An all sky image taken by Doug Zubenel just after local midnight
in Cherry County, Nebraska August
North is at top, Mars is rising at 10 o'clock and ruddy Arcturus
is low in the west at 3 o'clock.
Far away from light pollution, the Milky Way blazes across
Elsewhere, soft green bands stretch from east to west. This
is light emission from Earth's upper atmosphere - the airglow.
Earth’s nightside sky is never completely
dark. Long after sunset and even with no interference from artificial
lights, moonlight or aurorae the sky has
a soft glow.
To see it needs long dark adaptation.
Eventually, when stars are almost blinding and the Milky Way
is a bright convolved mass, the sky itself will be seen to
be luminous and a hand held towards the heavens will be a black
Unlike the aurora, the airglow is visible all
over the globe. Though brightest 10-15 degrees above
the horizon it fills the sky. It is strongly
coloured yet it is without colour to our unaided eyes because its light is below their
threshold of colour perception. From orbit it is a green
Look carefully and banded or uneven structure changing slowly
over minutes might be visible. The bands can appear
to cross the sky and converge towards two opposite points on
the horizon in the same way that crepuscular and anticrepuscular
rays do. The reason is the same, the glowing bands are parallel
and perspective effects makes them appear to converge.