|MadSci Network: Astronomy|
When the light from the Sun or the Moon shines through the atmosphere, it illuminates particles suspended in the air and produces a number of different phenomena depending on the kind of particles that are more abundant at that moment. Some of the more common are the haze and halos around the Moon and Sun, which are produced by ice crystals in the upper atmosphere, often associated with cirrus clouds. It is not well understood how these crystals form, but the fact that the halo forms at 22.5 degrees from the Moon or the Sun means that it is caused by them. There is a nice picture and some more explanations in http://antwrp.gsfc.nasa.gov/apod/ap000515.html Perhaps the most common cause of haze is produced by water droplets in humid weather. Other kinds of particles like dust, soot or volcanic ash can produce different phenomena when the light from the Sun or the Moon shines through them. You can read an interesting account of them in http://www.ast.cam.ac.uk/~ipswich/Miscellaneous/Weather_phenomena.htm Even Venus can produce atmospheric phenomena like the Venus pillars, which are vertical rays, which are produced by light being reflected from suspended ice crystals close to the ground. It is of course more common to see Sun or Moon pillars. For a more complete and beautiful account of atmospheric optics see http://www.sundog.clara.co.uk/atoptics/phenom.htm Vladimir Escalante Ramírez
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