Seeing
Astronomy Atmospheric seeing Astronomical seeing Image steadiness
Seeing describes astronomical image disturbance caused by Earth’s atmosphere. Good seeing makes stars and planetary detail appear steadier and sharper; it is not the same as clear visibility.
Why stars shimmer
Air layers with slightly different temperatures and densities bend light by different amounts. Moving layers and eddies continually change its path. Through a telescope, a star image jumps or spreads into a small blur. The star itself is usually holding perfectly still.
Small arcseconds are good here
Seeing is often expressed as the size of an atmosphere-blurred star image in arcseconds. One arcsecond is 1/3,600 of a degree. A value of 1″ indicates better conditions than 3″ when wavelength, viewing direction and measurement method match.
Do not confuse it with transparency
- Seeing: How much does the air blur the image?
- Transparency: How much light passes through?
A clear night can shimmer strongly, while slightly hazy air may be steady. Image stability is especially important for fine lunar and planetary detail.
The observing site also matters
Warm roofs, asphalt or a telescope that has not cooled to ambient temperature can create local turbulence. A position over grass may help, but cannot remove turbulence higher up. A seeing forecast may also be a model estimate rather than a direct measurement.