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I have always found September a perfect time for looking beyond my beloved atmosphere to the heavens above. The evenings are often still warm enough to linger comfortably outdoors and sit and gaze upward. Yet, sunset is early enough and darkness quickly complete enough to enjoy the celestial show without staying up half the night.
The weather, of course, plays an important role in star and planet watching since an overcast night kills any chance of seeing beyond the cloud deck. Air pollution, combined with light pollution, also affects what we see or how much we see. They are related as dust and other pollution particles in the air not only diminish the incoming starlight but can scatter urban light and render all but the brightest bodies invisible. But let's look at this topic from a location away from strong human influences. We'll also eliminate the clouds, although the right cloud-moon combination can bring many fascinating features to nighttime sky watching. What is the first thing you usually notice about stars on these clear nights? I'll give you a hint from our youth: "Twinkle, twinkle little star...." Yes, stars twinkle. The "little" part may refer to the fact that the planets, which have much of the same appearance as stars, do not twinkle. The technical term for twinkling is scintillation, the rapid variation in apparent position, colour or brightness of a luminous object when viewed through a turbulent media, in this case, the atmosphere. Stars, as we know, are large masses of glowing gas similar to our sun, but they are located so far away that they appear to us as bright pin-points. Their light travels relatively straight and true across the light-years of interstellar space, reaching the top of Earth's atmosphere as a steady point of light (how they would appear to viewers on the International Space Station). When starlight enters the relatively dense atmosphere (compared to the vacuum of space), its rays are diverted from their direct path by changes in air density on their way toward the surface. This is called refraction. If the atmosphere were just a dense immovable coating around the earth, stars would appear slightly off their true location due to the refraction of the atmosphere. (For more on this see my June 2002 article on the superior mirage.) The atmosphere, however, is in fairly constant motion and becomes increasingly dense as one moves closer to the surface, though not uniformly so. Light rays bend differently when they pass through cold air and hot air regions, and always bend toward the colder air. This is because cold air is more dense than warm air (assuming all other
The copyright of the article Twinkle, Twinkle: September Skies and Starlight in Meteorology is owned by . Permission to republish Twinkle, Twinkle: September Skies and Starlight in print or online must be granted by the author in writing.
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