Magnitude
Also called: apparent magnitude, mag, brightness.
The brightness of a sky object on a reversed scale: smaller numbers are brighter, and each step is 2.5× fainter.
The scale goes back to ancient Greece, where the brightest stars were “first magnitude” and the faintest “sixth”. Today it's exact: 5 magnitudes are a factor of 100, so each magnitude is 2.512× fainter. Very bright objects go negative — Sirius is −1.5, Venus up to −4.7, the full Moon −12.7.
Under a dark sky the eye reaches about magnitude 6.5; binoculars about 9.5; an 8-inch telescope about 14. For extended objects, the total magnitude can mislead — see surface brightness.
In AstroPilot
Magnitudes appear on every object page and in the sky chart, whose star sizes follow them.