The Astronomy Nexus

How severe an effect does poorly designed and placed lighting have on the night sky? City skies indeed have a washed-out look to them, and experienced observers can often make mental comparisons to skies they have observed elsewhere. However, a direct side-by-side comparison can be difficult to make.

Motivated in part by an interest in maintaining dark skies (prompted in part by some very spectacular skies at events like the Nebraska Star Party and observing sessions near my home town of Salt Lake City, Utah), I decided to take advantage of the Web and display simulations of the sky as seen under different lighting conditions.

Limiting Magnitudes

In astronomy, particularly among amateur observers, the quality of an observing location is often described by a "limiting magnitude." The magnitude of a star is a measure of its brightness; the magnitude scale is logarithmic, so that every five units correspond to a brightness ratio of 100, and with smaller numbers corresponding to brighter stars. Thus a star of magnitude 1.0 is 100 times as bright as one of 6.0. Really bright objects have negative magnitudes (e.g., the Sun is -26.7, Venus is between -3 and -4.5); by contrast, the dimmest stars seen by unaided eyes under a dark sky range from about +6.0 to +7.5, depending on the quality of the sky (dry sites at high elevations are best) and the observer's ability to see faint objects.

The limiting magnitude is simply the magnitude of the faintest star a person can see from a given place at a given time. In good viewing areas, values in the range +6 to +7 are typical, as described earlier. In cities, however, the values are often much lower -- +4 or even +2 in very severe cases (e.g., downtown region of a city with more than 1 million residents).

To illustrate the effects of light pollution on the number and apparent brightness of stars one can see, I've prepared a gallery of graphics images of the constellations Orion and Ursa Minor (the Little Dipper) with a planetarium program (Voyager II for the Macintosh). I chose these two since Orion is familiar to many people, and since Ursa Minor has a wide range of star brightnesses and is often used as a rough guide to evaluating limiting magnitudes. See what limiting magnitude is needed to see all the stars in the Little Dipper!

The images are meant to simulate the view of these constellations as they might be seen in places with various limiting magnitudes. According to Sky and Telescope, March 1991, p. 262-264, the following correspondences can be made:

• +6.5. Dark sites with little or no light pollution.
• +5.5. Minor light pollution -- enough to illuminate clouds and make them appear brighter than the sky, but not enough to completely wash out the Milky Way.
• +4.5. Moderate light pollution. The Milky Way is barely visible. This is typical of suburban locales in the United States; Sky and Telescope editors use this as a value typical for
  many of its readers.
• +3.5. Serious light pollution. Typical of locations in major cities.
• +2.5. Extreme light pollution. Downtowns of large cities with poor lighting practices or on hazy nights.

Under particularly bad conditions -- severe smog in Los Angeles, say -- even +2.5 may not be achievable. On the other hand, people working under exceptional conditions (e.g., high elevations in deserts), or who have good night vision, can sometimes see stars dimmer than +6.5. Just for fun, I've included an image for a limiting magnitude of +7.5, which can be obtained by an experienced observer at an excellent site (e.g., much of the high plains and mountain regions of the western U.S.). [Warning: Don't pick the magnitude +7.5 image of Ursa Minor, unless (a) you're pretty familiar with the sky, or (b) you've already seen some of the earlier images. Otherwise, you may not recognize the Little Dipper in that mass of stars for a long time...]