For years, astronomers have described the darkness of a night sky by referring to its “limiting magnitude.” The ‘limiting magnitude’ of any particular sky is based upon the magnitude of the faintest star visible to the unaided, naked eye. And the magnitude of that faintest star is based on a star brightness classification invented by the Greek astronomer, Hipparchus, more than 2,100 years ago.
Back in the 2nd Century BCE, Hipparchus studied some 860 stars he saw and assigned them to classes of brightness ranging from ‘magnitude 1’ for the brightest, to ‘magnitude 6’ for the faintest. English astronomer N. R. Pogson later refined that system when in 1856, he identified a typical magnitude 1 star as being 100 times brighter than a typical magnitude 6 star. (As a side note, many astronomers believe humans on Earth can no longer see those magnitude six stars as classified by Hipparchus in antiquity, due to light pollution.)
What all this means is that when we describe a night sky today as, e.g., a magnitude 6, we’re saying we can see all the brightest stars plus the faintest ones classified as magnitude six as they appear on modern magnitude charts. A magnitude 6 night sky is fairly dark and full of stars; a magnitude 2 night sky is opague with only the brightest stars visible. But, a magnitude 7 sky is amazingly dark! In other words, as the limiting magnitude number goes up, we are capable of seeing more and more faint stars. Take a look at Charts 3 and 7 for comparisons of limiting sky magnitudes.
Night sky technology has advanced quite a bit since Pogson’s time in 1856 and exponentially in just the past 10 years. Scientists and mathematicians have modernized night sky magnitude charts like those in Figures 1 and 2 by Jenik Hollan, who incorporates local sidereal time, and through the use of Sky Quality Meters manufactured by Unihedron, based on measuring the amount of light at the zenith (top of the sky). Sky limiting magnitude charts are even available in fraction and with negatives such that our sun, for example, is a Mag Minus 26.8!
Collection and analysis of today’s night sky data (we now include GPS coordinates, elevation, etc.), confirms that adults who live in heavily light-polluted cities such as Tokyo, New York, Los Angeles, etc., have never seen the Milky Way. In fact, the vast majority of adults who live in metropolitan areas with populations over 250,000 have never seen a meteor streak across the sky. The vast majority see only a few stars in the sky at night.
But, here in Silver City, on a good, moonless night, after a still day with no monsoon, and even better, if you’re just 7 miles outside the city limits, particularly to the northwest, you’ll see thousands & thousands of stars, a Milky Way that stretches from horizon to horizon, and meteors nearly every night. Take a look, for example, of an all sky image of the skies over Silver City a few miles north of downtown. This was taken by SCAS Board Member Bob Bryant just a few months ago.
Clear, dark & dry skies here run between the 5th and 7th limiting magnitude and our SQM readings run 19.3 to 21.7.
Did you know that our Earth travels through some 43 meteor showers in a year? Keep checking back on the Silver City Arts & Culture District website. We’ll be talking about those next!
[© 2012 Kathy Anderson, Silver City Astronomical Society]