MadSci Network: Astronomy
Query:

Re: How much light would a city add to starlight, for reading by starlight?

Date: Mon Jan 3 12:53:04 2005
Posted By: Chris Peterson, Faculty, Denver Museum of Nature and Science
Area of science: Astronomy
ID: 1095968917.As
Message:

If you assume no Moon and no light pollution, you can add up the intensities of all the things in the night sky that produce light: the stars, the Milky Way, and natural sky glow. If you do this, you get an illumination level on the ground of about 0.013 lux. The way the eye works, we see smaller details under brighter illumination. Above 2000 lux (a very cloudy day), the resolution of the eye is about one arcminute. This means that a person with good eyesight can read characters a half millimeter high from a distance of half a meter. But at very low light levels the resolution of the eye decreases. At 0.013 lux, most people can only read characters 50 mm high at a half meter. So it seems unlikely that your friend was able to read a map by starlight alone.

When there is a full Moon, the irradiance increases to 0.3 lux, and the human eye can read characters about 13 mm high at a half meter. It is certainly possible to get some useful information from a map held close at this brightness level.

The ground illumination can increase dramatically from light pollution, particularly when it is cloudy. Inside a major city, irradiance from lighting can easily exceed the brightness of the full Moon. So light pollution alone could be enough to let somebody read at night. However, light pollution probably does not play much role in the case of Tucson. Because of the many nearby observatories, Tucson has strict lighting control laws. It is easy to see the Milky Way even from the downtown area, something that is usually not possible in cities this size. From several miles away, and sheltered from the direct light dome by a canyon, Tucson will add very little to the overall irradiance. Southern Arizona has some of the most transparent skies in the world, so scatter from a city several miles distant will be small.

I wouldn't go so far as to say your friend is lying. There is a wide variation in visual acuity, and many things can produce an unusually bright sky. There are anecdotal cases of people reading by starlight alone, but most people will not be able to do so. During the next new Moon, try driving a few miles outside of town and see just how large print needs to be before you can read it.

References:
http://home.tiscali.se/pausch/comp/radfaq.html - brightness measurements
http://amper.ped.muni.cz/noc/english/canc_rhythm/PV_measur_Posch.pdf - a poster about the effects of light pollution on vision
http://www.lightpollution.it/dmsp/ - light pollution maps
http://www.darksky.org/ - International Dark-Sky Association


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