Magnitude (astronomy)

The apparent magnitude () of a celestial object is a number that measures its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value (i.e. inverse relation). The Sun, at apparent magnitude of −27, is the brightest object in the sky.
Magnitude is a logarithmic measure. It is measured in a specific wavelength or passband, usually in optical or near-infrared wavelengths. A full moon is −13 magnitude and the brightest planet Venus measures −5. The brightest man-made objects, Iridium flares, are ranked at −9 and the International Space Station measures −6.
History.
The Greek astronomer Hipparchus first invented our system of apparent magnitude. He gave the brightest stars a magnitude of 1 and increased the numbers for dimmer stars. About 300 years later Ptolemy of Alexandria created an encyclopedia of stars based on Hipparchus' work. The Persian astronomer Al-Sufi took Ptolemy's work and gave Arabic names to the stars some 800 years later. Medieval European astronomers simply translated Al-Sufi's work into Latin which is why many stars today have Arabic names.