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. It also quantifies the sky's transparency: better transparency means that fainter stars are visible. Limiting magnitude is used in meteor and deep-sky observing. 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 a star map and check which of them are visible. The easier way (originally invented by meteor observers) is to count visible stars inside known star squares and triangles. A number of these areas are shown below.

Clicking on a yellow section of the map below, or on the appropriate text link at the bottom, will take to to a page with a printable star chart. These charts also have tables which show how the number of stars you can see correspond to your own personal limiting magnitude.

Draco Perseus Ursa Major Gemini Aquila Pegasus - Andromeda Cepheus Taurus Leo Virgo Corona Borealis - Bootes Serpens - Libra - Ophiuchus Lyra - Hercules Cygnus Draco - Hercules Canes Venatici - Ursa Major Auriga Andromeda Draco - Ursa Minor Camelolopardalis Pisces Austrinus - Aquarius Lepus - Orion - Eridanus Corvus Libra Scorpius - Lupus Triangulum Astrinus - Ara - Centaurus Centaurus - Crux Carina Hydrus Tucana - Pavo

Click on the image map above or choose one of the links below

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 one of the links below.
  3. Count the total number of stars you see in the area including the corners.
  4. Look up that number in the table to determine your limiting magnitude.
  • You may see more (or fewer) stars than are plotted on the charts.
  • Different people will likely get slightly different results.
  • The area used to do this test should be choosen so that it is either near the direction you want to view, or the zenith, depending on observations and the situation. Meteor observers use the viewing direction. Deep sky observers (in Finland) choose an area near 45 degrees altitude.

This information and much of the text came from Bill Arnett's seds page on limiting magnitude, which in turn was derived from information created by Finnish amateur Veikko Mäkelä. The only reason for this additional LM page is to host my printer-friendly black on white images, all but one of which is under 30k in size.

All the star map images (except the image map above) were created by Guide 8 from Project Pluto.

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