The corona is observed around the sun as it is about to rise over the mountain ridge approximately
1.5 hours after sunrise. An extremely thin overcast of Altostratus or Cirrus cloud is present, but is
not thick enough to whiten the sun. The inside ring is green to violet, while the outer ring is red.
A faint secondary blue ring is visible outside the red ring (Medicine Bow Mountains, Colorado, USA,
photo by D.O. Blanchard)
Diffraction corona around the sun. Only a part of this phenomenon is visible. The star-shaped
appearance of the sun is caused by over-exposure in the camera. (photo by G.P. Können)
The unPolarized diffraction corona appears as a bright coloured aureole around a source of
light. It is much smaller than the halo (some degrees in diameter, just like the glory) and is
very bright. It is best seen near the moon or near a terrestrial source of light; near the sun it is
difficult to observe with the naked eye because of its brightness. Yet this is when it is at its
most beautiful. It is usually noticed near the moon and it is almost the only optical
phenomenon that maintains a wealth of colours by moonlight. One sees it appear when there
are thin clouds in front of the moon. It is formed by diffraction by the cloud particles; its centre
is vividly white, with a red edge round it. Round it appear coloured rings, which in favourable
circumstances can sometimes recur but then do so with a continually decreasing intensity of
light. The size of the rings is inversely proportional to the diameter of the particles, as in the
case of the glory. When a second cloud passes in front of the moon, a change in the
diameters of the rings ca be noted; near the edge of a cloud the corona is often slightly egg-
shape (this also occurs with the glory). During a foggy night one often sees a corona appear
around a lamp-post or around the head-lights of a motorcar. Coronas can also frequently be
observed through blurred window panes; then, however, the centre is dark. In this case they
are not formed by diffraction but rather by interference of light that has passed through several
little 'holes' between the drops. Atmospheric coronas can be formed in both water-clouds and
ice-clouds, but in the latter case the coloured rings are less well developed. Even around
bright planets like Venus or Jupiter the corona can be seen; here, however, it is only the
uncoloured centre that appears since the intensity of light in the coloured parts is too low for
any observation. On the other hand, a complete diffraction corona has sometimes been seen
around the totally eclipsed sun: then the solar corona was the source of light.
