False Dawn -- The Zodiacal Light

[HenryC]

Here's another unpublished article, I'm putting it here in case any of you has ever seen the Zodiacal Light or the Green Flash.  I'd like to hear your report.



Florida’s dark nights, transparent atmosphere and year-round clear skies offer much to the stargazer.  But its far-southerly location places it near the tropics, so the daily path of the rising and setting sun intersects the horizon at a steep angle.  Consequently, twilight is over in a hurry in our state, and the time between day and night is short.   In spite of this, Florida is one of the best places in the USA to see the Zodiacal Light, or the “False Dawn”, and unlike in our northern states, it can be seen throughout the year.

The Zodiacal Light is an astronomical phenomenon that is rarely seen nowadays because it demands perfect sky conditions, a dark moonless night, and is only visible for a short time just before sunrise or after sunset.  Under the right conditions, it can be a ghostly sight, a mysterious mist of light rising from the horizon. The glare of the newly rising or setting sun can cover up this faint sky glow, as can moonlight, the slightest light pollution, or even the Milky Way.  In fact, the Zodiacal Light is often mistaken for the Milky Way, which it resembles in appearance.  The difference is that the False Dawn is always near the position of the sun where it hides below the horizon, while the Milky Way is a river of stardust visible as a faint band from horizon to horizon across the whole sky, all night long.

The Milky Way’s faint glow is the combined light of the hundreds of billions of stars in our galaxy. The Zodiacal Light arises a bit closer to home, it is reflected sunlight off untold numbers of tiny dust particles floating in the plane of the solar system.  As a result, the glow seems to concentrate along the yearly  path of the sun and planets through the night sky, what astronomers call the ecliptic, or “Zodiac”, to use its ancient astrological name.

Those dust motes between us and the sun have their illuminated side facing away from us, so they are invisible.  Those a little further from the sun are closer to it, and brilliantly lit, but they are lost in the background glow of the sky. Those on the opposite side of the sky from the sun are fully illuminated, but they are further from the sun and much fainter.  To see this spooky light demands we look somewhere in between, at those dust specks near enough to the sun and us to be bathed in light, but far enough from it than we see them silhouetted against  a black sky.  So we perceive the Zodiacal Light as a faint glow along the ecliptic, a vaguely triangular shape of dim light against the darkness, with the point of the pyramid on the ecliptic, pointing away from the  position of the sun just under the horizon.  Like the Milky Way, the Zodiacal Light can be quite bright  and once you finally see it you will wonder why you never noticed it before.
The cloud of dust that is embedded in the plane of our solar system and which gives rise to the Light creates a faint glowing band all along the ecliptic.  Although brightest near the sun, as we have noted, this “light bridge” continues all the way around the sky, extremely faint, and very difficult to see unless conditions are perfect.   At that point in the sky directly opposite the sun the light bridge brightens and expands slightly into the gegenschein, German for “counter-glow”.  If we are lucky enough to see it (it is extremely difficult to spot, even by experienced observers under ideal conditions), it means we are looking at the faint reflections of all those tiny dust specks fully illuminated by sunlight because they are directly opposite the sun in the sky, their reflecting sides facing us. Both gegenschein and light bridge usually demand the darkest possible skies, like those visible from mountaintops in the high desert country.  But the False Dawn itself is easily visible from Florida, if you know where and when to look.

The important thing to remember when stalking the Zodiacal Light is that it is faint.  Although only visible at dusk, every trace of twilight must be absent, the sky totally clear and black, and the eye must be completely dark-adapted. There must be a good horizon, no moon and absolutely no artificial light anywhere, and the Milky Way and bright stars and planets should not be  nearby to distract or dazzle the observer. The best places to see it are from a deserted beach, barrier island, the deep Everglades, or a boat offshore.

There are other visual delights awaiting the skygazer at the sunrise and sunset hours.  As the sun slips below the horizon, the last little bit of the red disk sometimes flashes an unearthly green as it disappears below the edge of the world, the famous Green Flash.  For years it was thought to be a myth, because it is only seen when conditions are just right, the air is clear and transparent, and even then it doesn’t always happen. Today we know it is a real physical phenomenon and it has even been photographed. Although I’ve spent most of my life on the West Coast of Florida, probably the best place in the country to look for it, I’ve only seen it about a dozen times–most of the time it fails to materialize. I always make it a point to look for it when I’m at the beach at sunset, but  I lived in California for eight years and never saw it once on the Pacific.  Apparently the transparency of the air is just not right there.  There is almost certainly a green flash at sunrise on our Atlantic Coast, too, but since we don’t know the exact place and time when the sun first peeks over the horizon, we don’t get a chance to see it happen.

The green flash is caused by the refraction of the atmosphere, which is thickest between our eye and the sun when the latter is right at the horizon. Like sunlight refracted though a prism, or through water droplets in a rainbow, the white light  is broken up into its component spectral  colors; violet, blue, green, yellow, orange and red, which flash by our eyes one after another like the flipped pages of a book   But most of the colors are absorbed or scattered by the atmosphere, or lost in the red glare, and only the green comes through relatively unaffected.  So we see only a flash of beautiful emerald. It takes your breath away.

Skies at night and during the day are exquisite in Florida, but the sunrises and sunsets are legendary, the clarity of the air and the towering cumulus clouds paint the dawn and dusk in every conceivable variation of salmon, pink, and crimson against a background of violet and blue.  In winter, the mackerel skies and high cirrus clouds that foretell the coming of distant cold fronts contrast their scarlet tracery against the blue-black of early morning or late evening. The shadows of distant clouds  break up the hidden sun into spikes and rays, stabbing up from below the horizon.  The ancient Greeks, blessed with a sea and a sky not unlike ours,  imagined this as the hands of rosy-fingered Dawn, Eos, goddess of the morning, as she languidly climbed over the edge of the earth, her cloudy hair tumbling over her bare shoulders,  and sensuously rose over the wine-dark sea.