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.

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.