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Apparent magnitude
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{{Short description|Brightness of a celestial object observed from the Earth}} {{For|a more detailed discussion of the history of the magnitude system|Magnitude (astronomy)}} {{Use dmy dates|date=October 2023}} [[File:65Cyb-LB3-apmag.jpg|thumb|upright=1.4|Asteroid [[65 Cybele]] and two stars in the constellation [[Aquarius (constellation)|Aquarius]], with their magnitudes labeled]] '''Apparent magnitude''' ('''{{mvar|m}}''') is a measure of the [[Irradiance|brightness]] of a [[star]], [[astronomical object]] or other celestial objects like [[artificial satellite]]s. Its value depends on its intrinsic [[luminosity]], its distance, and any [[extinction (astronomy)|extinction]] of the object's light caused by [[interstellar dust]] along the [[sightline|line of sight]] to the observer. Unless stated otherwise, the word ''magnitude'' in astronomy usually refers to a celestial object's apparent magnitude. The magnitude scale likely dates to before the ancient [[Ancient Greek astronomy#Astronomy in the Greco-Roman and Late Antique eras|Roman astronomer]] [[Ptolemy|Claudius Ptolemy]], whose [[Star catalogue|star catalog]] popularized the system by listing stars from [[First-magnitude star|1st magnitude]] (brightest) to 6th magnitude (dimmest).<ref>{{cite book |last1=Toomer |first1=G. J. |title=Ptolemy's Almagest |date=1984 |publisher=Springer-Verlag |location=New York |isbn=0-387-91220-7 |page=16 |url=https://archive.org/details/ptolemysalmagest0000ptol/page/16/mode/2up}}</ref> The modern scale was mathematically defined to closely match this historical system by [[Norman Robert Pogson|Norman Pogson]] in 1856. The scale is reverse [[logarithmic scale|logarithmic]]: the brighter an object is, the lower its [[magnitude (astronomy)|magnitude]] number. A difference of 1.0 in magnitude corresponds to the brightness ratio of <math>\sqrt[5]{100}</math>, or about 2.512. For example, a magnitude 2.0 star is 2.512 times as bright as a magnitude 3.0 star, 6.31 times as magnitude 4.0, and 100 times magnitude 7.0. The brightest astronomical objects have negative apparent magnitudes: for example, [[Venus]] at −4.2 or [[Sirius]] at −1.46. The faintest stars visible with the [[naked eye]] on the darkest night have apparent magnitudes of about +6.5, though this varies depending on a person's [[Visual acuity|eyesight]] and with [[Horizontal coordinate system|altitude]] and atmospheric conditions.<ref>{{cite journal |last=Curtis |first=Heber Doust |year=1903 |orig-date=1901-03-27 |title=On the Limits of Unaided Vision |journal=[[Lick Observatory|Lick Observatory Bulletin]] |publisher=[[University of California]] |volume=2 |pages=67–69 |bibcode=1903LicOB...2...67C |doi=10.5479/ADS/bib/1903LicOB.2.67C |doi-access= |number=38}}</ref> The apparent magnitudes of known objects range from the Sun at −26.832 to objects in deep [[Hubble Space Telescope]] images of magnitude +31.5.<ref name="AAVSO">{{cite web |title=Magnitudes: Measuring the Brightness of Stars |first=Templeton |last=Matthew |date=21 October 2011 |publisher=American Association of Variable Stars (AAVSO) |url=https://www.aavso.org/magnitude |access-date=19 May 2019 |archive-url=https://web.archive.org/web/20190518084414/https://www.aavso.org/magnitude |archive-date=18 May 2019 |url-status=live }}</ref> The measurement of apparent magnitude is called [[Photometry (astronomy)|photometry]]. Photometric measurements are made in the [[ultraviolet]], [[Visible spectrum|visible]], or [[infrared]] [[Electromagnetic spectrum#Regions|wavelength bands]] using standard [[passband]] filters belonging to [[photometric system]]s such as the [[UBV photometric system|UBV system]] or the [[Strömgren photometric system|Strömgren ''uvbyβ'' system]]. Measurement in the V-band may be referred to as the '''apparent visual magnitude'''. [[Absolute magnitude]] is a related quantity which measures the [[luminosity]] that a celestial object emits, rather than its apparent brightness when observed, and is expressed on the same reverse logarithmic scale. Absolute magnitude is defined as the apparent magnitude that a star or object would have if it were observed from a distance of {{convert|10|parsec|ly km mi|abbr=off|lk=in}}. Therefore, it is of greater use in [[stellar astrophysics]] since it refers to a property of a star regardless of how close it is to Earth. But in [[observational astronomy]] and popular [[stargazing]], references to "magnitude" are understood to mean apparent magnitude. [[Amateur astronomy|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. This can be useful as a way of monitoring the spread of [[light pollution]]. Apparent magnitude is technically a measure of [[illuminance]], which can also be measured in photometric units such as [[lux]].<ref>{{cite journal |last1=Crumey |first1=A.|title=Human Contrast Threshold and Astronomical Visibility.|journal=Monthly Notices of the Royal Astronomical Society|date=October 2006|volume=442|issue=3|pages=2600–2619 |doi=10.1093/mnras/stu992 |doi-access=free|arxiv=1405.4209|bibcode=2014MNRAS.442.2600C }}</ref>
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