Once upon a time, before cities banished darkness and electrified the night, a glow on the horizon betrayed not the presence of civilization, but a hauntingly beautiful phenomenon known as the zodiacal light.
This triangular tower of light, also known as a "false dawn," is a fleeting specter, often appearing for less than an hour at the end of evening twilight or just before morning twilight. What's particularly fascinating about it, however, isn't just its ethereal glow, but what causes it to occur in the first place.
The origins of the zodiacal light have long been debated, with the first modern studies dating back to the 17th century. Italian astronomer Giovanni Domenico Cassini (the same man who inspired the name for NASA's spectacular Cassini mission to Saturn) believed it was due to cosmic dust reflecting off sunlight. Despite the clear images we've all seen from space, the solar system is a very dusty place. Asteroid collisions, off-gassing from comets, and other collisions within the solar system all contribute to the formation of interplanetary dust clouds.
In 2015, an ion dust spectrometer on board the ESA/Rosetta orbiter confirmed that the dust of the zodiacal light most likely comes from Jupiter-family comets during close passes to the sun. As the comets heat up, they expel an incredible amount of dust and particles. It's estimated that to keep the zodiacal light a constant presence in our sky, some 3 billions tons of matter must be injected into it every year by comets. Otherwise, like clouds at the mercy of wind in Earth's atmosphere, it would be quickly blown away by interplanetary forces.
The billions of grains of dust that make up this cosmic cloud all settle into a flattened disc spread out along the ecliptic — the annual path of the sky (or zodiac) that the sun appears to travel along. The cloud is so large that it radiates beyond the orbit of Mars and towards Jupiter.
From Earth, this interplanetary cloud actually extends across the entire sky. When observed after the setting sun is blocked by the horizon (or before rising at dawn), the angle of the light reflecting off the dust creates a towering pillar of light.
To spot the eerie glow of the zodiacal light, you'll need to travel to areas free of light pollution. Spring and autumn are the best times to observe it, when the path of the ecliptic makes the column of light stand nearly vertical in the twilight.
"It’s most visible after dusk in spring because, as seen from the Northern Hemisphere, the ecliptic – or path of the sun and moon – stands nearly straight up in autumn with respect to the western horizon after dusk," writes EarthSky.org. "Likewise, the zodiacal light is easiest to see before dawn in autumn, because then the ecliptic is most perpendicular to the eastern horizon in the morning."
During optimal viewing conditions, the zodiac can be seen for up to an hour after dusk ends or an hour before dawn.
In the 12th century, the beauty of the zodiac was immortalized in the poem "The Rubaiyat" by the great astronomer-poet Omar Khayyam of Persia.
"When false dawn streaks the east with cold, gray line,
Pour in your cups the pure blood of the vine;
The truth, they say, tastes bitter in the mouth,
This is a token that the ‘Truth’ is wine."
If you want to give yourself a serious challenge under the darkest of viewing conditions, try and spot the gegenschein. This faint concentration of oval light, which means "counter glow" in German, occurs opposite the sun in the middle of the night. Like the zodiacal, it's caused from sunlight reflecting off comet dust in the ecliptic plane.
Because the gegenschein is fainter than either the Milky Way or the zodiacal light, it's a phenomenon that is increasingly no longer visible from most inhabited regions of the world.