Auroræ: Their Characters and SpectraCapron, J. Rand
History
Auroræ: Their Characters and Spectra
Capron, J. Rand
Auroras
“Between four and eight o’clock P.M. a light fog, rising to the altitude
of six degrees, became coloured on its upper edge, being fringed with the
light of the meteor rising behind it. This border, becoming gradually
more regular, took the form of an arch, of a pale yellow colour, the
edges of which were diffuse, the extremities resting on the horizon.
This bow swelled slowly upwards, its vertex being constantly on the
magnetic meridian. Blackish streaks divided regularly the luminous arc,
and resolved it into a system of rays. These rays were alternately
extended and contracted, sometimes slowly, sometimes instantaneously,
sometimes they would dart out, increasing and diminishing suddenly in
splendour. The inferior parts, or the feet of the rays, presented always
the most vivid light, and formed an arc more or less regular. The length
of these rays was very various, but they all converged to that point
of the heavens indicated by the direction of the southern pole of the
dipping-needle. Sometimes they were prolonged to the point where their
directions intersected, and formed the summit of an enormous dome of
light.
[Sidenote: It ascends to the zenith. Reaches the zenith.]
“The bow then would continue to ascend toward the zenith. It would
suffer an undulatory motion in its light—that is to say, that from
one extremity to the other the brightness of the rays would increase
successively in intensity. This luminous current would appear several
times in quick succession, and it would pass much more frequently from
west to east than in the opposite direction. Sometimes, but rarely, a
retrograde motion would take place immediately afterward; and as soon
as this wave of light had run successively over all the rays of the
Aurora from west to east, it would return in the contrary direction
to the point of its departure, producing such an effect that it was
impossible to say whether the rays themselves were actually affected by
a motion of translation in a direction nearly horizontal, or if this
more vivid light was transferred from ray to ray, the system of rays
themselves suffering no change of position. The bow, thus presenting
the appearance of an alternate motion in a direction nearly horizontal,
had usually the appearance of the undulations or folds of a ribbon or
flag agitated by the wind. Sometimes one, and sometimes both of its
extremities would desert the horizon, and then its folds would become
more numerous and marked, the bow would change its character and assume
the form of a long sheet of rays returning into itself, and consisting
of several parts forming graceful curves. The brightness of the rays
would vary suddenly, sometimes surpassing in splendour stars of the
first magnitude; these rays would rapidly dart out, and curves would be
formed and developed like the folds of a serpent; then the rays would
affect various colours, the base would be red, the middle green, and the
remainder would preserve its clear yellow hue.
Public-domain text, read in full here on John Shaqi.
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