Fri. Feb 23rd, 2024
zodiacal light, gegenschein

The importance of having dark sky reserves cannot be underestimated.

In addition to the many benefits already described above on our blog, you can see many things that others cannot from the light-polluted skies of our urban and, increasingly, rural locations.

Zodiacal light and gegenschein are two phenomena known for centuries, but only visible in dark skies with a good western or eastern horizon.

zodiac light

This appears as a very faint glow before sunrise (the east) or after sunset (the west).

Many have referred to the light as “the false dawn.” In fact, it was clearly described in Islamic religious texts, so it was not confused with the actual dawn (astronomical twilight) that denotes the time of morning prayer.

Photo: ESO/Y. Beletsky – ESO CC BY 4.0

The zodiacal light seen at dusk follows the path taken by the sun throughout the spring. As such, it appears highest in the sky during March and can be seen about an hour after sunset.

You need a clear, dark, unobstructed view of the western sky, like that of Lake Superior Provincial Park.

The origin of this mysterious light has long been questioned, although scientists have recently confirmed the source. While it was long thought to be light reflected from dust within our solar system, it has recently been concluded that this dust is constantly being replenished with material from comets (rather than asteroids or other sources).

The opposite note

German for “counterbrightness,” genschein is a somewhat concentrated form of light seen in dark skies in rare circumstances.

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Occasionally you can observe this phenomenon in the night sky right in front of where the sun would be. So if you look south around midnight during spring or fall, you may be able to see a faint ball of concentrated light: the gegenschein. It is difficult to see in summer or winter when the Milky Way can eclipse this fainter light.

CounternotePhoto: ESO/Y. Beletsky – ESO CC BY 4.0

Like zodiacal light, gegenschein light comes from interplanetary dust. However, the dust produced by the gegenschein light is reflected directly from a point opposite the sun, known as “backscattering.”

What is special about this point?

Well, the combination of gravitational forces from the Sun, Earth, and Moon produces a series of stable gravitational zone points around the Earth, meaning that objects at these points tend to stay locked in at those points.

There are five of these “Lagrangian” points around the Earth. The second point (L2) is where the dust that produces the gegenschein is located.