Estimating Limiting Magnitude

Limiting magnitude is used to evaluate the quality of observing conditions. It tells the magnitude of the faintest star visible to the unaided eye. The limiting magnitude could also be observed by some instrument. It describes well the sky's transparency: better transparency means that fainter stars are visible.

Limiting magnitude is used eg. in meteor and deep sky observations. It can be used also to approximate light pollution.

The simplest way to evaluate limiting magnitude is to find suitable stars with known magnitudes from star map and check which of them are visible. A more clever way is to count visible stars inside known star squares and triangles. This method was originally invented by meteor observers.

The Method

  1. Wait for your eyes to dark adapt (at least 30 minutes).
  2. Choose one of the estimating areas from the image map above (or the larger version here) or one of the links below; the resulting page has a larger sky chart and a limiting magnitude table.
  3. Count the total number of stars you see in the area including the corners.
  4. Look up that number in the table and read off your limiting magnitude.
Notes:

The areas used in limiting magnitude estimation:

In observations the area should be choosen so that it is either near the view direction or the zenith, depending on observations and the situation. Meteor observers use the viewing direction. Deep sky observers (in Finland) use the area in 45 degrees altitude. 
Area   Corner stars                        Constellation
  1    Chi-Zeta-Delta-Xi Dra               Draco
  2    Beta-Delta-Zeta Per                 Perseus
  3    23-Theta-Beta UMa                   Ursa Major
  4    Alpha-Epsilon-Beta Gem              Gemini
  5    Zeta-Gamma-Delta Aql                Aquila
  6    Alpha And--Gamma-Alpha Peg          Pegasus-Andromeda
  7    Alpha-Beta-Delta Cep                Cepheus
  8    Alpha-Beta-Zeta Tau                 Taurus
  9    Alpha-Beta-Gamma-Delta Leo          Leo
 10    Alpha-Zeta-Gamma Vir                Virgo
 11    Alpha CrB--Gamma-Alpha Boo          Corona Borealis-Bootes
 12    Alpha Ser--Beta Lib--Delta Oph      Serpens-Libra-Ophiuchus
 13    Beta-Zeta Lyr--Theta-Nu Her         Lyra-Hercules
 14    Epsilon-Eta-Gamma Cyg               Cygnus
 15    Beta Dra--Tau-Pi Her                Draco-Hercules
 16    Alpha CVn--Epsilon-Eta UMa          Canes Venatici-Ursa Major
 17    Epsilon-Theta-Delta Aur             Auriga
 18    Mu-Gamma-Phi And                    Andromeda
 19    Kappa-Alpha Dra--Beta UMi           Draco-Ursa Minor
 20    42-Beta-Gamma Cam                   Camelopardalis
 21    Alpha PsA--98-Delta Aqr             Pisces Austrinus-Aquarius
 22    Beta Lep--Beta Ori--53 Eri          Lepus-Orion-Eridanus
 23    Delta-Gamma-Epsilon-Beta Crv        Corvus
 24    Beta-Gamma-Sigma-Alpha Lib          Libra
 25    Alpha-Epsilon Sco--Chi Lup          Scorpius-Lupus
 26    Gamma-Alpha TrA--Eta Ara--Alpha Cen Triangulum Astrinus-Ara-Centaurus
 27    Beta Cen-Alpha-Gamma Cru            Centaurus-Crux
 28    Beta-Epsilon-Iota Car               Carina
 29    Gamma-Alpha-Beta Hyd                Hydrus
 30    Alpha Tuc--Alpha-Epsilon Pav        Tucana-Pavo
European users may get faster access to the original version of this page which is part of the Finnish Amateur Astronomy Homepage.

Another version of this information (with nice black-on-white star charts) is available at the IMO Major-Shower Observations page.

This page was created by:
Veikko Makela Veikko.Makela@Helsinki.fi with additions from Rainer Arlt and major modifications by Bill Arnett