Full Lunar Eclipse - StarLog 10/08/2014

I decided to leave my large astronomy binoculars (25x100) back at my apartment because my well is going to be finishing up within the next couple of days and I figured I wouldn’t have an immediate need for them.  Turns out I was wrong. Shortly after unpacking my things from the truck I got a notification from Starwalk (an astronomy application) telling me that there was going to a full lunar eclipse tonight…
  

Time lapse of last night's lunar eclipse.

Fortunately you do not need highly sensitive optics to view the moon so I was content with my small pair of binoculars.  When the time came and the moon began to pass into the Earth’s shadow I stopped working, pulled up a lawn chair and sat back to enjoy the view.  I was within the region of the US that was able to get a clear view off the full eclipse and my skies were clear so I was very pleased with my view.  Earlier in the year during the last lunar eclipse a couple of friends and I drove an hour out of town in a snow storm in an attempt to catch a view of the eclipse.  Our outing ended in disaster when my friend’s car blew a head gasket, overheated and left us stranded.  We never caught so much as a glimpse of the event.  
  

While I was watching the moon pass into the Earth’s shadow I sent a text message out to a few friends and my family which was along the lines of, “If you’re awake there is a full lunar eclipse happening right now!”  As it so happened my Mom was driving to work and had a view through her front window, she called me up to talk about it.  Apparently they were talking about it on the radio, but my mom wondered why the moon turned red when in the presence of the Earth’s shadow.  I thought about it for a second and then I realized I wasn’t sure.  I figured that it had to do with the visible light spectrum and the differing wave lengths.  I gave a quick explanation of why the sky was blue and how that phenomenon could be explained by the scattering of the shorter blue wavelengths but I was still somewhat puzzled myself by why the moon was red.  If the moon is in the Earth’s shadow then the Moon must not be reflecting any of the light emitted from the Sun, right?  Wrong!  After doing a bit of research I realized that I was on the right track but I forgot to consider how light refracts or bends when it goes through a given medium such as the Earth’s atmosphere.
  

The celestial geometry of the Sun, Earth and Moon during a full lunar eclipse.

A full lunar eclipse is when the Sun, Earth and Moon are in perfect alignment and the Moon passes through the Umbra (or Earth’s shadow).  A normal argent full moon is very bright because it is reflecting direct sunlight back to Earth.  However, during a lunar eclipse it is no longer reflecting direct sunlight, but instead it is reflecting sunlight that has traveled through the Earth’s atmosphere and been refracted at a slight angle so that it hits the Moon.  
 

The visible light region of the Electromagnetic Spectrum.

A cartoonist’s recreation of Newton’s experiment in 1665 showing how a prism bends white light and that each color refracts at a different angle depending on the wavelength of the color.

If we think back to our science days we can remember what light really is, that being electromagnetic radiation.  However, because the cone-shaped cells in our eyes are tuned so precisely we can only see a narrow band of that spectrum.  We call this spectrum the visible light spectrum.  Issac Newton showed us that if we shine the full spectrum of visible light or white light through a prism, the wavelengths (A wavelength is the distance from crest to crest) separate into the colors of the rainbow.  This is because each color of the rainbow corresponds to a different wavelength.  Does the acronym ROYGBIV ring any bells?  Violet has the shortest wavelength, at around 380 nanometers and red has the longest wavelength, at around 700 nanometers.  
  

A visual depiction of wavelength as it corresponds to the visible light spectrum.

So how come the moon shines blood red during a full lunar eclipse?  Well for the same reason the sky shines blue on a clear day because sunlight is bouncing off the air in our atmosphere and preferentially bounces blue light.  This is because our atmosphere is made mostly of nitrogen and oxygen.  The shortest wavelengths, indigo and violet (blue light) are more efficiently scattered than the longer wavelengths because they are closer to the size of the molecules.  In other words, the particles that make up our atmosphere are about as small as the wavelength of blue light.  
 

But wait…
 

So how does the blue sky relate to the red moon during a lunar eclipse?  Well for the same reason the sunset is red!  The red seen at sunset is what is left after all of the blue light has been scattered out of the incoming sunlight.  Because at sunset the light is passing through our atmosphere lower on the horizon it passes through a much larger column of air than at noon.  What we see at sunset is the residue light after it has already scattered out all of its blue light, and in turn what is left is the longer red wavelengths. 
  

The Answer!
 

The moon shines blood red during a full lunar eclipse because the only sunlight that reaches its surface has passed completely through the Earth’s atmosphere where the blue light has been scattered out by the Nitrogen and Oxygen in our atmosphere leaving only a residue of red wavelengths.  This residue of sunlight then comes into contact with the Moon where it is reflected back towards Earth, passes again through our atmosphere further depleting it where the light is finally absorbed by passing through a small hole in our eyeball where it is received by a few cone shaped cells at the back of our eye processed by our brain where we measure the wavelength and have an aha moment, the moon is red!  Wait, why is the moon red?