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Will of the Wisps

The night sky contains more than the familiar stars, galaxies, planets, comets and meteors. It contains odd entities like the new Moon in the old Moon's arms, the gegenschein, the ashen light, noctilucent clouds, the green flash and the Earth's shadow. The atmosphere play a major role in all these effects.

At dawn (and less frequently at dusk) it is sometimes possible to see the Earth's shadow projected onto the sky. The shadow is best viewed in the mountains facing west when the air is heavy with the morning's dew. Just before sunrise you can sometimes make out the dim curved silhouette of the Earth against the sky. If there are cragged mountains to the East, you may see the most spectacular form of these shadows as the Sun peeks between the crags - the Pillars of Dawn. The Pillars last only a few moments before the Sun rises too high, but they are worth rising early if you have a chance to behold them.

The gegenschein [German for counter glow] has been reported from antiquity, but scientists didn't believe it existed, until surprise, surprise they began to pick it up on their photographs! I have looked for it for fifty years without spotting it, but I still have hopes. The gegenschein is composed of minuscule meteors and dust. When the Sun's light is focused by the Earth atmosphere like a giant lens it illuminates these particles something like dust in a shaft of sunlight. This faint glowing cloud is best seen at midnight.

The noctilucent clouds provide another nightly glow, but they aren't very mysterious. Most of the weather of the Earth occurs close to the surface, but sometimes other gasses concentrate at great altitudes. Exposed during the day to the intense solar ultraviolet radiation, these clouds fluoresce after the Sun sets. Even oxygen fluoresces faintly for several hours after sunset. This is why it is darker two hours before sunrise than two hours after sunset. Don't confuse noctilucent clouds with the aurora borealis [northern lights] or normal high altitude clouds caught in the last moments of sunlight.

The green flash is not some sort of comic book super hero but an effect of the Earth's atmosphere on a setting [and rarely a rising] Sun. The dense air near the horizon behaves like a prism. The white light of the Sun is spread out into partially overlapping multicolored disks. The disks of red and orange light bends downwards. The disk of yellow light [which constitute the bulk of the Sun's light] passes through in the center, and disks of green, blue and violet light bend upwards. This sequence is the same as you see in a rainbow. Just as the Sun's brilliant yellow disk drops below the horizon, you may see a momentary flash from the green disk - the green flash. Blue flashes occur at high altitudes. Blue light bends up more steeply than green light. As far as I know, no one has reported a violet flash. When a line of clouds lie just above a clear horizon at sunset, you may see a red or orange flash just below the clouds. A level horizon such as over an ocean enhances the effect greatly, explaining why green flashes are common on tropical islands.

The ashen light is the source of some controversy. The inner planets particularly Venus show distinct phases just like the phases of the Moon. When Venus is a crescent, the shaded area can sometimes be seen emitting a faint grayish glow. Whether this ashen light is sunlight bent by Venus' dense atmosphere or an optical illusion as a viewer's eye tries to fill in the crescent is debatable. Some scientists maintain it is an illusion, because the ashen light doesn't appear on photographs. Maybe so, but illusion or not, I am one of many observers who has seen the effect.

The crescent Moon sometimes can be seen with the dark area illuminated with a faint bluish light. While the new Moon in the old Moon's arms may look like the ashen light, it has another cause entirely. It was the renaissance genius, Leonardo Da Vinci, painter of the Mona Lisa, who first correctly explained this blue light in his note book sketches. It is simply sunlight reflecting from the Earth's oceans and atmosphere onto the Moon. Do not confuse a new Moon in the old Moon's arms with a Blue Moon.

After a frustrating Summer, where we had the foggiest cloudiest drought I can remember, we are coming into just about the best season for viewing - Fall. The gas giants will all be well placed for viewing. Yesterday, those of us who stayed late at Frosty Drew got to witness the transit of Io across the face of Jupiter. Both the moon itself and its shadow were clearly visible in spite of rather hazy conditions - thanks to our new large telescope. For years, I've gotten used to seeing Jupiter and Saturn as small jewel like discs marked by tiny and precise lines and shadows. I could scarcely believe the very large crisply detailed disc presented by our new telescope. We were able to see things like the Great Red Spot which has become so pale in recent years that it fades from most small telescopes. One of our members decided it should be renamed the Pale Peach Oval given its bland contrast to the surface.

Leslie Coleman
Leslie Coleman
Entry Date:
Sep 1, 1999
Published Under:
Leslie Coleman's Columns
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