The Atmosphere
Though reasonably transparent to visible light, the atmosphere may absorb as much as 60% of the visible and near-visible light. It is opaque to most other wavelengths, except certain fairly short radio waves. In addition to absorbing much light, our atmosphere bends light rays entering at a slant (for a given observer) so that the true position of a star close to the horizon is not what it seems to be. One effect is that we see the Sun above the horizon before it actually is. And the unsteady movement of the atmosphere causes the “twinkling” of the stars, which may be romantic, but is a nuisance when it comes to observing.
The composition of our atmosphere near the ground is 78% nitrogen and 21% oxygen, the remaining 1% consisting of other gases, most of it argon. The composition stays the same to an altitude of at least 70 mi (112 km) (except that higher up two impurities, carbon dioxide and water vapor, are missing), but the pressure drops very fast. At 18,000 ft, half of the total mass of the atmosphere is below, and at 100,000 ft, 99% of the mass of the atmosphere is below. The upper limit of the atmosphere is usually given as 120 mi (192 km); no definitive figure is possible, since there is no boundary line between the incredibly attenuated gases 120 mi (192 km) up and space.
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