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Without an atmosphere, the seeing disk would be determined by diffraction and would be inversely proportional to the diameter of the telescope. However when light enters the Earth's atmosphere, the different temperature layers and different wind speeds distort and move the image in various ways. The effects of the atmosphere can be modelled as cells of still air about 10 cm in diameter, and this limits the resolution of the telescope.

The distortion changes at a high rate, typically more frequently than 100 times a second. In a typical astronomical image with an exposure time of seconds or even minutes, the different distortions average out as a filled circle. The diameter of this circle is measured as the seeing disk.

It follows from this definition that seeing is always a variable quantity, different from place to place, from night to night and even variable on a scale of minutes. Astronomers often talk about "good" nights with a low average seeing, and "bad" nights where the seeing was so high that all observations were worthless.

Seeing is usually measured in arcseconds, abbreviated with the symbol ("). A 1.0" seeing is a good one for average astronomical sites. The seeing of an urban environment is usually much worse. Good seeing nights tend to be clear, cold nights with no wind. Warm air rises degrading the seeing as does wind and clouds.

Seeing is one of the biggest problem for Earth-based astronomy: while the big telescopes have theoretically milli-arcsecond resolution, the real image will never be better than the average seeing disk during the observation. This can easily mean a factor of 100 between the potential and practical resolution.

The effects of atmospheric seeing were indirectly responsible for the belief that there were canals on MarsFor a time in the late 19th and early 20th centuries, it was believed that there were canals on Mars . These were long straight lines that appeared in drawings of the planet Mars, first observed in 1877. The Italian astronomer Giovanni Schiaparelli called. In viewing a bright object such as Mars, occasionally a still patch of air will come in front of the planet, resulting in a brief moment of clarity. Before the use of CCDs, there was no way of recording the image of the planet in the brief moment other than having the observer remember the image and draw it later. This had the effect of having the image of the planet be dependent on the observer's memory and preconceptions which led the belief that Mars had linear features.

The first answer to this problem was NASAThe National Aeronautics and Space Administration NASA (established 1958) is the government agency responsible for the United States of America's space program and long-term general aerospace research. A civilian organization, it conducts (or oversees) re's Hubble Space TelescopeHubble Space Telescope Space Shuttle Discovery during the second servicing mission of the telescope, STS-82 Organization NASA, ESA Wavelength regimeoptical Orbit height600 km Orbit period100 min Launch date 24 April 1990 Deorbit datecirca 2010 Mass11,000: working outside the atmosphere, this telescope does not have any seeing problems, and always works at maximum resolution power. The difference is so great that, even being quite small (only a 2.4 meters mirror), it surpassed all Earth-based telescopes for many years, and it is still widely regarded as the best currently available telescope.

Starting in the 1990s, many telescopes have begun to develop adaptive opticsAdaptive optics is a technology to improve the performance of reflecting telescopes by reducing the effects of twinkling. Adaptive optics work by rapidly " adapting" and reshaping telescopic mirrors. The technique was only practical starting in the 1990s systems that partially solve the seeing problem, but none of the systems so far built or designed completely removes the atmosphere effect.