Moon Rainbow

Sat Night at 8.30pm I saw a very clear rainbow in a semi circle around the top half of the almost full moon.

Maby this happens frequently, but it is the first time I have ever seen anything like it. I was going to run into the house and get a camera but was afraid it might be gone by the time I got back. So I just stared and enjoyed the specticle. It remained clear for about a minuite and then started to fade and reduce in size over next five mins until it was gone.

Did anybody else see it? is this unusual?

Damon

hi Damon,

saw it myself as I was leaving work on Saturday night. Very nice colouration to it.

strictly speaking, the phenomenon is called a corona (or aureole). It's roughly the same principle as what causes a rainbow in that it is water droplets scattering light into its constituent colours (though in a rainbow, there are one or more internal reflections also).

as with an ordinary sunlight rainbow however, a Moon rainbow will form opposite the light source in the sky. Because Moonlight is fainter, lunar rainbows are generally whitish in colour though photography will tease out the pale colours.

I know of only two instances in the historic record of the last two hundred years where a rainbow was seen around the Sun. This would be what they call the third order rainbow; a normal rainbow is first order, a rainbow outside this, with colours reversed, is the second order. Each lower level order is only a tenth as bright as the previous level so to see a third-order bow is extremely rare. In the lab however, it is possible to reproduce rainbows to the 12th order in a droplet of water using laser light. Jeral Walker, in his "Amateur Scientist" column of Scientific American, described this in an issue from a good number of years ago (can't remember the year but I have the column on a CD archive if anyone is interested).

all the best to yourself, Sally and the gang for the Christmas,

John

I have seen a Lunar rainbow myself on a couple of occsions, but never thought it to be a unique sighting! Like what you saw Damon, each tim it was short lived. Probaby just an unusually 'wet' misty cloud passing by at the time. I do remember though, that the Moon was exceptionaly bright on each occasion. It certainly has been bright the past few nights! :shock:

Seanie.

hi Seanie,

yes, lunar rainbows are sort of infrequent. What yourself and Damon saw was a corona though. These is a diffraction phenomena when light cloud passes in front of the Moon, scattering the light into its constituent colours. A lunar rainbow on the other hand will be opposite the Moon in the sky. I jumped straight in to discussing rainbows in my earlier post without properly defining what people saw. Apologies for that.

John