Seeing

Seeing describes how steady the atmosphere is on a given night, and it is one of the two atmospheric factors, along with transparency, that determine how good a night is for observing. Turbulent layers of air at different temperatures bend light unevenly as it passes through, which makes stars twinkle and planetary detail shimmer and blur, especially at high magnification.

Seeing is largely independent of light pollution and can vary from hour to hour even at the same location, which is why a night near a city can sometimes offer sharper planetary views than a night in the countryside if the upper atmosphere happens to be calmer. Seeing tends to be better when the jet stream is not directly overhead and when a telescope has had time to cool to outdoor temperature, since warm air rising off the optics or tube also degrades the image.

Because seeing sets a ceiling on usable magnification, pushing a telescope past what the sky allows just produces a bigger, blurrier image rather than more detail. Experienced observers judge seeing by watching how steadily a bright star holds its point-like shape at high power, and they adjust magnification down on poor nights rather than assuming a soft image means something is wrong with the equipment.