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What Is “False Dawn”?

This month, you might be able to spot this ghostly glow extending up from the horizon before the sun rises. But you'll need to get someplace dark.

About a dozen years ago I had the pleasure of spending several nights in the southern Arizona desert where I had access to a dark and unpolluted sky.  On the first night, I arose around 3 a.m. and upon stepping outside, I immediately found myself under a magnificent star-spangled sky.  With a few hours to go before sunrise, I spent a long time admiring the beautiful array of winter stars, led by Orion, the Hunter high toward the south.  Looking toward the east, I could see the stars of Leo, the Lion slowly climbing up the sky.

It was also about this time that I noticed something that I apparently overlooked from earlier on in the evening.  Far below Leo and rather near to the eastern horizon was some sort of very faint, whitish, diffuse glow.  After about a half-hour later, when I again looked in the same direction, I could still see the glow, although it was now actually somewhat brighter and appeared to be reaching a bit higher into the sky.  After another half-hour, the glow seemed brighter still and was now well up into the eastern sky, actually reaching almost to the stars of Leo itself.  It was as if some local town or a distant city had suddenly materialized from beyond the nearby hills producing some sort of light haze projecting upwards against the sky.  It briefly crossed my mind that perhaps morning twilight had begun, but a quick perusal of my watch told me that, no dawn was still at least an hour away.

Suddenly, I realized what I was looking at.  “Of course! I’m seeing the Zodiacal Light.”

For someone who has spent the majority of his life in brightly-lit environments, this sighting of the Zodiacal Light was a real treat for me.

Why It’s Also Called False Dawn

I’m not the first to mistake the Zodiacal Light for the onset of morning twilight.  Countless others before me have been fooled as well.  In fact, the Persian astronomer, mathematician, and poet, Omar Khayyam (1048-1131) made reference to it as a “false dawn” in his one long poems, The Rubaiyat.

That faint ghostly glow was once thought to be solely an atmospheric phenomenon: perhaps reflected sunlight shining on the very high atmosphere of Earth.  We now know, however, that while it is indeed reflected sunlight, it is being reflected not off our atmosphere, but rather off of a non-uniform distribution of interplanetary material; debris left over from the formation of the planets.

These countless millions of particles – ranging in size from meter-sized mini-asteroids to micron-sized dust grains – seem densest around the immediate vicinity of the Sun, but extend outward, beyond the orbit of Mars and are spread out along the plane of the ecliptic (the path the Sun follows throughout the year).  Hence the reason for the name “zodiacal” light, is because it is usually seen projected against the zodiacal constellations.

Best Time To View Zodiacal Light

The best time to see the Zodiacal Light is when the ecliptic appears most nearly vertical to the horizon.  For those in the Northern Hemisphere, the best morning view in the eastern sky comes from late September into the early part of November; and in the western evening sky after sunset from early February to late March/early April.

For northerners at this particular time of the year, it is just before morning twilight begins (about 90 minutes before sunrise), that the Zodiacal Light should appear at its brightest and most conspicuous.

To the discerning eye, its diffuse shape resembles almost a tilted cone, wedge or slanted pyramid.  At the base of the cone, the light may extend some 20º to 30º along the horizon.  At its best, the display can approach or even equal the Milky Way in brightness, but yet it is so faint that even a small amount of atmospheric haze can obscure it.  On exceptionally clear nights, the tapering cone might be seen to stretch more than halfway up into the sky.

But you’ll have to get someplace very dark to see it. Most of us will find it hard to spot the zodiacal light due to light pollution.

Joe Rao is an astronomer and contributes regularly to the Farmers' Almanac.

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Mel

This morning I was admiring the moon and venus. It was so bright and eye catching. After standing in the yard for a lil while something caught my eye. At the time I had no idea what I was looking at,so I googled a description and Zodiacal light is what it brought me to. Until today I have never heard of the zodiacal light but I am very glad I witnessed it, and I hope to see it again over the next 2 wks

David Neaves

In writing books, I’ve made reference to false light, first light, and sunrise. False light was the light that comes early, enough to start hiding stars, barely get around, but not really very bright. First light can cast shadows, but no sun over the horizon yet. And sunrise, when the orb itself peeks over the horizon.

Janine

I’ve seen that but at night. I wondered what it was. Thank you for sharing

58florene

Thank you for the great article. I’ve never heard of this but I believe I’ve seen it and not realized what I was looking at. When it’s happened, I would think “there isn’t a city in that direction so what is that?” and then just brush it off as my lack of knowledge of the area (passing through unfamiliar states while driving). Fascinating to learn this.

Mary

Thanks for sharing! Really cool!

Nancy Mason

I noticed this when I visited my Aunt an Unk in Tucson as they had lived there for many years they knew what I was talking about and explained it to me
To me it looked like it never got dark

Babby660

(stretch, yawn) don’t know; I never get up that early:-)

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