THE SOUTHWORTH PLANETARIUM
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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.
[image:
https_blogs-images.forbes.com_jamiecartereurope_files_2018_11_Lights-Over-Lapland-Aurora-Moonbow-1200x795.jpg]
*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.
[image: Rainbow1.svg.png]
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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