Editing Apparent magnitude (section)

== History ==
{{More citations needed section|talk=talk:Apparent magnitude#Main Article Problems|date=May 2019}}
{|class="wikitable" style="float: center; margin-left: 1em; text-align: center;"
! Visible to<br />typical<br />human<br />eye<ref name="SIMBAD-mag6.5"/>
! Apparent<br />magnitude
! Bright-<br />ness<br />relative<br />to [[Vega]]
! Number of stars <br /> (other than the [[Sun]]) <br />brighter than<br />apparent magnitude<ref>{{cite web | url = http://www.nso.edu/PR/answerbook/magnitude.html | archive-url = https://web.archive.org/web/20080206074842/http://www.nso.edu/PR/answerbook/magnitude.html | archive-date = 6 February 2008 | title = Magnitude | publisher = National Solar Observatory—Sacramento Peak | access-date = 23 August 2006}}</ref><br />in the night sky
|-
| rowspan="9" | Yes
|−1.0 || 251% ||1 ([[Sirius]])
|-
|{{0|0}}0.0 || 100% ||5
<small>([[Vega]], [[Canopus]], [[Alpha Centauri]],</small>
<small>[[Arcturus]])</small>
|-
|{{0|0}}1.0 || 40% ||15
|-
|{{0|0}}2.0 || 16% ||48
|-
|{{0|0}}3.0 || 6.3% ||171
|-
|{{0|0}}4.0 || 2.5% ||513
|-
|{{0|0}}5.0 || 1.0% ||{{val|1602}}
|-
|{{0|0}}6.0 || 0.4% ||{{val|4800}}
|-
|{{0|0}}6.5 || 0.25% ||{{val|9100}}<ref>[[Bright Star Catalogue]]</ref>
|-
| rowspan="4" | No
|{{0|0}}7.0 || 0.16% ||{{val|14000}}
|-
|{{0|0}}8.0 || 0.063% ||{{val|42000}}
|-
|{{0|0}}9.0 || 0.025% ||{{val|121000}}
|-
|10.0 || 0.010% ||{{val|340000}}
|}

The scale used to indicate magnitude originates in the [[Hellenistic Greece|Hellenistic]] practice of dividing stars visible to the naked eye into six ''magnitudes''. The [[List of brightest stars|brightest stars]] in the night sky were said to be of [[first magnitude star|first magnitude]] ({{mvar|m}} = 1), whereas the faintest were of sixth magnitude ({{mvar|m}} = 6), which is the limit of [[human]] [[visual perception]] (without the aid of a [[telescope]]). Each grade of magnitude was considered twice the brightness of the following grade (a [[logarithmic scale]]), although that ratio was subjective as no [[photodetector]]s existed. This rather crude scale for the brightness of stars was popularized by [[Ptolemy]] in his ''[[Almagest]]'' and is generally believed to have originated with [[Hipparchus]]. This cannot be proved or disproved because Hipparchus's original star catalogue is lost. The only preserved text by Hipparchus himself (a commentary to Aratus) clearly documents that he did not have a system to describe brightness with numbers: He always uses terms like "big" or "small", "bright" or "faint" or even descriptions such as "visible at full moon".<ref>Hoffmann, S., Hipparchs Himmelsglobus, Springer, Wiesbaden/ New York, 2017</ref>

In 1856, [[Norman Robert Pogson]] formalized the system by defining a first magnitude star as a star that is 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today. This implies that a star of magnitude {{mvar|m}} is about 2.512 times as bright as a star of magnitude {{math|''m'' + 1}}. This figure, the [[Generalized continued fraction#Example 2|fifth root of 100]], became known as {{vanchor|Pogson's Ratio}}.<ref>{{cite journal |title=Magnitudes of Thirty-six of the Minor Planets for the first day of each month of the year 1857 |author-link=Norman Robert Pogson |first=N. |last=Pogson |journal=[[Monthly Notices of the Royal Astronomical Society|MNRAS]] |volume=17 |page=12 |date=1856 |bibcode=1856MNRAS..17...12P |doi=10.1093/mnras/17.1.12 |doi-access=free }}</ref> The ''1884 Harvard Photometry'' and 1886 ''Potsdamer Durchmusterung'' star catalogs popularized Pogson's ratio, and eventually it became a de facto standard in modern astronomy to describe differences in brightness.<ref>{{Cite book |last=Hearnshaw |first=John B. |title=The measurement of starlight: two centuries of astronomical photometry |date=1996 |publisher=Cambridge Univ. Press |isbn=978-0-521-40393-1 |edition=1. publ |location=Cambridge}}</ref>

Defining and calibrating what magnitude 0.0 means is difficult, and different types of measurements which detect different kinds of light (possibly by using filters) have different zero points. Pogson's original 1856 paper defined magnitude 6.0 to be the faintest star the unaided eye can see,<ref>{{Cite journal |last=Pogson |first=N. |date=1856-11-14 |title=Magnitudes of Thirty-six of the Minor Planets for the First Day of each Month of the Year 1857 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=17 |issue=1 |pages=12–15 |doi=10.1093/mnras/17.1.12 |doi-access=free |bibcode=1856MNRAS..17...12P |issn=0035-8711}}</ref> but the true limit for faintest possible visible star varies depending on the atmosphere and how high a star is in the sky. The ''Harvard Photometry'' used an average of 100 stars close to Polaris to define magnitude 5.0.<ref>{{Cite book |last=Hearnshaw |first=J. B. |title=The measurement of starlight: two centuries of astronomical photometry |date=1996 |publisher=Cambridge University Press |isbn=978-0-521-40393-1 |location=Cambridge [England] ; New York, NY, USA}}</ref> Later, the Johnson UVB photometric system defined multiple types of photometric measurements with different filters, where magnitude 0.0 for each filter is defined to be the average of six stars with the same spectral type as Vega. This was done so the [[color index]] of these stars would be 0.<ref>{{Cite journal |last1=Johnson |first1=H. L. |last2=Morgan |first2=W. W. |date=May 1953 |title=Fundamental stellar photometry for standards of spectral type on the revised system of the Yerkes spectral atlas |url=http://adsabs.harvard.edu/doi/10.1086/145697 |journal=The Astrophysical Journal |language=en |volume=117 |pages=313 |doi=10.1086/145697 |bibcode=1953ApJ...117..313J |issn=0004-637X}}</ref> Although this system is often called "Vega normalized", Vega is slightly dimmer than the six-star average used to define magnitude 0.0, meaning Vega's magnitude is normalized to 0.03 by definition.

{|class="wikitable" style="float: right; margin-left: 0.5em; text-align: center;"
|+ Limiting Magnitudes for Visual Observation at High Magnification<ref>{{cite book
 | title=Observing Variable Stars, Novae and Supernovae
 | first1=Gerald | last1=North | first2=Nick | last2=James
 | publisher=Cambridge University Press
 | date=2014 | isbn=978-1-107-63612-5 | page=24
 | url=https://books.google.com/books?id=IzoDBAAAQBAJ&pg=PA24 }}</ref>
|-
!Telescope<br />aperture<br />(mm)
!Limiting<br />Magnitude
|-
| 35
| 11.3
|-
| 60
| 12.3
|-
| 102
| 13.3
|-
| 152
| 14.1
|-
| 203
| 14.7
|-
| 305
| 15.4
|-
| 406
| 15.7
|-
| 508
| 16.4
|}
With the modern magnitude systems, brightness is described using Pogson's ratio. In practice, magnitude numbers rarely go above 30 before stars become too faint to detect. While Vega is close to magnitude 0, there are four brighter stars in the night sky at visible wavelengths (and more at infrared wavelengths) as well as the bright planets Venus, Mars, and Jupiter, and since brighter means smaller magnitude, these must be described by ''negative'' magnitudes. For example, [[Sirius]], the brightest star of the [[celestial sphere]], has a magnitude of −1.4 in the visible. Negative magnitudes for other very bright astronomical objects can be found in the [[#List of apparent magnitudes|table]] below.

Astronomers have developed other photometric zero point systems as alternatives to Vega normalized systems. The most widely used is the [[AB magnitude]] system,<ref>{{cite journal|last1=Oke|first1=J. B.|last2=Gunn|first2=J. E.|title=Secondary standard stars for absolute spectrophotometry|journal=The Astrophysical Journal |date=15 March 1983 |volume=266|pages=713–717|doi=10.1086/160817|bibcode=1983ApJ...266..713O}}</ref> in which photometric zero points are based on a hypothetical reference spectrum having constant [[Spectral flux density|flux per unit frequency interval]], rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zero point is defined such that an object's AB and Vega-based magnitudes will be approximately equal in the V filter band. However, the AB magnitude system is defined assuming an idealized detector measuring only one wavelength of light, while real detectors accept energy from a range of wavelengths.