Qu'est-ce (qui) est magnitude$46256$ - définition
![Typical seismogram. The compressive [[P-waves]] (following the red lines) – essentially sound passing through rock – are the fastest seismic waves, and arrive first, typically in about 10 seconds for an earthquake around 50 km away. The sideways-shaking [[S-waves]] (following the green lines) arrive some seconds later, traveling a little over half the speed of the P-waves; the delay is a direct indication of the distance to the quake. S-waves may take an hour to reach a point 1000 km away. Both of these are ''body-waves'', that pass directly through the earth's crust. Following the S-waves are various kinds of ''surface-waves'' – [[Love wave]]s and [[Rayleigh wave]]s – that travel only at the earth's surface. Surface waves are smaller for deep earthquakes, which have less interaction with the surface. For shallow earthquakes – less than roughly 60 km deep – the surface waves are stronger, and may last several minutes; these carry most of the energy of the quake, and cause the most severe damage.](https://commons.wikimedia.org/wiki/Special:FilePath/1906 San Francisco earthquake seismograph.png?width=200 "Typical seismogram. The compressive [[P-waves]] (following the red lines) – essentially sound passing through rock – are the fastest seismic waves, and arrive first, typically in about 10 seconds for an earthquake around 50 km away. The sideways-shaking [[S-waves]] (following the green lines) arrive some seconds later, traveling a little over half the speed of the P-waves; the delay is a direct indication of the distance to the quake. S-waves may take an hour to reach a point 1000 km away. Both of these are ''body-waves'', that pass directly through the earth's crust. Following the S-waves are various kinds of ''surface-waves'' – [[Love wave]]s and [[Rayleigh wave]]s – that travel only at the earth's surface. Surface waves are smaller for deep earthquakes, which have less interaction with the surface. For shallow earthquakes – less than roughly 60 km deep – the surface waves are stronger, and may last several minutes; these carry most of the energy of the quake, and cause the most severe damage.")
![Isoseismal map for the [[1968 Illinois earthquake]]. The irregular distribution of shaking arises from variations of geology and/or ground conditions.](https://commons.wikimedia.org/wiki/Special:FilePath/1968 Illinois earthquake.svg?width=200 "Isoseismal map for the [[1968 Illinois earthquake]]. The irregular distribution of shaking arises from variations of geology and/or ground conditions.")
Wikipédia
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
The scale is logarithmic and defined such that a magnitude 1 star is exactly 100 times brighter than a magnitude 6 star. Thus each step of one magnitude is times brighter than the magnitude 1 higher. The brighter an object appears, the lower the value of its magnitude, with the brightest objects reaching negative values.
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, its distance, and the extinction reducing its brightness. 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.
The Sun has an apparent magnitude of −27 and Sirius, the brightest visible star in the night sky, −1.46. Venus at its brightest is -5. The International Space Station (ISS) sometimes reaches a magnitude of −6.
Amateur astronomers commonly express the darkness of the sky in terms of limiting magnitude, i.e. the apparent magnitude of the faintest star they can see with the naked eye. At a dark site it usual for people to see stars of 6th magnitude or fainter.
Apparent magnitude is really a measure of illuminance, which can also be measured in photometric units such as lux.
