domingo, 2 de março de 2008

Watch Out For The Zodiacal Light!

Written by Tammy Plotner
Zodical Light courtesy of NASA
For northern hemisphere SkyWatchers, the next week or so (until the Moon returns in the early evening) is prime time to spot the Zodiacal Lights. No, it's not a visitor from outer space but the interplanetary dust from within our own solar system!
Now that the Moon has departed from the early evening sky, this is a good time in the northern hemisphere to watch the western skyline for the evening Zodiacal Lights. If you live in an area where light and air pollution isn't heavy, you stand a very good chance of seeing the interplanetary dust in the plane of our own solar system lit by the setting Sun. At this time of year in the northern hemisphere, the ecliptic plane extends upright from the western horizon. (If you live in the southern hemisphere, you need to watch the eastern horizon before dawn after New Moon.) When the Sun is just below the horizon, we can see a ghostly glowing pyramid. But, what is it?
Sunlight is back-scattered off small interplanetary dust particles, perhaps some of them from the very formation of our solar system itself. However, a lot of these tiny, millimeter sized splinters are from asteroids - or debris ejected from comets. Some of these particles are initially distributed in the trails that cause meteor showers, but whole lot of the dust eventually gathers along the ecliptic plane. Why?
For the ultra-tiny particles, the radiation and solar wind disperses them beyond the confines of our solar system. The larger particles spiral inwards, pulled towards the Sun by gravity and form a flattened disc - a very low density cloud of dust, coincident with the plane of the solar system. Sunlight absorbed by the particles is re-emitted as invisible infrared radiation. This re-radiation causes the particles to spiral slowly into Sun, thus requiring continuous regeneration of the dust particles composing this cloud. The reflective particle disc makes its home in the same path the planets take around the Sun - the ecliptic. This imaginary path across the sky is where we here on Earth see the Sun and Moon, and it's also home to the constellations of the zodiac!
Using the same celestial mechanics that give us times of solstice, equinox, lunar and solar eclipses, it only stands to reason there comes a time when the ecliptic plane appears nearly vertical from a certain vantage point. For the northern hemisphere it's west in the spring and east in the fall. For the southern hemisphere it's just the opposite! When the plane is near vertical, the thick air near the horizon doesn't block out relatively bright reflecting dust and we see the Zodiacal Light!
Head out to an open horizon area where you're away from man-made light pollution. As the skies grow dark, look for a faint pyramid of light spread out over a very large area of the sky. It won't be as dramatic as photos show it. Near its base at the horizon it can measures as broad 40 degrees (two handspans), and stretch up as high as 60-80 degrees under good conditions. The spectrum of the zodiacal light is the same as the solar spectrum, reinforcing the deduction that it is merely sunlight reflected by dust in the plane of the planets. If you think you see a ghostly glow, you're probably right!
Best of luck… And share your photos and stories!

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