PerEwpa, literally " things in the air," from yerb., beyond, and a€ipav, to lift up), a term originally applied by the ancient Greeks to many atmospheric phenomena - rainbows, halos, shooting stars, &c. - but now specially restricted to those luminous bodies known as shooting stars, falling stars, fireballs and bolides.
In the first class we have halos, and coronae, or "glories," which encircle the luminary; the second class includes rainbows, fog-bows, mist-halos, anthelia and mountainspectres, whose centres are at the anti-solar point.
Here it is only necessary to distinguish halos from coronae.
Halos are at definite distances (22° and 46 °) from the sun, and are coloured red on the inside, being due to refraction; coronae closely surround the sun at variable distances, and are coloured red on the outside, being due to diffraction.
Luminous arcs (T), tangential to the upper and lower parts of each halo, also occur, and in the case of the inner halo, the arcs may be prolonged to form a quasi-elliptic halo.1 The physical explanation of halos originated with Rene Descartes, who ascribed their formation to the presence of icecrystals in the atmosphere.
The impurity of the colours (due partly to the sun's diameter, but still more to oblique refraction) is more marked in halos than in rainbows; in fact, only the red is at all pure, and as a rule, only a mere trace of green or blue is seen, the external portion of each halo being nearly white.
The two halos are the only phenomena which admit of explanation without assigning any particular distribution to the ice-crystals.
AUgUSte Bravais's celebrated memoir, "Sur les halos et les phenomenes optiques qui les accompagnent" (Journ.
According to a local legend, Athamas was king of Halos in Phthiotis from the first (Schol.
Halos was added to the number of Early Iron Age sites in Thessaly in 1912 (Wace and Thompson).
Formerly classified by the ancient Greeks with halos, rainbows, &c., under the general group of "meteors," they came to receive considerable attention at the hands of Descartes, Christiaan Huygens, and Sir Isaac Newton; but the correct explanation of coronae was reserved until the beginning of the 19th century, when Thomas Young applied the theories of the diffraction and interference of light to this phenomenon.
Prior to Young, halos and coronae had not been clearly differentiated; they were both regarded as caused by the refraction of light by atmospheric moisture and ice, although observation had shown that important distinctions existed between these phenomena.
Thus, while halos have certain definite radii, viz.
These appear ancos differ from halos and coronae inasmuch as their centres are at the anti-solar point; they thus resemble the rainbow.
The fourth essay is a systematic treatment of the nature of colour, with a description of many curious experiments and a discussion of the rainbow, halos, parhelia, diffraction, and the more purely physiological phenomena of colour.
The chromatic halos which frequently encircle these images are due to diffraction.
That Newton must have begun early to make careful observations of natural phenomena is sufficiently testified by the following remarks about halos, which appear in his Optics, book ii.