08/01/1995 07:00:00

Caltech Question of the Month: Visible Stars

Question: How many stars can a person see at night with the naked eye?

Answer: Under ideal conditions, about 3,000 stars should be visible at night with the unaided eye, but many factors can reduce this number.

In the best of circumstances, about 6,000 stars are bright enough to be visible to the naked eye. But of course, only half of these are visible at any one time; no one can see the stars below the horizon.

A full moon reduces the number of stars that are visible. The sunlight reflected from the moon's surface brightens the entire sky and obscures the fainter stars. A similar problem is created by the artificial lights of towns and cities. That's why the telescopes on Mount Wilson aren't able to see as well as they might; city lights brighten the sky and mask the faintest stars.

Certain atmospheric conditions can also make stars more difficult to see. Smoke from fires, dust kicked up by wind, and high humidity all make stars harder to see. And of course clouds block them out altogether.

So for the best viewing, go out on a dry, clear, calm, moonless night far from the city, and you should be able to see nearly all the 3,000 visible stars.

This is a monthly feature produced by the Media Relations Office at the California Institute of Technology, in collaboration with Caltech's faculty, to answer commonly asked or particularly intriguing questions about science and the natural world.

The Media Relations Office will accept any questions you may have.