Computed for selected Southwest U.S. cities
2008 January 1
Twilight gives way to night on a southern California winter evening. Photo taken with a Nikon D70 digital SLR and Nikkor 18-70mm AF-S lens. Copyright 2006, Brian Webb.
When the sky will be completely dark after sundown? The answer depends on the brightness of the sky background due to manmade light pollution and moonlight. On moonless nights, suburban skies are their darkest about 75 minutes after sunset until about the same interval before sunrise. At remote locations far from city lights, night begins later after sunset and ends earlier before sunrise.
A good measure of when night begins and ends is astronomical twilight. It occurs when the center of the Sun has an altitude of -12° or more with respect to the horizon. If the Sun's altitude is below this amount, it does not add to sky illumination.
The following links lead to tables of daily astronomical twilight times for the current year.
Copyright © 2005-2008, Brian Webb. All rights reserved.