Seeing Conditions

Mike Swanson

Other than the obvious difference between a clear and a cloudy night, most folks don't realize how variable our view of the night sky really is. Some nights you can make out great detail on Jupiter, while other nights you are lucky to see two bands. Other nights you can see a faint globular cluster quite clearly, other nights it might be invisible. Various factors affect our seeing conditions, but three are most critical: seeing, transparency and light pollution.

Seeing is mainly our observation of the distortion caused by different layers of air going different directions due to wind, the jet stream, temperature differences, etc. It causes images to waver. I tend to visualize this as the hot air wavering above a blacktop road in the summer. In the night sky, it causes the stars to twinkle. In the eyepiece you can easily see the wavering air when you pump up the magnification on a planet or the moon but it affects all objects viewed, especially in larger aperture telescopes.

Transparency is our observation of the distortion caused by particles in the air. Clouds are obviously the extreme, but dust, smog, moisture, etc., are other particles that limit the transparency of the air. This is the same factor that affects what pilots call visibility ('today visibility is 10 miles'). When observing from a dark site, transparency causes objects to appear less bright. When observing from a site with any appreciable light pollution it is many times worse because the particles reflect light back down to the ground. This results in a 'glowing' sky and makes faint objects very difficult or impossible to see as the effect on contrast (difference in light levels) is to make the black velvety sky behind the object not black, but potentially gray. This factor, unlike seeing, affects small telescopes more than those with larger apertures. Contrast is critical for faint objects and nice to have on bright objects.

Light pollution is generally considered to be any man-made light that is directed upward. Houses, cars, storefronts and street lights are all prime sources of light pollution. Any level of light pollution will decrease the contrast between celestial objects and the hopefully black sky. Naturally the problem is worse around cities, but even small towns have appreciable levels of light pollution. The truth is, there are very few populated places on Earth that still offer truly dark skies. Growing up in rural Indiana, on very dark and clear nights I could actually see the the 'light dome' above Chicago from more than 80 miles away! Some communities have adopted local regulations to cut down on light pollution. In fact Connecticut has passed a law requiring full-cutoff lights - those that do not send light upward - and Colorado passed a similar law. Besides spoiling the night sky, all light that goes upward is simply wasted energy. To learn more, visit the International Dark-Sky Association's web site:

Copyright 2002-2006,
Ryukyu Astronomy Club
Contact the Webmaster: