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   Zodiacal Light
  Pre Dawn Zodiacal Light imaged by Dominic Cantin (site) in September 2003. The ghostly white cone is sunlight scattered by interplanetary dust particles. The red glow near the horizon is an aurora and much closer. Image ©Dominic Cantin, shown with permission.
   


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.

  
      
  Particles 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 all directions.  
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.

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 band' crossing the sky and connecting the cone of zodiacal light to the gegenschein.

These 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.