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The Bortle dark-sky scale (usually referred to as simply the Bortle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution .
Reaching the requirements of the two chosen, overlapping images is simple. The principal points (central point of the image in geometry) of the two photos must be in different locations on the terrain. [2] Another restriction is that the scale of the images must be the same. [2] The flying routes of the planes and the time of day are not ...
minimum brightness [42] +1.33: star Alpha Centauri B: seen from Earth +1.86: planet Mars: seen from Earth minimum brightness [42] +1.98: star Polaris: seen from Earth mean brightness [49] +3.03: supernova SN 1987A: seen from Earth in the Large Magellanic Cloud (160,000 light-years away) +3 to +4: Faintest stars visible in an urban neighborhood ...
It includes all stars brighter than magnitude +2.50 in visible light, measured using a V-band filter in the UBV photometric system. Stars in binary systems (or other multiples) are listed by their total or combined brightness if they appear as a single star to the naked eye, or listed separately if they do not.
So far, 131 such objects have been found. Only 22 are bright enough to be visible without a telescope, for which the star's visible light needs to reach or exceed the dimmest brightness visible to the naked eye from Earth, 6.5 apparent magnitude. [1] The known 131 objects are bound in 94 stellar systems.
This is the modern magnitude system, which measures the brightness, not the apparent size, of stars. Using this logarithmic scale, it is possible for a star to be brighter than “first class”, so Arcturus or Vega are magnitude 0, and Sirius is magnitude −1.46. [citation needed]
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(The S 10 unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec −2.) The total sky brightness in zenith is therefore ~220 S 10 or 21.9 mag/arcsec² in the V-band. Note that the contributions from Airglow and Zodiacal light vary with the time of year ...