- Ulloa circle
- Also known as circle of Ulloa, Ulloa ring, Ulloa's ring, Ulloa's halo, Bouguer's halo, white rainbow, and fogbow. The eponym Ulloa circle refers to the Spanish naval officer, explorer, and astronomer Antonio de Ulloa y de Torre-Giral (1716-1795). It is used in meteorology to denote a rare *physical illusion consisting of a white luminous ring or arch that can sometimes be seen in mountainous regions, typically in foggy weather, while facing an area opposite the Sun (i.e. the antisolar point). Inside the Ulloa ring smaller rings or arches can sometimes be seen. These are known as glories or * Ulloa's bows. Ulloa is commonly credited with having been the first to record both phenomena, after having observed them during an expedition in what is now Ecuador, between 1735 and 1739. As recounted by the French astronomer and author Nicolas Camille Flammarion (1842-1925), "Ulloa, being in company with six fellow-travellers upon the Pambamarca at daybreak one morning, observed that the summit of the mountain was entirely covered with thick clouds, and that the Sun, when it rose, dissipated them, leaving only in their stead light vapours, which [were] almost impossible to distinguish. Suddenly, in the opposite direction to where the Sun was rising, 'each of the travellers beheld, at about 70 feet from where he was standing, his own image reflected in the air as in a mirror. The image was in the centre of three rainbows of different colours, and surrounded at a certain distance by a fourth bow with only one colour. The inside colour of each bow was carnation or red, the next shade was violet, the third yellow, the fourth straw colour, the last green. All these bows were perpendicular to the horizon; they moved in the direction of, and followed, the image of the person whom they enveloped as with a glory.' The most remarkable point was that, although the seven spectators were standing in a group, each person only saw the phenomenon in regard to his own person, and was disposed to believe that it was repeated in respect to his companions." The mediation of the Ulloa circle is not fully understood. A central role is attributed to the interaction of sunlight and droplets of water less than 10 (im in radius suspended in the air. Flammarion classifies the Ulloa circle as an * anthelion. Today both atmospheric phenomena are generally classified as physical illusions. Because of its lack of a tangible substratum in the extracorpo-real world, the Ulloa circle is also classified as a *fiction illusion. A physical illusion quite similar to the Ulloa circle is known as Buddha's light or *Buddha's halo. It should not be confused with *heiligenschein.ReferencesFlammarion, C. (1873). The atmosphere.Translated by Pitman, C.B. Edited by Glaisher, J. London: Sampson Low, Marston, Low, & Searle.Juan, J., de Ulloa, A. (1748). Relacion historica del viaje a la America Meridional hecho de orden de su Majestad. Madrid: Juan de Zuniga.Lynch, D.K., Futterman, S.N. (1991). Ulloa's observations of the glory, fogbow, and an unidentified phenomenon. Applied Optics, 30, 3538-3541.Lynch, D.K., Livingston, W. (1995). Color and light in nature. Cambridge: Cambridge University Press.
Dictionary of Hallucinations. J.D. Blom. 2010.