Shallow Sky Object of the Month: The Clear Daytime Sky

Original article appears in GuideStar November, 2020.

HoustonSky.PNG

Originally Published in the November 2016 Guidestar

by Bill Pellerin

Object: The Clear Daytime Sky
Optics needed: Unaided eye

Why is the sky blue instead of some other color, or no color? Most of us know why, in general, the sky is blue, don’t we?  The short, quick answer is that it has something to do with the scattering of blue light from the by the atmosphere.

SO, WHAT HAPPENS BETWEEN THE TIME THE LIGHT LEAVES THE PHOTOSPHERE OF THE SUN AND THE TIME IT REACHES US? THE MOST SIGNIFICANT EVENT IN THAT JOURNEY IS THE LAST 10 OR 20 MILES OF ITS TRIP, IN THE ATMOSPHERE OF PLANET EARTH.

Light of different colors is electromagnetic radiation of different wavelengths. You may remember the name Roy G. Biv from school… the name was to help you remember the color components of ‘white’ light in order of decreasing wavelength (higher frequency).   Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Wavelength and frequency are inversely proportional; the longer the wavelength the lower the frequency and the shorter the wavelength the higher the frequency.

This relationship is:                   

λ = v/f where:  
λ = the wavelength  
v = the velocity of light (300,000 meters/sec or 186,000 miles/sec)  
f = the frequency of the light wave (cycles/second)

The Sun is a G2 star, with a temperature at the photosphere (what  we think of as the ‘surface’ of the Sun) of 5800 Kelvin. It’s accurate  (enough) to say that the light that is radiated from the Sun is white  light and that the light includes all the colors you see – all the colors of the rainbow (literally). So, what happens between the time the  light leaves the photosphere of the Sun and the time it reaches us? The most significant event in that journey is the last 10 or 20 miles of its trip, in the atmosphere of planet earth.

The formal name for this scattering phenomenon is Rayleigh Scattering named after the British physicist Lord Rayleigh. The intensity of the Rayleigh scattering is proportional to the diameter of the particle and inversely proportional to the wavelength. That is, larger particles at shorter wavelengths (blue  light) produce more scattering. 

Here is a simplified way to think about what happens: Consider a white light beam leaving the Sun. This light beam includes all colors (wavelengths) of light, but when it slams into the  atmosphere we know that the shorter wavelength blue light is more likely to get scattered. That is, the blue light hits an air molecule and, instead of coming straight down to your eyes it goes, let’s say,  left or right. The blue light continues its bumper pool path by hitting another molecule which redirects light to your eyes. In reality, it may take several bounces for the light to be sent to your eyes, and some of the blue light will be reflected up and away from the planet, which makes our planet look like a blue marble.  

This simplification isn’t perfect, but it gives you the idea.  In truth, the blue light acts to set up an oscillation in the particles, and this oscillation causes re-radiation of the blue light in an arbitrary direction.

Sunrises and sunsets are red for the same reason that clear skies are blue. When the Sun is near the horizon, there’s  much more atmosphere between the Sun and the observer. Since atmosphere scatters the blue light, very little of the light from the Sun makes it to your eyes at sunrise or sunset. So, the sky looks red.

One more thing. You’ve probably noticed that when children draw the Sun in the sky it’s often a yellow circle. It’s not yellow, though, it’s closer to white. Since even at noon, the blue light from the Sun  is scattered and doesn’t follow a direct path to the earth, the light  that’s left looks a bit more yellow.

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