THE SOUTHWORTH PLANETARIUM

207-780-4249   www.usm.maine.edu/planet

70 Falmouth Street   Portland, Maine 04103

43.6667° N                   70.2667° W 

Altitude:  10 feet below sea level

Founded January 1970

Julian date: 2458749.5

2019-2020:  XVI

            "Always try to be a little kinder than necessary, for everyone you meet is fighting a hard battle."       -J.M. Barrie

THE DAILY ASTRONOMER

Tuesday, September 24, 2019

Moonbows

exist, but they're rare and, understandably, quite faint.   The image below shows a moonbow arcing across a sky bedazzled by aurora light over a village in northern Sweden.

Moon bows and northern lights:  this photograph shows a rainbow formed by moonlight through raindrops.  The colors are distinct in this image because the camera captured the image over a brief period of time as opposed to snapping the photo in an instant.    This photographic technique, well known to astrophotographers, is called "Time exposure."  Image:  Lightsoverlapland.com

Moon bows, like rainbows, form when light enters a raindrop in such a way as to cause the incidental white light to "split" into its component colors.   As we can see from the graphic below, the rainbow forms at an angle that is 42 degrees opposite from the position of the light source.    Light waves experience this splitting, properly called refraction, because the speed of light changes in different media, such as glass and water.  When the light enters the rain drop (which we assume to be spherical, but isn't), the wavelengths propagate through the water as speeds proportional to their wavelengths.   Red moves more slowly than the violet, as the latter is of slightly higher energy than the former.      As a result, these different color waves follow divergent paths in the droplet so that when they reflected out of the drop, they appear as a continuum of colors.

A rainbow can only form when the light source, Sun or moon, is low in the sky so that its light strikes the water droplets in the air at the correct angle.      During the day, one will find rainbows either in the western morning sky or eastern late afternoon sky.      One will not see a rainbow at noon.       

A moonbow is more rare than a rainbow because not only does the moon have to be lower in the sky, it also has to be bright and therefore either full or in a late gibbous phase.     Moreover, when a moonbow does form, it will be quite faint because even a full moon will be significantly dimmer than the Sun.*      When a moonbow does form, it will generally appear white because our eyes cannot perceive distinctive colors at low light levels. (For this reason, we don't see colors in most stars.)      The colors we saw in the first image were noticeable because that moonbow was captured in a time exposure photograph.   

As the moon is passing through the waning crescent phase today and will be about 25% illuminated, we won't expect to see a moonbow anytime soon.  However, if the moon is at least 75% illuminated and is low on the horizon when the sky is clearing after a rainstorm, gaze toward a region opposite that of the moon to find a faint arc of white light:  a rare, but beautiful, lunar rainbow.

*Even though the full moon is nine times brighter than the quarter moon and is much closer to us than the Sun, its angular diameter is equal to that of the Sun and it reflects only about 7% of the Sun's light

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