The Earth at night is a blaze of lights. Cities, towns, natural gas fields, and even fishing fleets send their lights into the sky and off into space, announcing our presence to the universe.
This is how the continents of North and South America appear from space: lights from coast to coast. The large cities appear as sprawling jewels in the night.
(Visit http://www.ngdc.noaa.gov/dmsp/download_iss_movies.html to see a movie showing many cities from space.)
While we may gain from lighting the night, we also lose some things: particularly the night sky. The brighter the lights on the ground around us, the less we see of the stars and the more we lose connection with the universe around us. Consider this computer simulation of the loss of visibility of the night sky as city lights grow brighter around us.
Astronomers are particularly affected by increasing lights. Most of the objects astronomers are interested in observing with GSMT are very faint, such as galaxies at the edge of the universe, planets orbiting other stars, or stars just beginning to form. Thus an important requirement for a possible observatory site is a dark sky. While other factors contribute to a dark sky, the main one is the absence of human-made light sources: building lights, factories, airplanes and automobiles. Thus dark skies are found in isolated places far from any city or town. Unfortunately, as the population of the world grows, it gets harder and harder to find dark-sky locations. And sites that were dark in the past are getting brighter as nearby cities get larger.
As an example, when Kitt Peak National Observatory in southern Arizona was started in 1958, nearby Tucson had a population of only about 100,000 people. As seen from the top of Kitt Peak (see figure below), Tucson’s lights were fairly dim and far away. But the lights became more prominent as Tucson grew to nearly a million people today, despite one of the world’s most progressive light-reduction city ordinances.
The lights surrounding Kitt Peak today are most clearly seen from space. The night-time satellite image below, centered on Kitt Peak, shows the lights of Tucson to the upper left, and the lights from the southern edge of the Phoenix metropolitan area at top center. Other towns can be identified from the matching map in the lower panel.
The lights that can be directly seen from the top of a mountain depends on its elevation – the higher the mountain, the more distant the lights are than can be seen. Kitt Peak is about 7100 feet in elevation, so lights about 100 miles away can be seen, which is about the width of the image north and south from Kitt Peak. The relationship between elevation and the distance to the horizon is shown in the graph below.
Using this figure and maps of night lights from the Defense Meteorological Satellite Program, one can estimate the amount of light visible at a proposed observatory site from its elevation. A quantitative measure of the light can be obtained in the following activity.
For more information about dark skies and their preservation, visit the International Dark-Sky Association.