Has the darkness of the night sky changed at your observing site? Would you like to know your naked-eye magnitude limit? Find out using both traditional and digital ways.
We all like to know how dark the sky is at our favorite observing sites. It makes it easy to compare one place to another and helps us decide how deep we might go with our telescope that night. A set of stars with stable brightnesses (no variables please!) running the gamut of magnitudes from bright to faint and positioned at a comfortable viewing angle fits the bill. The faintest one visible determines the NELM, or "naked-eye limiting magnitude," for that location.
Transparency can be highly variable, dependent on the amount of haze, smoke, and humidity in the air as well as your elevation. Long-term monitoring of limiting magnitude can reveal information about trends in local and regional light pollution or even the variability of airglow, caused by the recombination of atoms and electrons in the upper atmosphere that were ionized by the Sun's UV light earlier that day.
Determining the best part of the sky to pick a standard set of stars isn't always easy. Ideally, you should have a different set for each season. Maybe even a set for each direction since for most of us the sky is rarely uniform but instead brighter here and darker there due to variations in local light pollution levels.
The object's altitude is also important so we must factor in atmospheric extinction. The closer a star is to the horizon the more air you have to look through and the more its light is dimmed or extinguished by the atmosphere and its brew of natural and synthetic aerosols.
The amount of air directly overhead is called 1 air mass. For a star 30° above the horizon, you're looking through 2 air masses. At 10°, 5.6 air masses, and for an object on the horizon, 40 air masses. A star seen 5° above the horizon will be dimmed by 3.5 magnitudes compared to the same star at the zenith assuming identical, ideal atmospheric conditions.
With this in mind, let's pick a set of stars of known magnitude at a high enough altitude (to avoid the worst of atmospheric extinction) and visible over much of the year. The celestial poles are ideal for mid-latitude observers, which is why you'll often see Ursa Minor used as an example. But to be fair to the Southern Hemisphere and with a nod to observers around Earth's middle, I've included three maps: Ursa Minor, Octans (Ursa Minor's equivalent in the Southern Hemisphere), and a familiar seasonal asterism, the Great Square of Pegasus.
I encourage you to create your own limiting magnitude maps by selecting what you feel is more appropriate for your location. Avoid the zenith, which has an unfair advantage, and areas within 40° of the horizon. At 40°, extinction amounts to about 0.2 magnitudes.
A darker sky means more satisfying views of our favorite cosmic objects. If you live where light pollution gets you down, check out the interactive Light Pollution Atlas to help you find the darkest possible area in your region reachable by car. Color-coded zones of light pollution overlay detailed road maps that you can zoom into in great detail. Click anywhere and you'll get details about the site including its Bortle class — our next topic. Very handy!
Determining your NELM and including it in descriptions of what you see through your telescope provides a standard for comparing observations. A more comprehensive standard that uses limiting magnitude as its basis is the Bortle Dark-Sky Scale created by amateur astronomer and Sky & Telescope writer John Bortle. It first appeared in the February 2001 issue of Sky & Telescope magazine. Bortle considered NELM alone too dependent on individual eyesight and the variable amounts of time people put into the effort of seeing the faintest star.
His nine-level scale bundles several factors including NELM, the appearance of the zodiacal light and Milky Way, telescope magnitude limit, and naked-eye visibility of familiar deep-sky objects like M33 and M31. Observer Nirvana is rated Class 1 with stars of magnitude 7.6–8.0 visible and M33 obvious with the naked eye. A rural sky, the best many of us who live near cities can reach, rates a Class 3. A Class 9, or inner-city sky, has a limiting magnitude of 4.0 or less.
There are also new apps for both Android and iPhone that let you use the phones' cameras to help navigate to and determine how faint a star you can see. For more information, check out the Dark Sky Meter (iPhone) and the Loss of the night (Android) on Google Play. Observers have the option with either app of sending their observations to international databases used to monitor night-sky brightness across the planet.
Any of these methods makes us more aware of what's happening to our sky, information we can use to better educate city councils on managing lighting to preserve the night or simply to inform our observing.