Atmospheric Ceilings and Skylights

The information—the news—we get from space is censored by the several layers of Earth's atmosphere. In effect, our Earth is surrounded by a ceiling pierced by two skylights. A rather broad range of radio waves readily penetrates our atmosphere, as does a portion of infrared and most visible light, in addition to a small portion of ultraviolet. Astronomers speak of the atmosphere's radio window and optical window, which allow passage of electromagnetic radiation of these types. To the rest of the spectrum—lower-frequency radio waves, some lower-frequency infrared, and, fortunately for us, most of the energetic ultraviolet rays, x-rays, and gamma rays—the atmosphere is opaque, an impenetrable ceiling.

In many ways, the partial opacity of our atmosphere is a very good thing, since it protects us from x-ray and gamma radiation. An atmosphere opaque to these wavelengths, but transparent to visible light and some infrared, is a big reason why life can survive at all on Earth.

For astronomers, however, there is a downside to the selective opacity of the earth's atmosphere. Observations of ultraviolet, x-ray, and gamma ray radiation cannot be made from the surface of the earth, but must be made by means of satellites, which are placed in orbit well above the atmosphere. No wonder that the advent of the space age has led to such an explosion in the amount of information that we have about the universe.

Close Encounter

Close Encounter

Has this ever happened to you? You're on a cross-country car trip, pass through a city, and find an FM radio station you really like, only to lose it within a few miles. On the other hand, many AM stations (with programming that can range from dull to downright frightening) seem to go on forever. The reason? FM waves are at a higher frequency and correspondingly shorter wavelength than AM. The earth's atmosphere is usually transparent to high frequency FM radio waves, but opaque to lower frequency AM. As a result, FM signals do not follow the curvature of the earth, but go off on a tangent and into outer space, whereas AM signals can be reflected downward by the atmosphere and tend to follow the earth's curve for a fair distance. If alien life forms are picking up our radio signals, they're getting mostly FM and television (which broadcasts in the upper end of the FM band), but not AM. They are, therefore, mostly denied the "pleasure" of Rush Limbaugh and Howard Stern.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

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