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I have seen a large circular formation of cloud around the full moon sometimes.

Have anyone seen the same? What is the reason behind this formation? Due to lunar attraction?

Most of the sky is empty and clear, and no clouds at all, only around the moon, far from the moon, not just very close, that cloud circle formed. I saw this many times, like the below drawing:

Enter image description here

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    $\begingroup$ Do you have an image handy of what you're talking about? $\endgroup$ – Undo Apr 20 '15 at 18:23
  • $\begingroup$ Maybe a moonbow? Even I have created faint versions of them at night in the moonlight, with a garden hose. $\endgroup$ – LocalFluff Apr 20 '15 at 18:52
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    $\begingroup$ If you can find a copy of it, Color and Light in Nature is a good book that goes into the various aspects of all the different halos, bows, and such. $\endgroup$ – user595 Apr 20 '15 at 21:54
  • $\begingroup$ I see these all the time, including around streetlights (even close up). I always figured it was caused by caused by my perscriptive lenses, or maybe the weather (it's constantly very humid where I live). $\endgroup$ – QuestionC Apr 21 '15 at 14:03
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That sounds very much like a 22° halo.

It's not an astrophysical phenomenon; it results from the refraction of light by ice crystals in Earth's atmosphere.

If this is what it is, it should be a fuzzy but regular circle centered on the Moon, with a radius of about 22 degrees. If you hold your fist out at arm's length with the thumb extended, the angular distance between the base of your fist and the tip of your thumb is typically close to 15 degrees; you can use that for visual comparison. Remember that 22° is the radius of the halo; the diameter is 44°, close to three time the width of your fist+thumb.

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    $\begingroup$ Aw man, you beat me to it. But +1 for the guide to measuring its size. $\endgroup$ – pela Apr 20 '15 at 18:59
  • $\begingroup$ thanks for the clear description. I will try to measure the way you said when I see the halo again. $\endgroup$ – Ganapathy C Apr 21 '15 at 1:47
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I'm guessing what you see is the moonlight being scattered by the hexagonal ice crystals in cirrus or cirrostratus clouds, which lie at very high altitudes, 5-6 km and above. The light is scattered by roughly 22º, and because of the slight wavelength dependence, the halo actually has rainbow-like colors, although often they are so faint that you just perceive it as white.

Read more here.

Moonhalo

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  • $\begingroup$ I think you labelled the wrong angles as 22 degrees. This is the minimum deviation of light passing through the crystals. Larger angles are possible. $\endgroup$ – Rob Jeffries Apr 20 '15 at 21:09
  • $\begingroup$ @RobJeffries: Wow… $\endgroup$ – pela Apr 20 '15 at 21:45
  • $\begingroup$ @RobJeffries: Corrected. Thanks a lot for pointing this out. $\endgroup$ – pela Apr 20 '15 at 21:55
  • $\begingroup$ I don't understand why don't you simply mark the outbound angle (from the observer towards the halo) as 22° one on your drawing? That's the angle we're measuring here. The two angles you marked now could be anything, it's just a coincidence that the Sun and the Moon cover more or less the same solid angle (their angular diameter is 31.6 to 32.7 and 29.3 to 34.1 minutes, respectively) when observed from the surface of the Earth. Plus, the angles you mark aren't 22°. Considering Sun's and Moon's distance, they would be nearly identical to half their angular diameter. $\endgroup$ – TildalWave Apr 20 '15 at 22:07
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    $\begingroup$ @TildalWave: Ah okay, I understand. You're right; in reality, the part of the solid lines between the Moon and the clouds should be drawn as parallel. I think I can live with my drawing not being to scale, though. If you think it's very confusing, I can make another one tomorrow. But now it's 0:30 in Denmark, so I gotta sleep :) $\endgroup$ – pela Apr 20 '15 at 22:31

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