question: Why rainbows are always circular // arch shaped.
comment: A rainbow occurs as [light is refracted]( URL_0 ) in raindrops. This occurs at a specific angle, however, so in order to observe a rainbow you must be located as to make a 40-42 degree angle between the sun, the raindrop, and you. This condition is satisfied along a curved path, creating the curved rainbow. [Here's a diagram.]( URL_1 ) This is why you can see the full circle if you are on top of a large building or in a plane.
passages: In theory, every rainbow is a circle, but from the ground, usually only its upper half can be seen. Since the rainbow's centre is diametrically opposed to the sun's position in the sky, more of the circle comes into view as the sun approaches the horizon, meaning that the largest section of the circle normally seen is about 50% during sunset or sunrise. Viewing the rainbow's lower half requires the presence of water droplets "below" the observer's horizon, as well as sunlight that is able to reach them. These requirements are not usually met when the viewer is at ground level, either because droplets are absent in the required position, or because the sunlight is obstructed by the landscape behind the observer. From a high viewpoint such as a high building or an aircraft, however, the requirements can be met and the full-circle rainbow can be seen. Like a partial rainbow, the circular rainbow can have a secondary bow or supernumerary bows as well. It is possible to produce the full circle when standing on the ground, for example by spraying a water mist from a garden hose while facing away from the sun.
   A circular rainbow should not be confused with the glory, which is much smaller in diameter and is created by different optical processes. In the right circumstances, a glory and a (circular) rainbow or fog bow can occur together. Another atmospheric phenomenon that may be mistaken for a "circular rainbow" is the 22° halo, which is caused by ice crystals rather than liquid water droplets, and is located around the sun (or moon), not opposite it.
  
  Section::::Variations.:Supernumerary rainbows.
   Section::::Geographic Distribution.
   In Book I of "Naturales Quaestiones" (c. 65 AD), the Roman philosopher Seneca the Younger discusses various theories of the formation of rainbows extensively, including those of Aristotle. He notices that rainbows appear always opposite to the sun, that they appear in water sprayed by a rower, in the water spat by a fuller on clothes stretched on pegs or by water sprayed through a small hole in a burst pipe. He even speaks of rainbows produced by small rods (virgulae) of glass, anticipating Newton's experiences with prisms. He takes into account two theories: one, that the rainbow is produced by the sun reflecting in each water drop, the other, that it is produced by the sun reflected in a cloud shaped like a concave mirror; he favours the latter. He also discusses other phenomena related to rainbows: the mysterious "virgae" (rods), halos and parhelia.
   In certain circumstances, one or several narrow, faintly coloured bands can be seen bordering the violet edge of a rainbow; i.e., inside the primary bow or, much more rarely, outside the secondary. These extra bands are called "supernumerary rainbows" or "supernumerary bands"; together with the rainbow itself the phenomenon is also known as a "stacker rainbow". The supernumerary bows are slightly detached from the main bow, become successively fainter along with their distance from it, and have pastel colours (consisting mainly of pink, purple and green hues) rather than the usual spectrum pattern. The effect becomes apparent when water droplets are involved that have a diameter of about 1 mm or less; the smaller the droplets are, the broader the supernumerary bands become, and the less saturated their colours. Due to their origin in small droplets, supernumerary bands tend to be particularly prominent in fogbows.
   Judith Bauer Stamper's 1987 Find Your Fate book "Jem and The Holograms #3: The Secret of Rainbow Island ", involves the reader in saving Rainbow Island, a tropical island that frequently has rainbows.
  
  American legend retelling "The Rough Face Girl" involves the heroines seeing an invisible Chief, who wears a rainbow as a sash.
  
  Stephen King's 1985 book 'It" features a scene where Ben Hanscom sees a spray bow while fighting with Henry Bowers in The Barrens, and comments on finding gold at the end of it.
   Glories arise due to wave interference of light internally refracted within small droplets.
  
  Section::::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.
   Fog bows should not be confused with ice halos, which are very common around the world and visible much more often than rainbows (of any order), yet are unrelated to rainbows.
  
  Section::::Variations.:Circumhorizontal and circumzenithal arcs.
  
  The circumzenithal and circumhorizontal arcs are two related optical phenomena similar in appearance to a rainbow, but unlike the latter, their origin lies in light refraction through hexagonal ice crystals rather than liquid water droplets. This means that they are not rainbows, but members of the large family of halos.
   Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.
  
  Section::::Visibility.
   In addition to the common primary and secondary rainbows, it is also possible for rainbows of higher orders to form. The order of a rainbow is determined by the number of light reflections inside the water droplets that create it: One reflection results in the first-order or "primary" rainbow; two reflections create the second-order or "secondary" rainbow. More internal reflections cause bows of higher orders—theoretically unto infinity. As more and more light is lost with each internal reflection, however, each subsequent bow becomes progressively dimmer and therefore increasingly harder to spot. An additional challenge in observing the third-order (or "tertiary") and fourth-order ("quaternary") rainbows is their location in the direction of the sun (about 40° and 45° from the sun, respectively), causing them to become drowned in its glare.
   Droplets (or spheres) composed of materials with different refractive indices than plain water produce rainbows with different radius angles. Since salt water has a higher refractive index, a sea spray bow doesn't perfectly align with the ordinary rainbow, if seen at the same spot. Tiny plastic or glass marbles may be used in road marking as a reflectors to enhance its visibility by drivers at night. Due to a much higher refractive index, rainbows observed on such marbles have a noticeably smaller radius. One can easily reproduce such phenomena by sprinkling liquids of different refractive indices in the air, as illustrated in the photo.
   Tertiary and quaternary rainbows should not be confused with "triple" and "quadruple" rainbows—terms sometimes erroneously used to refer to the—much more common—supernumerary bows and reflection rainbows.
  
  Section::::Variations.:Rainbows under moonlight.
   BULLET::::- The Sumu of Honduras and Nicaragua "may simply refer to the rainbow as "walasa aniwe", ‘the devil is vexed’". These people hide their children in their huts to keep them from looking or pointing at the rainbow.
  
  BULLET::::- The Fang of Gabon (Africa) are initiated into the religion by a "transcendent experience when they arrive at the rainbow's center, for there they can see both the entire circle of the rainbow and of the earth, signaling the success of their vision." The Fang also prohibit their children from looking at the rainbow.
   Ngalyod, first-born son of Yingarna, who is the Rainbow Serpent creator of the Kunwinjku in western Arnhem Land in Australia—sucks up water during the dry season and spits it out as rain during the wet season. Like Iris, Ngalyod helps to ensure fertility with rains, but he can destroy as well as nurture. Yingarna, the creator of the Kunwinjku people, is "nominally female" and androgynous like her son Ngalyod. She possesses cunningly ambivalent form; as she combines elements of snake, fish, crocodile, catfish, emu, and kangaroo. The Estonian Rainbow Serpent, like the Aborigines' Ngalyod, sucks up water and spits rain, and also has a confused identity – it possesses an ox's head on its serpent body.
   The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky. Each rainbow reflects white light inside its coloured bands, but that is "down" for the primary and "up" for the secondary. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.
  
  Section::::Variations.:Twinned rainbow.
   The Irish leprechaun's secret hiding place for his pot of gold is usually said to be at the end of the rainbow. This place is impossible to reach, because the rainbow is an optical effect which depends on the location of the viewer. When walking towards the end of a rainbow, it will appear to "move" further away (two people who simultaneously observe a rainbow at different locations will disagree about where a rainbow is).
   Section::::Variations.:Fogbow.
   Rainbow
  
  A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
  
  Rainbows can be full circles. However, the observer normally sees only an arc formed by illuminated droplets above the ground, and centered on a line from the sun to the observer's eye.
   The circumhorizontal arc is sometimes referred to by the misnomer "fire rainbow". In order to view it, the Sun or Moon must be at least 58° above the horizon, making it a rare occurrence at higher latitudes. The circumzenithal arc, visible only at a solar or lunar elevation of less than 32°, is much more common, but often missed since it occurs almost directly overhead.
  
  Section::::Variations.:Rainbows on Titan.
   Rainbows are the result of a combination of internal reflection and dispersive refraction of light in raindrops. A single reflection off the backs of an array of raindrops produces a rainbow with an angular size on the sky that ranges from 40° to 42° with red on the outside. Double rainbows are produced by two internal reflections with angular size of 50.5° to 54° with violet on the outside. Because rainbows are seen with the sun 180° away from the centre of the rainbow, rainbows are more prominent the closer the sun is to the horizon.
  
  Section::::See also.
   The cause of a twinned rainbow is the combination of different sizes of water drops falling from the sky. Due to air resistance, raindrops flatten as they fall, and flattening is more prominent in larger water drops. When two rain showers with different-sized raindrops combine, they each produce slightly different rainbows which may combine and form a twinned rainbow.
   Section::::Background.
   Light pillars can also form around the moon, and around street lights or other bright lights. Pillars forming from ground-based light sources may appear much taller than those associated with the sun or moon. Since the observer is closer to the light source, crystal orientation matters less in the formation of these pillars.
  
  Section::::Circular halo.
   BULLET::::- Rainbows. These result from a combination of internal reflection and dispersive refraction of light in raindrops. Because rainbows are seen on the opposite side of the sky from the sun, rainbows are more visible the closer the sun is to the horizon. For example, if the sun is overhead, any possible rainbow appears near an observer's feet, making it hard to see, and involves very few raindrops between the observer's eyes and the ground, making any rainbow very sparse.
  
  Other phenomena that are remarkable because they are forms of visual illusions include:
  
  BULLET::::- Crepuscular rays,
  
  BULLET::::- Anticrepuscular rays, and
   The Church of Jesus Christ of Latter-Day Saints founder and prophet Joseph Smith stated that the second coming of the Christ would not occur in any year in which a rainbow is seen.
  
  In the Dreamtime of Australian Aboriginal mythology, the rainbow snake is the deity governing water.
  