GraphDark Hint - Zodiacal Light

  Sometimes called the 'false dawn' the Zodiacal Light should be familiar to sky watchers who live in the tropics. Unfortunately most people living at higher latitudes, particularly in more populated areas, have probably never seen, or even heard of it.

 
    There is a very diffuse glow all along the ecliptic caused by dust in the solar system, probably left over from comets and meteor streams.

Because of the way dust scatters sunlight it appears brightest to us in the general direction of the sun but much of this is, of course, hidden by daylight and twilight.

The main part of what remains visible shows up as the Zodiacal Light, a broad cone of light in the western evening sky after twilight has ended - or in the morning sky before twilight begins.
 

   

There is also a very slight brightening directly opposite the sun called the Gegenschein, or counterglow. It is very broad and faint but can be seen with the naked eye under excellent conditions, when it is high enough in the sky and not in front of the Milky Way etc.

Some dust is faintly visible all along the ecliptic with the faintest part being between the Gegenschein and the main Zodiacal Light. This is called the Zodiacal Band and requires exceptionally dark skies to be seen with the naked eye.

 
 
The best times for viewing Zodiacal Light are at the end of evening twilight and just before morning twilight, at times of the year when the ecliptic makes the steepest angle with the horizon. Under good conditions, just before dawn at low latitudes, it shows up as an easily visible, diffuse cone of light which reaches high up the sky along the ecliptic. Near the horizon it is tens of degrees wide and comparable to the brightest parts of the Milky Way. The name false dawn is very appropriate, but the real approaching dawn soon merges with it and drowns it out.

Because it reaches tens of degrees along the ecliptic the top of the Zodiacal Light cone can be seen for several hours after dusk and before dawn. The Gegenschein of course is highest in the sky around midnight, but its altitude and visibility will also depend on the latitude and time of year.

For those living at higher latitudes, for example in the United Kingdom, the Zodiacal Light is not so obvious, but it was easily recorded in the days before light pollution and can still be seen from darker parts of the country. Part of the problem is knowing what to look for and when, and having sufficiently clear and dark conditions at the right time of year.

Examples using GraphDark are given on the next few pages.
 

   Richard Fleet 2004

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