Glance at the sky on a clear night, and it looks as though you can see millions of stars. In fact, your eye shows you no more than few thousand stars in all. By taking photographs with large telescopes, howeer, astronomers can now literally count stars by the million. In the 1860s, the Prussian astronomer Friedrich Argelander measured the positions of all the stars he could se from Bonn, with a telescope that had a lens 3in (75mm) across. The final catalogue he produced contained some 458,000 stars. Astronomers in Argentina later measured stars too far south to be seen from Germany, and extended the total by almost three times as much, to 1,072,000 stars.
About 9000 stars in the sky are visible to the naked eye. At any one time and place only half the sky can be seen, and haze reduces the number of stars that are seen near the horizon, so a keen sighted person can see no more than 3000 stars even on a very dark, clear night.
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