Seeing or "astronomical seeing" is used in reference to the effects of atmospheric turbulence on astronomical observations. The most common example of this can be seen when viewing a bright a star near the horizon. The star will seem to flash varying colors. Another common example is the blurring effect that occurs when viewing a planet through a telescope.

Light traveling in initially unaffected wavefronts travels through layers of the atmosphere and encounters turbulence, at which point, the wavefronts become distorted due to changes in the refractive index. This effect is more prominent near an observers horizon, because the light must travel through more atmosphere to arrive at the observer. Seeing is usually referred to as being good or bad. To get the best possible seeing, astronomers try to observe objects near Zenith.

All Earth-based telescopes are affected, in some way, by these distortions. Most observatories utilize high mountaintops to get above much of the low atmospheric turbulence. With the invention of adaptive optics, many observatories have successfully been able to correct for a significant amount of the distortions of incoming wavefronts.

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