Star Points for September, 2004; by Curtis Roelle
How Light Polluted are Carroll's Skies?

Last moth we discussed sky factors that can have adverse or positive effects on astronomical observing. We talked exclusively about the effects of weather, namely (1) obscuring cloud cover, (2) transparency or atmospheric clarity, and (3) seeing or how steady and calm the atmosphere is. You can review last month's column on line at the Star Points web site.

One other major factor for determining the quality of view you may expect with or without a telescope for celestial objects does not involve nature at all. Light pollution does not occur naturally -- it is man made. Light pollution is waste light escaping skyward causing the sky to glow and thus diminishing the visible starry realm.

Most nighttime outdoor lighting is intended for illumination of items on the ground -- parking lots, intersections, security for buildings or walkways. Unfortunately for the consumer as well as for astronomers inefficient and cheaply made lighting fixtures waste light through their failure to control the light beam. Glare light spills out sideways and trespasses onto adjoining property and also leaks upward into the sky. Wasted light squanders energy and hence wastes money.

When the humidity is high, as it is in our area, the water molecules suspended in the atmosphere scatter the light pollution resulting in sky glow. This sky glow reduces contrast and causes fainter stars as well as the Milky Way to become invisible for many miles around metropolitan areas.

How does one measure light pollution? A scale invented by and bearing the name of comet hunter John Bortle is the latest means of objectively identifying the amount of damage inflicted by light pollution. Here is an abridged version of the Bortle Scale:

Bortle 1 & 2: Excellent and truly dark sky site. Zodiacal light, gegenschein and zodiacal band are all visible. Brighter galaxies such as the "Pinwheel" M33 in the constellation Triangulum are easily visible to the naked eye. The summer Milky Way is visible all the way to the horizon showing much structure, marbling, and other detail in its star clouds and regions of dark obscuring matter. The airglow which occurs naturally in our atmosphere is visible up to about 15° above the horizon.

Bortle 3 & 4: Rural and rural/suburban transition sky. Domes of light pollution from towns are visible along the horizon. Clouds overhead show faint illumination near the horizon but are dark overhead. The summer Milky Way is still somewhat impressive but lacks all but the most obvious structure.

Bortle 5: Suburban sky. Milky Way is weak and almost washed out except high overhead. Horizon has a glow all the way around. Clouds visible in any part of the sky are brighter than the sky background.

Bortle 6 & 7: Bright suburban sky and suburban/urban transition. If the Milky Way is seen at all it is directly overhead and very washed out. Any clouds are fairly bright and strongly pinkish.

Bortle 8 & 9: Entire sky is whitish grey or orangish even when entirely clear. Only the brightest stars are visible and nearly all constellations are unrecognizable. The only telescopic objects that look pleasing at all are the moon, planets, and brightest star clusters (if they can be found).

The town of Westminster suffers from a bright suburban Bortle 6-7 sky. Same with Mt. Airy proper and Eldersburg including Piney Run Park which all fall within the envelope of the light polluted Washington-Baltimore metropolitan area. Most of the rest of the county, including my observatory five miles from New Windsor as well as the Bear Branch Nature Center, have a suburban Bortle 5 sky. The best and darkest skies that "keep it country" Carroll County has to offer is a small patch of suburban (Bortle 4.5) sky south of Harney. The rest of the county is all Bortle 5 or worse.

Next month we will explore a web site that features useful information for determining when observing conditions will be right as well as tools to help you locate darker sites more suitable for observing.