Which astronomer initially categorized naked-eye stars into six magnitude classes around the second century BC?

Hipparchus

The historical foundation of the magnitude system dates back to the work of the Greek astronomer Hipparchus, who lived around the second century BC. Hipparchus performed an initial cataloging of stars visible to the unaided eye, dividing them into six distinct classes or magnitudes. The brightest stars he could perceive were placed in the first magnitude class, while the faintest stars that remained visible to him under clear conditions were assigned to the sixth magnitude class. This initial system established the essential framework where lower numerical values signified higher observed luminosity. Although modern astronomy has transformed this into a precise, quantitative scale involving specific mathematical ratios, the core structure established by Hipparchus—where smaller numbers mean brighter—remains the defining characteristic of stellar magnitude measurement.