This phenomenon is caused by the refraction of solar light enabling
it to diffuse itself gradually over our hemisphere, obscured by the
shades of night, long before the sun appears, even when that luminary
is eighteen degrees below our horizon. It is towards the poles that
this reflected splendour of the great luminary is longest visible,
often changing the whole of the night into a magic day, of which the
inhabitants of southern Europe can form no adequate conception.