The Portland daily press. [volume] (Portland, Me.) 1862-1921, 1876-11-02
[From the Waltham Free Press.1
SundOgS.

The sum- moon, and more brilliant stars and
planets sometimes appear to be surrounded by
luminous colored circles near the luminous
body! These circles are called the corona.
They may be noticed around lights seen
through frosty, misty or even dusty panes Of
glass, and are produced in the sky by drops of
water or particles of snow, in clouds or mists,
acting simply as opaque bodies, that is, by the
light coming past them or among them. The
size of the corona is greater when the particles
of water are smaller, and thus by measure
ment of the diameter of the corona the dame
ter of the mist particles is found to be in sum-
ner about three Of the ten thousandths of an
inch, and in winter sometimes nearly three
times as large, viz: .0008 Of an inch.
Rainbows are, on the other hand, produced
by drops of water acting as transparent bodies,
that is, the light of the rainbow has passed
through the drops of rain, and suffered re-
fraction. Halos and sundogs are produced BY
particles of snow, or ice, floating in the air or
acting as transparent bodies; that is by light
passing through the ice crystals and suffering
refraction.
The parhelion circle is produced by refiec-
ton from the surfaces of such crystals. The
ice crystal is a hexagonal needle, and these
needles are sometimes built together in beauti-
ful stars, &C. Sometimes the needles are SO
short as to make them as broad as they are
long, and sometimes they are so very short as
to become mere six-sided scales.
When the crystals fly, turning at random
every way, they produce one or two circles
about the luminous body, which L will call
circle A and circle B. Circle A is the common
large circle or halo SO familiar to every one.
Its diameter is about 43 degrees, that is, will
reach nearly half way from the zenith to the
horizon. Circle B is rarer and of little more
than twice the diameter Of A.. saw it in the
year 1924, think it was, aad it made power
full impression on my childish Imagination.
Both A and u are produced BY light passing in
at one face of the ice prism and passing out at
the next face but one.
When large proportion of the crystals are
long in proportion to their diameter, and the
air is still, these long crystals in falling assume
from the resistance of the air. vertical posi-.
ton; and the passage of the light through the
end OF the crystals and out at the sides, or vice
versa, produces third circle. which L will call
circle O; while the passage of the light as for
A and B producessundogs
Sundays are bright colored spots Just outside
OF circle A, 1.-THE two brightest on a level with
the luminous body with feebly coored tails
pointing from the sun. They are also called
mock suns, and parhelia When they are visi-
BILE let the observer always look also for the
beautiful circle c. so conspicuous in Waltham
on the 9th of January, 1870, produced by the
same vertical position of the crystals. Circle
@, when the sun is low, has its centre at the
zenith, and is usually only half circle towards
the sun, but in favorable circumstances as ON
January 8, becomes q complete circular rain
bow about an apparently cloudless zenith If
the sun is higher the circle diminishes; at a
certain altitude of the un it contracts to a
point and vanIshes; at a higher altitude it re-
appears with its centre in the horizon directly
under the sun.
The Parhelion circle produced by reflection
from these vertical ice needles, is circle of
white light RAINING through the sun and two
principal suudogs and going completely round
the sky, parallel to the horizon.
When any large proportion of the ice crys-
tails are hexagonal plates or scales and are fall
ing IN still air, they assume a vertical posi-
ton by the resistance of the atmosphere, and
this makes the axis of the hexagOnal prism
horizontal. Such particles will by reflection
brighten the parhelion circle; but by refrac-
ton increase the brightness of circle G, and
also produce a new curve, not circle, nor a |
part of circle, which will call D d is an
arc resting inverted on the upper side OF circle
A, SO as to make a pair of horns to it.
As circles A, b and G, the curve D. the sun
dogs, and the parhelion circle are all produced
by frozen vapor floating ia the air, they are
more common in winter than summer, and
more common as we approach the Pole They
are seldom all seen at once. My main OBJ
ject 10 this note is to advise my readers when
they see sundogs to LOOK out for circle G,
aud curve ID; and when they see the large cir
Gle A, to look tor traces of the still larger B.
T. H.