<P> Astronomers use two different definitions of magnitude: apparent magnitude and absolute magnitude . The apparent magnitude (m) is the brightness of an object as it appears in the night sky from Earth . Apparent magnitude depends on an object's intrinsic luminosity and its distance . The absolute magnitude (M) describes the intrinsic luminosity emitted by an object and is defined to be equal to the apparent magnitude that the object would have if it were placed at a certain distance from Earth, 10 parsecs for stars . A more complex definition of absolute magnitude is used for planets and small Solar System bodies, based on its brightness at one astronomical unit from the observer and the Sun . </P> <P> The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values . The Sun has an apparent magnitude of − 27, the Moon − 13 at full phase, the brightest planet Venus up to − 5, and Sirius, the brightest visible star in the night sky, − 1.5 . Apparent magnitudes can also be assigned to artificial objects in Earth orbit . The brightest satellite flares are ranked at − 9, and the International Space Station (ISS) appears at a magnitude of − 6 . </P> <P> The scale is logarithmic and defined such that each step of one magnitude changes the brightness by a factor of the fifth root of 100, or approximately 2.512 . For example, a magnitude 1 star is exactly a hundred times brighter than a magnitude 6 star, as the difference of five magnitude steps corresponds to 2.512, or 100 . </P> <P> The magnitude system dates back roughly 2000 years to the Greek astronomer Hipparchus (or the Alexandrian astronomer Ptolemy--references vary) who classified stars by their apparent brightness, which they saw as size (magnitude means "bigness, size"). To the unaided eye, a more prominent star such as Sirius or Arcturus appears larger than a less prominent star such as Mizar, which in turn appears larger than a truly faint star such as Alcor . In 1736, the mathematician John Keill described the ancient naked - eye magnitude system in this way: </P>

How many times brighter is a first magnitude star than a third magnitude star
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