Expert: Tom Whiting - 12/13/2007

Question
Hello Tom, I often use to observe the sky with naked eye and with a telescope. Amateur I should say. While observing moon with naked eye, I have seen a ring around it(not like the one Saturn has) which has different diameters for different days. Its merely a light, but I have always wondered how this forms around moon. Not all days I have seen this, occasionaly I see this and also during rainy seasons. All the reason I have given may not be correct ! Can you explain how this gets formed.
Thanking you in expectation.
Mohan

Answer
Hi Mohan,
Yes, your analysis is quite correct, relating the ring
(or halo as we generally call it, if it's farther away
from the moon) to weather. (It's called a corona if it's
right up against the moon).
Anytime you see a change from the normal appearance of ANY object in the sky (sun, moon, stars)...maybe it's rings,
haloes, corona's, different colors, different shape from
the normal, twinkling of the stars, stars not pin-points but fat blobs in the sky, etc. etc..... that is all our atmosphere doing it. If we didn't have an atmosphere, every object would not change any appearance or color at all.  No orange-red sunsets, no flattened sun on the horizon, no orange-yellow rising moon, no twinkling of the stars, no haloes or corona's around the sun or moon.....and everything would stay at its full brightness until it set behind the western horizon....in other words, there would be no horizon dimming due to the light passing through a thicker atmosphere. But of course, we do have an atmosphere, so we see all those effects, on occasion.

Normally it's high thin cirrostratus clouds causing the big stuff, like corona's, sun dogs (mock suns), haloes, sun
and moon pillars when low on the horizon.... as it's those
clouds' ice crystals that is causing the effect that you
see sometimes, mainly by refraction (bending) of the light.  And since cirrostratus clouds usually always precede a warm front, then you can expect rainy weather within 24 to 48 hours.  In fact, the Native North American Indians used these lunar and solar haloes as a pretty accurate indication that rain was coming to their area soon. So you are correct...all that stuff is due to our atmosphere.

Oh, another good test at night is the naked-eye cluster, the Beehive open star cluster in Cancer (Messier # 44) if you are familar with it.  Same rule, if you can easily see it naked eye, then the sky is completely absent of high thin clouds, so you can expect a nice sunny day to follow.  But if you can't see it naked-eye, or it's very
difficult to see, then obviously there is something, some unseen high, thin cloud cover, blocking the dim light from the Beehive cluster, so expect rainy weather sometime the next day.


Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA