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8/02/2010

Limiting Magnitude Sky Features

First of a couple of posts today. I came upon this some time ago from another web site and I did not write or keep a link so I cannot give proper credit to the site outside of acknowledging this.
So I post it here hoping it will give some of you some guidelines for Limiting Magnitude.

LM Sky Features
4.5 - 5.0 Milky Way and Zodiacal light are invisible. Typical conditions found in suburbs of
major cities. Passing clouds are easily seen due to being lighted up from surrounding
lights.

5.1 - 5.5 The indistinct Milky Way is faintly visible only near zenith. Zodiacal light is still
invisible. M31, the Andromeda Galaxy is barely discernible.

5.6 - 6.0 The Milky Way is now more easily seen, but lacks detail. M13, the Great Hercules
globular star cluster can now be just glimpsed when near zenith. The Zodiacal light
is still invisible. The winter Milky Way from Auriga through Orion is still invisible.

6.1 - 6.5 The Milky Way is now obvious and some detail can be glimpsed. The Zodiacal light is
now barely visible, but not obvious. The winter Milky Way from Auriga through
Orion is faintly visible. There is still noticeable skyglow along the horizon due to
distant towns and cities.

6.6 - 7.0 Much sructure is visible in the Milky Way above. The Zodiacal light is an obvious
cone of light. The major constellations are less obvious due to "noise" caused by the
large number of faint stars now visible. Passing clouds appear as dark moving
masses as they block the natural skyglow from the Milky Way. A few sources of
skyglow are still visible along the horizon.

7.1 + INCREDIBLE! The Milky Way contains enormous amount of structure all the way to
the horizon and you can easily see your way around by its light. The Zodiacal light
now encircles the entire ecliptic. There are no sources of skyglow along any part of
the horizon. Meteors are easily visible.

It's rough, I'm not sure I agree with it all but it is a guideline and I guess if taken that way, it can help to evaluate sky conditions. I prefer actual weather conditions and the SQM or Sky Quality Meter (google it if your not sure what a SQM is).

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