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Heavenly Glows - Zodiacal Light & Gegenschein

by Perry Pezzolanella, MVAS

A truly dark sky is hard to find, but for those of us still lucky enough to live where the Milky Way is visible and sky glow from towns and cities is nearly non-existent, there are two interesting phenomena that are visible, although still challenging.

The zodiacal light is a faint, roughly triangular-shaped glow of white light extending above the horizon after sunset or before sunrise in the same direction where the sun will rise or just set. This light is actually caused from sunlight reflecting off fine dust grains that orbit the Sun in the plane of the planets. It is best to look for zodiacal light before the start of morning twilight or after twilight ends in the evening, or about two hours before sunrise or two hours after sunset. Look for a triangular glow extending about 30 to 40 degrees high slanting from the horizon appearing about 15 degrees wide at the base at the horizon and tapering to about 5 degrees near the top. It will lean to the right in the morning sky and to the left in the evening sky. Under ideal conditions with no Moon or haze it can appear as bright as the Milky Way. Because it is shaped like a tilted cone of light, it cannot be confused with actual twilight which appears as a flat uniform glow along the horizon. The best times of year to see the zodiacal light are March-April in the evening sky and September-October in the morning sky. At these times of the year the ecliptic is nearly perpendicular with the horizon therefore making the zodiacal light stand more upright. This reduces the chance of it being lost in the dust and haze closer to the horizon.

The gegenschein (German for "counter glow") is a faint, whitish patch of light in the night sky opposite the Sun, which naturally is well below the horizon. It is also caused by sunlight reflecting off fine dust grains in the plane of the planets. It is usually oval and often a few degrees wide and up to 10 to 15 degrees long. In the exact middle of the night it can be seen in the constellation that is crossing the meridian during that time of year. The best times of the year to see the gegenschein are also during March-April and September-October when it cannot be confused with the Milky Way. During these times of the year the Milky Way is not dominating the southern half of the night sky. The gegenschein might be visible in Virgo during the spring nights or in Pisces during the autumn nights. It is fainter than the zodiacal light and the Milky Way, therefore it is important to avoid any stray light such as streetlights, city glow, the Moon, bright planets, or even the Milky Way. The slightest haze will also render the gegenschein invisible.

When viewing these challenging, but fascinating glows in the heavens, keep in mind that the particles causing the zodiacal light and gegenschein are comet dust and dust generated by collisions among the asteroids; thus, they are like looking into the evolution and history of the Solar System.