Harvest Moon: This weekend's full Moon (Sept. 14/15) has a special name--the Harvest Moon. It's the full moon closest to the northern autumnal equinox (Sept. 22). In years past, farmers depended on the light of the Harvest Moon to gather ripening crops late into the night. Post-Edison, we appreciate it mainly for its beauty.

Be alert in the nights ahead for Harvest Moon halos:

Moon Halos
©David Cartier
Halos are not purely daytime happenings. Look for them whenever a bright moon is veiled by thin cirrus cloud. A full or nearly full moon is best.

22º halos often encircle the moon. More rarely, because the moon is relatively dim, it is possible to see moondogs and other halos. Colours are faint or non existent because their light is barely strong enough to excite the colour sensors of our eyes.

Much smaller coloured rings sometimes surround the moon. These are not a halo but a corona produced by the diffraction of light by the water droplets of clouds. And of course moonlight creates a rainbow although to the unaided eye it is usually a wan creature devoid of colour.


Coronas:

Image
©David Cartier

Lunar coronae are much more familiar than those around the sun. They are seen when the clouds are thin enough that each single corona light ray reaching the eye is scattered or diffracted by only one droplet. Of course, the whole corona is made by a great many droplets individually scattering the moonlight.

Sometimes as clouds pass over the moon the corona shrinks and swells as different sized droplets mould it. Small droplets make the largest coronae with aureoles a few moon diameters across.

And 'dogs':

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©David Cartier