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Zodiacal light is a faint, roughly triangular, diffuse white glow seen in the night sky that appears to extend up from the vicinity of the Sun along the ecliptic or zodiac.〔(Internet Encyclopedia of Science ) Accessed April 2010〕 It is best seen just after sunset in spring, and just before sunrise in autumn, when the zodiac is at a steep angle to the horizon. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders it invisible. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible in large part〔Reach, W. T. (1997). ("The structured zodiacal light: IRAS, COBE, and ISO observations", page 1 (in Introduction) )〕 for the total skylight on a moonless night. Another phenomenon—a faint, but slightly increased, oval glow directly opposite the Sun—is called the gegenschein. The dust forms a thick pancake-shaped cloud in the Solar System collectively known as the zodiacal cloud, which occupies the same plane as the ecliptic. The dust particles are between 10 and 300 micrometres in diameter, most with mass around 150 micrograms. == Viewing == In the mid-latitudes, the zodiacal light is best observed in the western sky in the spring after the evening twilight has completely disappeared, or in the eastern sky in the autumn just before the morning twilight appears. The zodiacal light appears as a column, brighter at the horizon, tilted at the angle of the ecliptic. Since the light scattered from extremely small dust particles is strongly forward scattering, although the zodiacal light actually extends all the way around the sky, it is brightest when observing at a small angle with the sun. This is why it is most clearly visible near sunrise or sunset, when the sun is blocked, but the dust particles nearest the line of sight to the sun are not. The dust band that causes the zodiacal light is uniform across the whole ecliptic. The dust further from the ecliptic is almost undetectable except when viewed at a small angle with the sun. Thus it is possible to see more of the width at small angles toward the sun, and it appears wider near the horizon, closer to the sun under the horizon. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Zodiacal light」の詳細全文を読む スポンサード リンク
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