I understand magnitude is a brightness rating we give stars, but how do you know what magnitude your skies are, I always hear people say "i live with mag 5 skies" or whatever how do you rate what your skies are? and how do we know the magnitude of a star? i realize software like TheSky will tell us the mag of an object but who makes these numbers, and is your magnitude 9 star the same as mine? or would it be more 6 to me??

Smoke Eater:

I understand magnitude is a brightness rating we give stars, but how do you know what magnitude your skies are, I always hear people say "i live with mag 5 skies" or whatever how do you rate what your skies are? and how do we know the magnitude of a star? i realize software like TheSky will tell us the mag of an object but who makes these numbers, and is your magnitude 9 star the same as mine? or would it be more 6 to me??

You will need a planetarium program or star chart that lists stars dimmer than what your naked eye can see.

Then stand outside and allow your eyes to become dark adapt.. Look up, without squinting or using averted vision, see what the faintest star is that you can see.  Then look at your chart or planetarium program to determine the magnitude of that star.  Generally under clear dark skies you should be able to see a mag 6 or 6.5 star. If however you have exceptionally good visual acuity you might be able to see dimmer stars than that.

Have A Nice __________

i see, so its a personal thing, if you are blind as a bat you might only be able to see the bright stars, and never anything below 7.. I understand there is no unit of measurement like candlepower or anything, its just a general rating number assigned to a star's brightness

thanks!

Smoke Eater:

i see, so its a personal thing, if you are blind as a bat you might only be able to see the bright stars, and never anything below 7..

Smoke Eater:

 

I understand there is no unit of measurement like candlepower or anything, its just a general rating number assigned to a star's brightness

In ancient times it was a general rating system in which the brightest stars were said to be of first magnitude and the dimmest visible stars were assigned sixth magnitude.  In modern times the system became more precise based on photometric data.  By definition a magnitude 1.00 star is exactly 100 times brighter than a magnitude 6.00 star.

Among the many significant accomplishments of the early Greek astronomer Hipparchus was the creation of the first comprehensive stellar atlas.  He plotted the positions of some 850 stars and catagorized those stars into classes of magnitude or brightness.

Many centuries later, the English astronomer Norman Robert Pogson discovered that the brightness scale developed by Hipparchus and refined later by Ptolemy was in fact logarithmic.  Each step in magnitude is equal to a brightness difference of 2.512 times. This means that a first magnitude star is (2.512)5 or 100 times brighter than one of sixth magnitude.  In 1856, he proposed standardizing stellar magnitude using this relationship, which became known as the Pogson Ratio.  This magnitude relation is given by m2 − m1 = − 2.5log10(L2 / L1), where m equals stellar magnitude and L is luminosity.

For more on the topic, browse http://curious.astro.cornell.edu/question.php?number=569

Another way of determining the local visual limiting magnitude is to do star counts in selected areas of the sky.

http://www.phys-astro.sonoma.edu/observatory/observers/limiting_magnitude.html

http://www.imo.net/visual/major/observation/lm

VLM is affected by a number of factors besides the visual acuity of the observer.  See http://www.go.ednet.ns.ca/~larry/astro/vislimit.html for a VLM calculator.

A few sharp-eyed observers have seen stars dimmer than seventh magnitude under ideal conditions.

Nowadays many amateur astronomers use an electronic device known as a Sky Quality Meter to quantify sky brightness.

Dave Mitsky

 And when you are determining your particular sky's magnitude limits, be sure to test the sky at the zenith (close to straight up), where the sky is usually the darkest and clearest. Mr Q