نجوم و کیهان‌شناسی

The early astronomers recorded the positions of the stars on the celestial sphere

and their observed brightness. The fi rst known catalogue of stars was made by the

Greek astronomer Hipparchos in about 130–160 BC. The stars in his catalogue

were added to by Ptolomy and published in 150 AD in a famous work called the

Almagest whose catalogue listed 1028 stars. Hipparchos had grouped the stars

visible with the unaided eye into six magnitude groups with the brightest termed

1st magnitude and the faintest, 6th magnitude. When accurate measurements

of stellar brightness were made in the nineteenth century it became apparent

that, on average, the stars of a given magnitude were approximately 2.5 times

brighter than those of the next fainter magnitude and that 1st magnitude stars

were about 100 times brighter than the 6th magnitude stars. (The fact that each

magnitude difference showed the same brightness ratio is indicative of the fact

that the human eye has a logarithmic rather than linear response to light.)

"Introduction to astronomy and cosmology" page 5