Online Encyclopedia

AURORA POLARIS (Aurora Borealis and A...

Online Encyclopedia
Originally appearing in Volume V02, Page 927 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
AURORA POLARIS (Aurora Borealis and Australis, Polar Light, Northern 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. The curtain often shows several conspicuous folds, and the lower
End of Article: AURORA POLARIS (Aurora Borealis and Australis, Polar Light, Northern Lights)
[back]
AURORA (perhaps through a form ausosa from Sansk. u...
[next]
AURUNCI

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.