It's not obvious to the unaided eye, but the blue sky is polarized, with the polarization being strongest at an angle of 90 degrees to the sun. This polarization becomes very clear when taking pictures of the sky with a polarizer on a very wide angle lens, like the examples below. The parts with the strongest polarization become the darkest and the effect is at its strongest during twilight.
(2013-09-21)
[ polarizer || no polarizer ] ⇛ Just as light from the sun scattered in the sky is polarized, so is light from the moon at night. In this photo the polarization is strongest in the darkest part in the upper left corner. Compare it to the same photo taken without a polarizer to see the difference (2014-04-11)
[ polarizer || no polarizer ] ⇛ (2015-09-18)
[ polarizer || no polarizer ] ⇛ (2015-12-26)
A stitch of three photos to show that the polarization of the sky is perpendicular to the sun (2015-12-26)
A stitch of three photos to show that the polarization of the sky is perpendicular to the sun, this time with the sun in my back (2015-12-26)
A warped version of the previous two combined (2015-12-26)