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Breaking false dawn

By Melissa Babcock

Publication: The Day

Published September 08. 2013 4:00AM

The morning sky will be without a moon for the next two weeks, presenting an opportunity to observe false dawn.

False dawn is another name for the zodiacal light, an eerie, cone-shaped glow, rising from the eastern horizon in autumn, well before dawn. It will appear slightly dimmer, but "milkier," than the Milky Way, and it's caused by sunlight reflecting off of dust particles that travel along the ecliptic, the same path traveled by Earth, the moon, and the other planets orbiting our sun.

From the northern hemisphere, the time surrounding the autumnal equinox on Sept. 22 is ideal for observing the elusive, pre-sunrise glow. In spring, everything reverses, and you'll have to look west well after sunset for false dusk. To increase your chances of seeing the light, you'll need to go far away from any cities (which emit their own brand of "false glow" into the sky).

From now until the moon returns to the morning sky mid-month, look at approximately 5 a.m. for a pyramid-shaped glow that points nearly straight up from the eastern horizon. It will resemble the glow of a distant city. On Sept. 18, the moon returns in waxing gibbous form and drowns out the zodiacal light.

Good views abound because the ecliptic stands almost directly above the eastern horizon before dawn, like an upside-down letter T. As seen from the southern hemisphere, the same is true of the western horizon after true darkness falls.

The zodiacal light is visible for about 120 to 80 minutes before sunrise, or up to an hour before true dawn begins to break, and that's about half an hour before sunrise. You'll be able to tell the difference between the zodiacal light and dawn because the former has no rosy glow. The reddish sky at dawn and dusk is caused by particles in Earth's atmosphere, and the zodiacal light, while also created by particles, originates far outside of our atmosphere.

Gazing at the zodiacal light means looking into the solar system, across the invisible circles along which each planet travels. If any planets were in the east and visible just before dawn, you'd find them nestled within the glow.




Sept. 8: Conjunction of the moon, Venus and Saturn. The moon will pass within half a degree of Venus low in the west during early evening. Meanwhile, Venus will sit within three degrees of Saturn. The three will be visible for about two hours after sunset.

A few hours later, another major player arrives late to the party. By 4 a.m., Mars appears low in the east-northeast. Be sure to look at Mars on Sept. 8 or 9 with binoculars or a telescope, when it hovers against the backdrop of the beautiful Beehive star cluster (M44) in the constellation Cancer.

Sept. 19: Full moon. This one is also known as the Harvest Moon.

Sept. 22: September equinox at 4:44 p.m. local time. The sun will shine directly on the equator, creating nearly equal amounts of day and night throughout the world. This marks the first day of fall (autumnal equinox) in the northern hemisphere and the first day of spring (vernal equinox) in the southern hemisphere.

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