Night Sky Brightness at Cerro Pachon

MODEL CALCULATIONS

Dr. Roy Garstang has calculated the light pollution at Cerro Pachon using the following inputs:

• latitude, longitude and elevation of Cerro Pachon
• latitude, longitude and elevations for La Serena-Coquimbo, Vicuna, Andacollo and Ovalle. The cities are treated as point sources.
• Population estimates for each center (1990, 2000, 2010, 2020, 2030, 2040).
• Light output per person of 150L (1990) and 300L (others).

Note that the model does NOT include:

• The effects of intervening mountains. Almost all the direct light (see Figure 1) is scattered at angles just above the horizontal, so a substantial amount of the direct light contamination is removed by intervening mountains. In our case Ovalle is not directly visible, and large fractions of Vicuna and La Serena-Coquimbo are occulted.

• On approximately 50% of nights La Serena and Coquimbo are totally covered by low cloud which reduces light contamination from these cities to near-zero. Until 2010 La Serena-Coquimbo contribute around half of the light contamination, and more in later years.

• The model calculates V magnitude light output at zenith. Since the street light are all Sodium and there are virtually no Mercury lamps, the V band is a good test case. We also have some calculations for 2010 (ie near next solar minimum) looking at zenith distance 45 degrees to and away from La Serena-Coquimbo.