magnitude
  • Measure of the brightness of any object in the sky, according to a geometrical scale set up by Hipparchos in about 120BC and refined in the 19th century after the development of photometry. The basis of the magnitude system is a 100-fold difference in brightness between objects separated by five magnitudes. This means that each magnitude is equivalent to a change of just over 2.5-fold in the amount of light received by an observer. The numbers get smaller as the objects get brighter. The original basis was that the brightest star in the night sky would be set at zero magnitude, although in fact Sirius, the brightest star in the sky, has a magnitude of –1.4 and there are two other negative-magnitude stars, Canopus and Arcturus. The Sun has a magnitude of –26.8 and the full Moon –12.6. There are about 2000 stars of naked-eye magnitude, generally set at 6.5, although at many dark sites, fainter stars can be observed. The largest telescopes now in use will reach objects below the 25th magnitude and the Hubble Space Telescope can stretch this to about the 31st magnitude. The magnitude system is a means of assessing the full influx of light from a celestial object. For extended objects, like comets or the Moon, or moving or continuous ones, like meteors, the total amount of light must be added up as if from a single point source. The magnitude system was devised to allow stars to be compared as seen from the surface of the Earth. Since the distances to celestial objects began to be known these apparent magnitudes have been supplemented by absolute magnitudes, the magnitude an object would have at a standard distance of 10 parsecs.
  • A measure of the extent, influence, size, or significance of something.
  • A numerical value, such as an amplitude.
  • A physical property, such as temperature, that can be quantified.
  • A logarithmic measure of the absolute or relative luminance of a celestial body, such as the sun or moon. The smaller the number, the greater the luminance.

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