What is a Ring or Circle Around the Moon?
Subject: RE: Circle Around Moon?
6 May 2004-0400
From: "Canali, Eric"
Although we often see a small hazy ball of scattered light surrounding
the Moon or the Sun as veils of thin cloud pass over between them and
us, the large "bright" circle or halo we see at times is almost always
of a radius of 22 degrees, measured from the sun or moon at its center,
out to its edge.
Measuring the apparent size of things in the sky in degrees may be a new
concept to you but it really is much simpler than it seems. Surely you
remember from school that a "right angle" (like the angles in each
corner of a square) has 90 degrees? Applied to the sky then we can fit
that same angular spread from the horizontal to the vertical; from the
horizon, up, up, up to directly overhead, to the "zenith". Your FIST,
at arm's length from your eye, takes up TEN DEGREES of your view of the
sky. The next time you see one of those big Sun or Moon haloes, try it
out: you'll find that the distance in the sky from the Moon or Sun to
the halo is a wee bit more than two "fists" at arm's length, 22 degrees
Scientists now understand the specific reasons that we see this effect
in the sky, which I will summarize here with the simplest, yet accurate
statement I've ever been able to come up with for it:
The 22 degree halo we see at times around the Sun or Moon in the sky is
caused by the fact that much of the light coming to our eyes is
scattered away from our line of sight to the Sun or Moon a MINIMUM of
22 degrees. From that 22 degree margin outward there is a much higher
concentration of the scattered or deflected light being diffused throughout
the sky. Inside that margin there is an oddly -lower- degree
of scattered light being directed to our eyes, even though that is so
much closer to the Sun or Moon in the sky. So, in a sense, it's almost
more a case of the halo being the bright margin of a "hole" in the
scattered light we see around the sun or moon.
What's scattering the light, and why a "minimum" of 22 degrees?
Billions of tiny needle like ice-crystals, far-up in the Earth's atmosphere.
They're way up there, mostly found in the zone between the highest and the next
to highest clouds. A kind of cloud-veil known as cirro-stratus is the most common
backdrop for halo-displays. The old-timers knew that haloes presaged a change in
the weather and they were right. Often when good weather is about to turn bad,
the highest clouds arrive first and then the progressively lower clouds...
The distinctive shape of the microscopic ice crystals is the reason for
the 22 degree dispersion angle. These water-ice crystals are mostly shaped as
hexagonal prisms (like a common wooden pencil). As the light
of the Sun or Moon passes through these prisms it is bent
("refracted") at least 22 degrees away from our line of sight. The
bending and the amount of bend are due to the combination of the
refractive index of water ice (how much ice bends light passing through
it at an angle) and the angles present in the hexagonal
Formerly of Buhl Planetarium & Tour Guide at
.. Pittsburgh PA.
Not a Scientist & Not from Adler Planetarium.
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meteorological question, was answered by a very experienced amateur
astronomer, who also works as a part-time Tour Guide at the Allegheny Observatory
. Allegheny Observatory provides public tours two nights a week from Pittsburgh
April through October; pre-registration is necessary, but the tours are free-of-charge.
For many years, Eric G. Canali was Floor Manager for the original Buhl Planetarium
and Institute of Popular Science in
. He also was a long-time member of the Pittsburgh
Amateur Astronomers' Association of Pittsburgh and is Founder of the South Hills
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