Astronomy News
Dr. Neal Sumerlin keeps us abreast of happenings in the night sky and the progress of the new Belk Astronomical Observatory.
Blogs we recommend
Other Posts:
Water on the Moon! (11/16/2009)
Leonid Meteor Shower (11/16/2009)
The Moon on YouTube (11/16/2009)
2012--Movies and Real Life (11/16/2009)
Describing the Indescribable (07/22/2009)
Total Solar Eclipse in July (06/07/2009)
Other Earths (05/20/2009)
Save Those Old Computers! (04/13/2009)
Play With Pictures from Mars! (04/13/2009)
Saturn in 2009 (04/13/2009)
The New Worlds (02/04/2009)
Christmas at the Moon (12/10/2008)
Potpourri of Space News (12/10/2008)
Night Sky Happenings (11/17/2008)
Power Sources for Space Probes (11/17/2008)
R.I.P., Mars Phoenix Lander (11/17/2008)
Pictures of Planets (11/17/2008)
Ice Geysers of Enceladus (09/22/2008)
Constellations (09/22/2008)
Happy Equinox Day! (09/22/2008)
More News from Mars (06/04/2008)
Search (but no rescue) on Mars (05/20/2008)
We lose a friend (05/03/2008)
Quiz winner! (04/29/2008)
Seeing and Patience (04/22/2008)
The World at Night (03/31/2008)
New Stars that are Really Old (03/14/2008)
Latest From Planetary Spacecraft (03/14/2008)
Lunar Eclipse Update (02/18/2008)
Aiming a Telescope (02/18/2008)
Observatory Update (02/04/2008)
Venus and Jupiter in the Morning (02/04/2008)
Total Lunar Eclipse (02/04/2008)
Messenger Mission to Mercury (02/04/2008)
Moon Halos
Posted on 12/02/2009Can you see a rainbow at night? If you were out last night in Lynchburg, you most certainly could and probably did. The full moon was at the center of a rainbow-colored ring of light-a halo-with a radius of 22 degrees, red on the inside of the ring and blue on the outside.
Two things are required for this phenomenon-a bright light source, and high cirrus clouds containing ice crystals between you and the source. Light is refracted (or bent) as it passes through these hexagonal ice crystals; the most likely angle of refraction is 22°. White light is made up of light of different wavelengths, and each of these is bent at a slightly different angle. This results in red light (longer wavelength, bent less) appearing on the inside of the ring of light.
You see these 22° halos around the sun when the necessary cirrus clouds are present. It rarer to see them around the moon, because the moon is much less bright, especially when it is not in its fully illuminated phase. When the moon is less than full, you may see a ghostly ring around it, but it will probably not be bright enough to trigger your eye's color sensors and will therefore just look white. A full moon is usually required to bring out the rainbow effect.
What you are seeing is reflected sunlight, after all. The full moon looks very bright against a dark sky, but you can safely look directly at it. It is a very poor mirror, reflecting only about 12% of the sunlight that falls on it. Even that low percentage is enough to let you read a newspaper by the light of the full moon!