The steadiness of telescopic images due to conditions in the Earth's atmosphere. Seeing is bad when air currents and temperature differentials cause the image to twinkle or undulate, or appear blurred or distorted - typically when the barometer is low or falling. The seeing is good when the air is still and the image appears sharp and steady - as is the case when there's a high pressure ridge over the observing site. Poor seeing affects the resolution of a telescope, putting an upper limit on the maximum usable magnification on any given night. On most nights, seeing conditions limit the resolution of even large telescopes to no better than five arc seconds or so and bloat small Airy disks into "seeing disks" three or four arc seconds in diameter.

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