Scientific Background for Teachers

When selecting a location to build a telescope, the most important requirement is visibility. Great sites have more than 300 clear nights every year. The next requirement is that the site be accessible. It isn't easy to get to a remote location, and some places are more remote than others. However, it is ill advised to build a telescope within one hundred miles of a large city as there will be too many lights (known as light pollution). 

Probably the most complicated issue is that of "seeing." When stars twinkle, it is because a very slender beam of starlight gets disrupted by the Earth's atmosphere--the scientific term for twinkling is scintillation. The less stars twinkle, the better and more stable the long exposure photographs of dim stars will be. When there is very little turbulence in the atmosphere and the stars don't twinkle, then astronomers say that the "seeing" is excellent. 

Earth at Night Poster available from ASP at