Notice the nice
blue sky, contrasting
with the white clouds as viewed from my garden. Notice also the
eerie appearance from all the lights in the valley. Yes, this
photo was taken at about 9:30 at night during the full moon of March
25, 2005. The full moon, high to the photographer's right,
provides enough illumination that a 15 second time exposure with a
digital camera is enough to bring out landscape details. What is
more interesting is that the sky is blue similar to a midday sky.
The blue sky in the daytime is caused by the greater scattering of blue
light from the sun by the air molecules in the sky (also called
Rayleigh scattering). We also only get blue skies when the
atmosphere is relatively free of pollutants - fitting for an Earth Day
photo. At night during a bright moon, the
comes from sunlight) is the illuminating source and the Rayleigh
scattering of moonlight makes the moonlit sky appear blue.
Visually we don't see the blue color of moonlit skies because our eyes
are less sensitive to color when the light is less intense. Just
above the white cloud in the nighttime photo one can barely see the
star epsilon Bootis.
The Photo at the right is the same scene taken in the early afternoon
the following day. Notice that the sky and the clouds are about
the same colors as the moonlit photo above, but the foreground colors
are much more "natural" without the street lights. Based on the
exposure values, aperture settings, the effective film speeds for the
two photos, and the pixel values, the daylight photo is illuminated
with about 166,000 times more light than the nighttime photo
illuminated by the full moon. This means the full moon on a clear
night is about 166,000 times less bright than the sun on a clear
day. This also means that the full moon reflects about 30% of the
solar visible light back to the earth, allowing for the scattering in
all directions after the light hits the rough surface of the moon and
the distance between the moon and the earth.. This also agrees
fairly well with the close-up reflectivity of the lunar surface as
about 10%. The wonders of information and measurements possible
with digital cameras! All photos by Donald F. Collins.
Physics Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
Department. These photos
feature an interesting phenomena in
the world around us. Students, faculty, and others are invited to
submit digital (or film) photographs for publication and
explanation. Atmospheric phenomena are especially welcome.
Please send any photos to firstname.lastname@example.org.
here to see all Physics Photo of the Week for 2005
to see all Physics Photo of the Week for 2004.