Some celestial wonders visible in the night sky are more elusive than others, but they’re no less fascinating to hunt down and observe.
One of the most mysterious of these astronomical events is the ghostly glow called the zodiacal light.
On display in the Northern Hemisphere for the next two weeks, this phenomenon will be visible above the western horizon as a faint cone of light that extends halfway up the sky for about an hour after local sunset.
—Picture courtesy ESO
Ancient Romans thought this spooky haze was due to far-off campfires below the horizon, while the ancient Greeks said that it must be caused by distant volcanic eruptions.
In the mid-16th century some speculated that the zodiacal light was the outstretched atmosphere of our sun.
Today we know that the zodiacal light is actually caused by sunlight scattering off countless grains of microscopic interplanetary dust spread out to beyond the orbit of Mars.
The vast majority of this dust is concentrated within the plane of the inner solar system near the sun, making the grains’ combined light appear along the ecliptic, the path in the sky each planet follows.
This faint light show is best seen during early spring and autumn, when the ecliptic line is nearly perpendicular to the horizon, as seen from the North Hemisphere.
Visible from the tropics to mid-latitudes, the zodiacal light is a special sight reserved for really dark rural locations far from the blinding light pollution of cities and towns.
Your best bet to catch the zodiacal light will be on the upcoming moonless nights, looking toward the western sky after dusk. It’s best to drive out to a dark site by sunset with a comfortable camping chair, blankets … and maybe some hot chocolate!
Leave all white lights turned off to let your eyes adjust to the approaching darkness as sunset fades to dusk, which takes about an hour. Without any binoculars or telescopes, look for a pyramid of diffuse light.
Planet buffs should also take note that Mercury is putting on its best show now, as the planet has reached what’s called its greatest elongation—when it moves the farthest away from the sun in our evening sky.
Mercury is still a bit tricky to track down in the first half hour after sunset. Look for a faint star shining low in the western horizon within the first hour after local sunset. Sweeping the sky with binoculars will help pinpoint the planet.
—Illustration courtesy Starry Night Software
Better hurry though, because by next week the little world will start sinking and fading fast as it approaches the sun once more.
Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.