"The DFS Today in History/DFS Web Stuff" TWO CD set contains Mega versions of "Today in History" and "The History Chronology" as well as content from all of the active websites, plus stuff that used to be published on the web!

About the World Sunrise and Sunset Almanac

Sunrise, sunset, swiftly flow the days

If you occasionally check the newspaper or television for times of sunset, it might never have occurred to you to ask the question: Just what is sunset, anyway?

Sunset (and sunrise) can actually be defined in a number of ways. Those at the U.S. Naval Observatory in charge of calculating such matters define it as the time when the upper edge of the sun's disk appears on the unobstructed horizon. An exact time is tough to calculate because it depends on atmospheric conditions, elevation, the terrain of landscape over which you look and more.

And what about "twilight"? What exactly does that mean? This refers to the period between sunset and darkness (or darkness and sunrise) where sky brightness decreases (or increases); some outdoor activities can continue without artificial lighting, but only for so long. Naturally, several limiting practical definitions have evolved over the years.

"Civil twilight" is said to begin in the morning - and to end in the evening - when the center of the sun lies six degrees below the horizon. In the morning before the beginning of civil twilight and in the evening after the end of civil twilight, artificial illumination is normally required to carry on ordinary outdoor activities.

"Nautical twilight" is defined to begin in the morning - and to end in the evening - when the center of the sun lies 12 degrees below the horizon. At the beginning or end of nautical twilight, under good atmospheric conditions and in the absence of other illumination, general outlines of ground objects may be distinguishable, but detailed outdoor operations are not possible, and the horizon is indistinct.

And, finally, "astronomical twilight" begins in the morning - and ends in the evening - when the center of the sun lies 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening, the sun does not contribute to sky illumination. For a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.

For mid-northern latitudes, each of these periods lasts between 20 and 30 minutes, but this varies throughout the year. To read a more detailed and comprehensive discussion or to calculate these (and other astronomically important) data, visit the U.S. Naval Observatory's Web site at

http://aa.usno.navy.mil/data

Sources For DFS Today in History