The zodiacal light is
not part of our atmosphere, it is produced between the planets.
Nonetheless it is directly linked to this site’s first
topic - the scattering of sunlight by dust particles.
The zodiacal light is a softly luminous cone of white light visible
from an hour or so after sunset or before dawn. It extends from
where the sun is located beneath the horizon outwards and upwards
along the ecliptic, the path of the sun across the stars. It is
of similar brightness to the Milky Way.
It is best seen when the
ecliptic or zodiac makes a steep angle with the horizon. In
the Northern Hemisphere this is after sunset in Spring and before
dawn in Autumn. The light moves with the stars and catching it
is a balance between detecting its radiance early against a darkening
sky or waiting until the sky is really dark when the tip of the
cone is getting low. A location in the tropics, or low to middle
latitudes, where the cone is more upright helps considerably.
The soft glow is the collective light
of dust particles orbiting the sun out to the orbit of Jupiter and
perhaps beyond. They are 1- 300 micron (0.001 - 0.3 mm) across and
probably each several miles from its neighbours. Some dust is hypothesised
to originate from comets,
other from remnants of very rare collisions between asteroids between
the orbits of Mars and Jupiter. Orbital resonances with Earth could
also lead to a dust ring at Earth’s distance from the sun.
large compared to the wavelength of light (top) scatter
light strongly forward with a lesser backward peak. Very
small particles (bottom) scatter light more equally in
The particles are large compared with visible light wavelengths
and scatter light strongly forward i.e. close to the original sunlight
direction thus producing the brightest glow close to the sun.
They also scatter light backwards, although with much reduced
intensity. The faint backscattered light is visible in very dark
skies as a faint glow at the antisolar point, the position in the
sky directly opposite that of the sun. The glow is the ‘gegenschein’ or ‘counter
glow’. Exceptionally dark and clear skies reveal the 'zodiacal
crossing the sky and connecting the cone of zodiacal light to the gegenschein.
faint glows need 'averted vision'. With your eyes thoroughly dark adapted
look away from the point where you expect to see the glow. It then becomes
more easily visible because the eye's rod detectors away from our central
vision are more sensitive.
More viewing hints and images on OPOD.