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.
- Wait for your eyes to dark adapt (at least 30 minutes).
- 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.
- Count the total number of stars you see in the area including the corners.
- Look up that number in the table and read off your limiting magnitude.
- You may see more (or fewer) stars than are plotted on the charts.
- Different people may get different results.
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.
Another version of this information (with nice black-on-white star charts) is available at the IMO Major-Shower Observations page.