Astronomical seeing is the blurring and twinkling of celestial objects, such as stars and planets, caused by turbulence in the Earth's atmosphere.
The worse the seeing conditions, the poorer the quality of the image being observed by a telescope. It's a bit like looking into a clear sea on a windy day. The more disturbed the sea is, the poorer the image of objects beneath the surface. Good seeing would be the equivalent of a flat calm sea.
Although a clear sky may appear calm, there are various updrafts and turbulent layers that can still mess about with the light passing through. The result can be rapid changes in the quality of images, as shown right.
All ground-based observatories are affected by seeing, but the effect can be reduced by putting observatories on the top of high mountains, so that telescopes are looking through less atmosphere. Astronomers have also developed adaptive optics technology to try and correct for seeing effects, but the best option is to launch your telescope into orbit above the Earth's atmosphere.
If you fancy learning more about astronomical seeing, and seeing the effects for yourself, why not try out our Seeing Workshop.