What is the baseline for determining the magnitude scale of celestial objects? Why do brighter objects have negative numbers?

The first observer to catalog differences in star brightnesses was Greek astronomer Hipparchus. He created a catalog around 135 b.c. of roughly 850 stars divided into six ranges. He called the brightest 1st magnitude and the faintest 6th magnitude. Observers noted that 1st-magnitude stars varied greatly in brightness. Also, to assign a magnitude to the brightest planets, the Moon, and especially the Sun, scientists would have to work with negative numbers.