A strange triangle of light appears in late winter skies. Here’s what causes it.

On clear late-winter nights, a dim light appears in the sky after twilight fades. It’s not the aurora, nor a lingering sunset, nor the inner core of the Milky Way Galaxy. Known as the zodiacal light—sometimes called “false dusk” or “false dawn”—this ghostly pyramid-shaped glow is one of the solar system’s most understated spectacles. As the spring equinox approaches, the alignment of Earth and the ecliptic makes the zodiacal light more visible—offering a rare chance to see the dusty plane of our solar system with the naked eye.

What is the zodiacal light?

Visually, the zodiacal light appears as “a cone-shaped wedge of light rising from the sunset point in the evening or sunrise direction in the pre-dawn hours,” says Brian Skiff, a research scientist and astronomer at Lowell Observatory. It’s a very diffuse glow that does resemble the earliest moments of dawn and the last ones of dusk.

Physically, the zodiacal light is scattered sunlight reflecting off a cloud of interplanetary dust “roughly the size of cigarette smoke particles,” says Skiff. “It is some mixture of debris from comet tails and trails, as well as from collisions of asteroids.” This dust lies along the plane of the solar system, the same one in which the planets orbit the sun. That’s why it’s called “zodiacal”—the glow traces the ecliptic, or the path of the sun through the zodiac constellations from the perspective of Earth.

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In 2021, scientists from NASA’s Juno mission published a paper suggesting that some of the dust particles involved with the zodiacal light might actually be from Mars. En route to Jupiter, the Juno spacecraft unintentionally measured the density of interplanetary dust in our inner solar system, finding the dust cloud’s densest region between Earth’s orbit and the Asteroid Belt, where Mars orbits. While Mars has frequent dust storms, scientists aren’t sure how Martian dust could have escaped the planet’s gravity to form an interplanetary cloud.

Why late winter is the best time to see the zodiacal light after dusk

The zodiacal light is best spotted in the weeks surrounding the spring and autumnal equinoxes, when the alignment of the ecliptic provides the best viewing conditions. “The ecliptic is tilted nearly vertically to the horizon, so the band suffers the least effects from any interfering glow closer to the horizon,” Skiff says. “The band is also well away from the Milky Way, so the natural sky background is darkest.”

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In late winter, the zodiacal light is visible in the evening in the Northern Hemisphere (false dusk) and in the pre-dawn morning in the Southern Hemisphere (false dawn). In early autumn, the timing switches between the hemispheres.

How to see the zodiacal light

While the zodiacal light is best seen at lower latitudes, its visibility extends to the mid-latitudes near the equinoxes.

To see the zodiacal light this season, head somewhere with truly dark skies—far from any light pollution—and a clear, unobstructed western horizon. Time your viewing for about 90 minutes to two hours after sunset and look west. The zodiacal light will appear to form a V shape with the Milky Way.

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“For the morning apparition in autumn, look toward the east,” says Skiff. At this time of year, it is better to do so 90 minutes to two hours before sunrise. “In this case, the Milky Way will be nearly overhead, stretching from northwest to southeast, the two bands forming a huge X-shaped pattern of light in the sky,” says Skiff.

How can you tell the difference between the Milky Way and the zodiacal light? “The Milky Way starclouds have a clumpy or mottled and grainy texture, whereas the 'zodi' is very smooth and diffuse,” says Skiff. “Remarkably, the Milky Way is not as bright as the zodiacal light. Even thin clouds or moonlight will wash it out.”