he sees the shadow of an enormous dark figure advancing toward him threateningly—the Brocken spectre created by the shadow of Robert sneaking up behind him. In other words, the "good" George is surrounded by a glory, while the "evil" Robert appears as a dark spectre.
In Masami Kurumada's Saint Seiya comic book, which is inspired by Greek mythology, warriors belonging to divine armies battle each other for possession of the Earth. The main characters, warriors known as Saints, belong to Athena's army, and one of the antagonistic armies they face belongs to the Olympic Gods, composed of warriors called the Angels. As the Saints wear protective armors called Cloths, which represent the 88 astronomical constellations, the Angels don similar attire, their armors being known as Glory, which Kurumada named after the optical phenomenon in reference to it being traditionally associated to angels in religious imagery, and the Glory armors represent angels in different poses and sizes.
Glory encountered during a hike in An Teallach, Scotland.
19th century drawing of a glory observed from a hydrogen balloon. From: G. Tissandier, Histoire de mes ascensions (1887), p. 133.
|Wikimedia Commons has media related to Glories.|
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