Why is the Sky Blue?

To understand the answer to this question, first understand what the sky looks like on
the moon. The moon is 81 times lighter than the earth (yet it has 28% of the diameter so
you will weigh 15% of what you do on earth on the moon).  It is not massive enough and
therefore does not have a strong enough gravitational field to hold an atmosphere.
Because most air molecules travel really fast (most at supersonic speeds and lighter
elements travel even faster)the moon’s small gravity is not enough to keep them from
shooting off into space.  So it has no atmosphere and thus the day time sky from the
lunar surface looks just like our night time sky.  You can see the stars for 24 hours a day
on the moon.  

The earth’s gravitational field is strong enough to hold an atmosphere, so our skies are
filled with all but the lightest gases (low mass atoms like those of hydrogen and helium
are light enough to escape the earths gravitational field). During the day time we cannot
see the stars because the gas molecules in our atmosphere disperse much of the light
from the sun in random directions. This is ambient light.  A very small percentage of the
total light that travels through our atmosphere is actually absorbed by it.  However,
enough light is scattered by the atmosphere to visually drown out the light from daytime
stars. During the night time we can see the stars because the sun is no longer present
to illuminate the atoms in our atmosphere.  
The light that makes up the ambient light in our atmosphere comes from the sun, which
emits white light. White light contains all of the wavelengths of light that are in the visible
spectrum (the ones that we can see). The nitrogen and oxygen atoms in our
atmosphere absorb many of these different wavelengths of sunlight.  They absorb less
and scatter more of the higher frequency wavelengths (bluish light) and therefore there
is more blue light bouncing around from atom to atom in the sky.

The difference in wavelength causes blue light to be scattered nearly ten times more
than red light. Lord Rayleigh of England (b. 1842) observed this phenomena and so it is
called Rayleigh scattering (and is also known as the Tyndall effect).  The color that is
scattered most is violet because it has the shortest wavelength of any visible light. The
sky is blue to us only because we have better receptivity in our retinas for blue light than
for violet light.


Organization for the Advancement of  
Interdisciplinary Learning
Physics
Organization for the Advancement of
Interdisciplinary Learning