NASA Astronomy Picture of the Day Collection

A Halo Around the Moon

Have you ever seen a halo around the Moon? This fairly common sight occurs when high thin clouds [ http://ww2010.atmos. uiuc.edu/(Gh)/wwhlpr /cirrus.rxml?hret=/g uides/mtr/opt/ice/ha lo/22.rxml ] containing millions of tiny ice crystals [ http://dspace.dial.…] cover much of the sky. Each ice crystal [ http://kristall.uni…] acts like a miniature lens. Because most [ http://dspace.dial.…] of the crystals have a similar elongated hexagonal shape, light entering one crystal face and exiting through the opposing face refracts 22 degrees, which corresponds to the radius of the Moon Halo [ http://ww2010.atmos. uiuc.edu/(Gh)/guides /mtr/opt/ice/halo/22 .rxml ]. A similar Sun Halo [ http://antwrp.gsfc.…] may be visible during the day. The town in the foreground of the above picture [ http://www.skylook.…] is San Sebastian [ http://www.donsnsn.…], Spain [ http://www.odci.gov…]. The distant planet Jupiter [ http://antwrp.gsfc.…] appears by chance on the halo [ http://dspace.dial.…]'s upper right. Exactly how ice-crystals form [ http://www.public.i…] in clouds remains under investigation [ http://adsabs.harva…].

Juan Carlos Casado [ mailto:jccasado@skyl ook.net ]

Moon

halo

atmosphere

Spain

Jupiter

Goddard Space Flight Center (GSFC)

Moon

Sun

Jupiter

KRISTALL

moon

Advanced Communication Technology Satellite (ACTS)

http://antwrp.gsfc.…

SPD-APOD-ap000515