Astronomical brightness measured on a logarithmic scale, based on the ancient practice of noting that the brightest stars in the sky were of "first importance" or "first magnitude", the next brightest being "second magnitude" etc. The human eye is a logarithmic detector, and in 1854 Pogson formalized this scale and defined a difference of 5 magnitudes to be exactly a factor of 100 in brightness. The scale is calibrated to the bright star Vega which is defined to have a magnitude of 0. For reference, the faintest naked-eye magnitude visible from a dark site is +6. Comet C/1996 B2 Hyakutake reached magnitude 0 in late March 1996.