AURORA POLARIS (Aurora Borealis and Australis, Polar

See also:

• LIGHT

Light,

See also:

• NORTHERN

Northern

See also:

• LIGHTS, CEREMONIAL USE OF

Lights)  , a natural phenomenon which occurs in many forms, some of great beauty . r . Forms.—Various schemes of classification have been proposed, but none has met with universal acceptance; the following are at least the principal types . (1) Arcs . These most commonly resemble segments of circles, but are not in-frequently elliptical or irregular in outline . The ends of arcs frequently extend to the horizon, but often one or both ends stop short of this . Several arcs may be visible at the same time . Usually the under or concave edge of the arc is the more clearly defined, and adjacent to it the sky often seems darker than elsewhere . It is rather a disputed point whether this dark segment—through which starlight has been seen to pass—represents a real atmospheric condition or is merely a contrast effect . (2) Bands . These may be nearly straight and regular in outline, as if broken portions of arcs; frequently they are ribbon-like serpentine forms showing numerous sinuosities . (3) Rays .

Frequently an arc or

band is visibly composed of innumerable short rays separated by distinctly less luminous intervals . These rays are more or less perpendicular to the arc or band; sometimes they are very approximately parallel to one another, on other occasions they converge towards a point . Longer rays often show an independent existence . Not in-frequently rays extend from the upper edge of an arc towards the zenith . Combinations of rays sometimes resemble a luminous fan, or a series of fans, or part of a hollow luminous cylinder . Rays often alter suddenly in length, seeming to stretch down towards the horizon or mount towards the zenith . This accounts for the description of aurora as " Merry Dancers." (4) Curtains or Draperies . This form is rare except in Arctic regions, where it is sometimes fairly frequent . It is one of the most imposing forms . As a rule the higher portion is visibly made up of rays, the light tending to become more continuous towards the lower edge; the combination suggests a connected whole, like a curtain whose alternate portions are in light and shade .