The statistic is based on counts of visible stars begun at the turn of the 20th century, when telescopes were less powerful. (scientificamerican.com)
Gosta Lynga began a catalog of all optically visible star clusters in 1977 («Plans for a Computer - Readable Cluster Catalogue») and it has since been continued by others, most recently in the «New catalogue of optically visible open clusters and candidates» by Dias, Alessi, Moitinho, and Lépine. (galaxymap.org)
Choose your observing site carefully to avoid urban light pollution, which can reduce the number of visible stars by 95 percent, says Kelly Beatty, senior contributing editor to Sky & Telescope magazine. (discovermagazine.com)