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Autumn’s false dawn

The next few weeks offer a wonderful opportunity to catch the mysterious zodiacal light – aka the false dawn – of autumn. The moon is out of the morning sky for the next two weeks, leaving it dark to see this elusive phenomenon. The zodiacal light can be seen in the east, preceding dawn’s first light. Look over the sunrise point on the horizon about 120 to 80 minutes before sunrise.

If you’re in the Southern Hemisphere, where it’s the spring equinox (rather than the autumn equinox) that happened a week ago, the zodiacal light appears in your western sky, beginning about an hour after the sun goes down. However, the moon is about ready to enter the evening sky, so watch for the zodiacal light about 80 to 120 minutes after sunset.

This light can be noticeable and easy to see from latitudes relatively close to Earth’s equator, for example, like those in the southern U.S. To those in rural locations, it’s often visible at this time of year while driving a lonely highway far from city lights, in the hour or so before true dawn begins to light the sky. In that case, the zodiacal light can resemble the lights of a city or town just over the horizon.

Meanwhile, skywatchers in the northern U.S. or Canada sometimes say, wistfully, that they’ve never seen it, although in recent years we’ve seen many photographs of the zodiacal light taken from northerly latitudes.

Here's the zodiacal light as captured on film in Canada.  This wonderful capture is from Robert Ede in Invermere, British Columbia.

Here’s the zodiacal light as captured on film in Canada. This wonderful capture is from Robert Ede in Invermere, British Columbia.

zodiacal light

Zodiacal light seen from Paranal Observatory in Chile. From latitudes closer to the equator, the zodiacal light appears brighter and easier to see. Image Credit: Wikipedia Commons

You need a dark sky location to see the zodiacal light, someplace where city lights aren’t obscuring the natural lights in the sky. The zodiacal light is a pyramid-shaped glow in the east before dawn (or after twilight ends in the evening, if you’re in the Southern Hemisphere now). It’s even “milkier” in appearance than the starlit trail of the summer Milky Way. It’s most visible before dawn at this time of year because, as seen from the Northern Hemisphere, the ecliptic – or path of the sun, moon and planets – stands nearly straight up with respect to the eastern horizon before dawn now. As seen from the Southern Hemisphere, the same is true of the western horizon after true darkness falls.

The zodiacal light can be seen for up to an hour or so before true dawn begins to break. Once again, look for it about 120 to 80 minutes before sunrise. Unlike true dawn, though, there’s no rosy color to the zodiacal light. The reddish skies at dawn and dusk are caused by Earth’s atmosphere, and the zodiacal light originates far outside our atmosphere. When you see the zodiacal light, you are looking edgewise into our own solar system. The zodiacal light is actually sunlight reflecting off dust particles that move in the same plane as Earth and the other planets orbiting our sun.

The constellation Leo adorns the eastern predawn sky at this time orf year, and the zodiacal light runs astride the ecliptic - Earth's orbital plane projected onto the constellations of the zodiac. As darkness gives way to dawn, watch for the planet Mercury to finally climb over the horizon.

The constellation Leo adorns the eastern predawn sky at this time orf year, and the zodiacal light runs astride the ecliptic – Earth’s orbital plane projected onto the constellations of the zodiac. As darkness gives way to dawn, watch for the planet Mercury to finally climb over the horizon.

Bottom line: No matter where you are on Earth, your local autumn is the best time to see the zodiacal light before dawn. Spring is the best time to see it in the evening.

An almanac can help you find the clock time for sunrise in your sky

Everything you need to know: zodiacal light or false dawn

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Bruce McClure

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