Corona

Corona (Lat. for “crown”), in astronomy, the exterior envelope of the sun, being beyond the photosphere and chromosphere, invisible in the telescope and unrecognized by the spectroscope, except during a total eclipse (see Sun; Eclipse).

Corona Borealis, also known as the Corona septentrionalis, and the Northern Crown or Garland, is a constellation of the Northern hemisphere, mentioned by Eudoxus (4th cent. B.C.) and Aratus (3rd cent. B.C.). In the catalogues of Ptolemy, Tycho Brahe, and Hevelius, eight stars are mentioned; but recent uranographic surveys have greatly increased this number. The most interesting members are: σ Coronae, a binary consisting of a yellow star of the 6th magnitude, and a bluish star of the 7th magnitude; R Coronae, an irregular variable star; and T Coronae or Nova Coronae, a temporary or new star, first observed in 1866. Corona Australis, also known as Corona meridionalis, or the Southern Crown, is a constellation of the Southern hemisphere, mentioned by Eudoxus and Aratus. In Ptolemy’s catalogue thirteen stars are described.

In physical science, coronae (or “glories”) are the coloured rings frequently seen closely encircling the sun or moon. 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. 22° and 46°, the radii of coronae vary very considerably; also, halos are coloured red on the inside, whereas coronae are coloured red on the outside (see Halo).

It has now been firmly established, both experimentally and mathematically, that coronae are due to diffraction by the minute particles of moisture and dust suspended in the atmosphere, and the radii of the rings depend on the size of the diffracting particles. (See Diffraction of Light.)

Other meteorological phenomena caused by the diffraction of light include the anthelia, and the chromatic rings seen encircling shadows thrown on a bank of clouds, mist or fog. These appearances differ from halos and coronae inasmuch as their centres are at the anti-solar point; they thus resemble the rainbow. The anthelia (from the Greek ἀντί, opposite, and ἥλιος, the sun) are coloured red on the inside, the outside being generally colourless owing to the continued overlapping of many spectra. The diameter increases with the size of the globules making up the mist. The chromatic rings seen encircling the “spectre of the Brocken” are similarly explained.

The blue colour of the sky (q.v.), supernumerary rainbows, and the gorgeous sunsets observed after intense volcanic disturbances, when the atmosphere is charged with large quantities of extremely minute dust particles (e.g. Krakatoa), are also explicable by the diffraction of light. (See Dust.)

See E. Mascart, Traité d’optique (1899-1903); J. Pernter, Meteorologische Optik (1902-1905).

In architecture, the term “corona” is used of that part of a cornice which projects over the bed mould and constitutes the chief protection to the wall from rain; it is always throated, and its soffit rises towards the wall. The term is also given to the apse or semicircular termination of the choir; as at Canterbury in the part called “Becket’s crown.” The large circular chandelier suspended in churches, of which the finest example is that given by Barbarossa to Aix-la-Chapelle, is often called a corona. The term is also used in botany of the crown-like appendage at the top of compound flowers, the diminutive being coronule.

* * *