Expert: Philip Stahl - 2/4/2005

Question
Hi, I have experienced this a number of times and so far cannot find anyone else who has seen the same thing nor have I been able to find what might cause this. My question is 'Why does the entire disc of the sun appear to be blue when the whole globe of the sun seems to be just over the horizon at sunrise?' Any input would be appreciated. Thanks, Keith

Answer
Hello.

Your observation of a "blue Sun" at the time you describe sounds truly fascinating. Especially as I've never had the experience.

Consequently, I've had to do some research to get at the answer - though I suspect it has something to with the scattering of particles in the atmosphere.

M. Minnaert in his 'Nature of Light and Color in the Open Air' (Dover, 1954) makes a brief reference to the phenomenon in his sub-section 'Green and Blue Sun' on page 223-24.

He notes the former is typically visible through steam or an atmosphere highly saturated with moisture.

The latter, or 'blue Sun' - was evidently noted for the first time during the years following the explosion of Krakatoa in 1883, and reported in the British journal 'Nature'.

In effect, the color was caused by scattering of sunlight amidst "extremely fine volcanic dust hurled into the highest layers of the atmosphere."  Minnaert goes on to note that the "blue Sun" has also been seen during "sand storms".

Putting all this together, it seems safe to say that wherever it is that you've made this observation has high concentrations of dust in the atmosphere. This dust scatters the sunlight, say at sunrise, and - to your eyes- makes the entire disk appear blue.