The nineteenth-century English poet John Keats famously described autumn as the “season of mists and mellow fruitfulness / close bosom-friend of the maturing sun”-- a welcomed time of harvest beneath golden afternoon light. Autumn customarily heralds the appearance of falling leaves, ripe pumpkins, and wool sweaters. But also, it occasions an elusive apparition in the nighttime sky, a celestial ghost showing up for Halloween— the Gegenschein.
The Gegenschein is a hazy patch of faint light, slowly rising in the east after sunset. Its unusual name is German, translated as “Counterglow”. Near the start of the 19th-century, the explorer Alexander von Humboldt christened the phenomenon, after observing that it always appears directly opposite the sun, like the full moon. But the Gegenschein is far dimmer than the moon, or even the Milky Way. That’s why it’s elusive.
In the pre-Industrial Age, the Gegenschein was just about visible to the naked eye in most inhabited regions. But in our modern era of streetlights and smog, you need to seek out very clear, dark skies to see it. However, we often can capture the Gegenschein by taking a long exposure with a sensitive digital camera.
What causes the Gegenschein? In the plane of the Solar System lies a belt of interplanetary dust, the Zodiacal Cloud, continually generated by the disintegration of comets, and perhaps, asteroids. Sunlight reflects directly from this dust, and back to Earth. The Gegenschein is essentially a shiny spot in the belt. Sunlight also reflects obliquely off parts off this belt, forming a more luminous apparition called the Zodiacal Light, which we on the Western Slope can see rising from the eastern horizon before dawn in the early fall. Under exceptional viewing conditions, both can be seen together— clearly connected as parts of a single structure.
While the Gegenschein is present year-round, Earth’s orbital position favors viewing it in October. Some night soon, consider venturing far from city lights (warm mug of cider in hand,) and try to catch the Counterglow yourself. To view it is to be well rewarded by an astronomical beauty.
You’ve been listening to Western Slope Skies, produced by members of the Black Canyon Astronomical Society. This episode was written by Michael Williams and voiced by Art Trevena.