Jim's Corner Blog

Rating the Night Sky from your Favorite Observing Site

In the process of selecting a suitable observing site for the Shenandoah Astronomical Society’s members-only star party, I had the opportunity to visit some candidate state parks. Many could have worked, but ultimately I placed a priority on a dark site as being more appealing than accessibility to club members.

Ironically, I choose my own backyard! I discovered that the criteria I used for my search for a site for the star party was similar to the criteria that I originally used for my search for a new home in the Winchester VA area eight years ago: namely little or no trees and a dark sky.

The tree criteria is easily understandable. But the definition of a dark sky is not.

The darkest sky I’ve ever experienced was on a drive from Las Vegas NV to the San Bernadino Mountains to attend the now-defunct Riverside Telescope Makers Convention (RTMC), formerly held at a Boys and Girls campground just outside Big Bear Lake, CA. After crossing the state line and entering California, there is a long stretch of highway through the desert before reaching the base of the mountains. While driving in the middle of the night, my friend and I stopped to view the night sky. Incredible! The night sky was extraordinarily dark. There were so many stars that it was hard to pick out the constellations. M31 the Andromeda Galaxy, M33 the Triangulum Galaxy, and NGC 7000 the North America Nebula were clearly visible with the naked eye.

Fast forward to 2013 when my wife and I had decided to move from Bowie, Maryland and its light polluted skies and were searching for country home with dark skies. We eventually settled on our current home. No trees around the house that would block my view of the horizon. And save for a slight light dome from Winchester, dark skies from my backyard allow me to see the Milky Way across the summer sky.

In the magazine Sky and Telescope February 2001 issue, the Bortle scale was introduced as a rating system for the night sky brightness of a particular observing location. John E. Bortle created the scale that quantifies the observability of astronomical objects and the interference of light pollution.

The Bortle scale ranges from Class 1 representing the darkest skies on Earth to Class 9 representing inner-city skies. The following are the definitions of each class:

Class 1 – An excellent dark sky site, representing the almost mythical descriptions of the night sky described by veterans returning from desert wars and my own experience in the desert of California. The naked eye limiting magnitude is 7.6 to 8.0! Phenomena such as the zodiacal light, gegenschein and the zodiacal band are clearly visible. Light domes of distant cities can be seen. The Milky Way in the regions of Sagittarius and Scorpio clearly display the dark cloud shadows. Constellations are barely recognizable due to the abundance of stars. Many Messier globular clusters, open clusters, and bright galaxies are naked eye objects! M33 the Triangulum Galaxy is a direct eye visibility object. The night is so srak that a bright Venus and Jupiter can affect your night vision!

Class 2 – A typical dark sky site. Magnitude limit from 7.1 to 7.5. A yellowish zodiacal light is visible. Some sky domes are visible. Fainter constellations are barely recognizable amid the the abundance of stars. Clouds and structural surroundings are visible as dark holes and silhouettes in the sky. The summer Milky Way appears highly structured. Many Messier objects and globular clusters are visible, with M33 still visible with the naked eye.

Class 3 – Rural sky, with a limiting magnitude of 6.6-7.0. The zodiacal light is visible in the spring and autumn. Some light pollution is weakly detected near the horizon. Clouds are dark overhead but illuminated near the horizon.Surrounding trees and buildings are vaguely visible. Although the Milky Way still appears complex, only the brightest globulars, such as M13, M22, M15, M4, and M5 are naked eye visible, with M33 an averted eye object.

Class 4 -Rural/suburban sky. Limiting magnitude 6.1-6,5. The zodiacal light is visible but does not extend halfway to the zenith. Light pollution is clearly visible is several directions. Clouds are illuminated by light sources but dark overhead. Near and distant surrounding are visible. The Milky Way is visible but lacks detail. M33 is a difficult avert vision object.

Class 5 – Suburban sky. Limiting magnitude 5.6 -6.0. The zodiacal light is just hunted during the autumn and spring. Light pollution is visible in all directions. Clouds are noticeably brighter in the sky. The Milky Way is not detectable in the horizon and is washed out overhead. The night sky is more dark blue than it is black.

Class 6 – Bright Suburban sky. Limiting magnitude of 5.1-5.5. No zodiacal light . Light pollution extends to 35°from the horizon. Clouds are brightly lit. Binoculars and telescopes are needed to see Messier objectsThe sky appears blue.

Class 7 – Suburban/Urban transition. 4.6-5.0 limiting magnitude. Light pollution makes the night sky appear grey. Strong lights source are visible in all directions.The Milky Way, forget it, invisible. A telescope is needed to view anything, with any detail being washed out.

Class 8 – City Sky. Limiting magnitude 4.1-4.5. The sky is a light grey or orange. Many constellations are invisible. Even a telescope is unable to detect some of the dimmer Messier objects.

Class 9 – Inner city. <4.0 magnitude visible. For example, in Las Vegas, only the Moon can be seen. In Las Vegas, the only stars visible are Celine Dion, Penn & Teller, and Elvis impersonators. Most inner cities, the astronomer is limited to viewing the Moon, Jupiter, Saturn, Venus and the Pleiades.

So with the Bortle scale, how do you rate your own back yard? I rate my backyard somewhere between a Class 3/Class 4, mostly due to the light and proximity from Winchester VA and locally the community of Lake Holiday. My front yard is a solid Class 3 view. Completely acceptable for an amateur astronomer.