At certain times of the year, when the sun has almost completely disappeared down a
low, unobstructed horizon, the tiny portion of light still visible turns bright green.
Because it flickers for just an instant, it is called a green flash. It can be seen only
when the atmosphere is so clear that the air particles bend sunlight like a prism,
separating the rays into its component colors.
These colors are layered one above the other with purple at the top and red at the
bottom. However, the violet and blue colors are easily scattered by the atmosphere, so
they are made practically invisible. The red light that is closest to the ground bends
only slightly, so it is the first color to be blocked by the earth as the sun sets. At
the right moment, the fading light is stripped of every color except green, which
flashes for only an instant before it disappears below the horizon.
On certain nights in very rare instances, when the air is extremely clear and light
scattering is minimal, the green color can turn instantaneously blue, then purple, and
finally a brilliant violet before giving way to night.