By Skywatching Columnist
updated 6/13/2008 1:13:56 PM ET 2008-06-13T17:13:56

On Friday, June 20, media outlets across North America will announce that summer officially begins. Indeed, the point in the sky the sun reaches at 8 p.m. EDT is still widely called the summer solstice. But inveighing against Northern Hemisphere chauvinism, I prefer instead to refer to it as the "June Solstice."

Many publications note that at the time of the June Solstice, for Northern Hemisphere localities, the days are longest and the nights shortest, but in the strictest sense this statement is only half true. As an example, from Philadelphia (latitude 40 degrees north), the sun on June 20 will be above the horizon for 15 hours. But can we really refer to the remaining 9 hours as "night?"

Not if we factor twilight into this equation. Certainly, after the sun disappears beyond the western horizon during the evening hours, the sky does not immediately become completely dark, nor is it totally dark until the moment that the sun emerges from the eastern horizon during the morning. We can thank our atmosphere for that. Twilight is defined as the diffused light from the sky during the early evening or early morning when the sun is below the horizon and its light is refracted by the Earth's atmosphere.

During the course of the year, the length of twilight can vary, but at this time of the year, for those in the Northern Hemisphere, it persists for the longest interval of time. And it may surprise you to learn that astronomers categorize twilight into three specific stages:

Civil twilight is defined to begin in the morning, and to end in the evening when the center of the sun is geometrically 6 degrees below the horizon (your clenched fist held at arm's length measures roughly 10 degrees in width), about 30 to 40 minutes before sunrise or after sunset. Civil twilight is the limit at which illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished. At the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the very brightest stars and planets (such as Sirius, Jupiter and Venus) are visible under good atmospheric conditions. 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.

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Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon, about 60 to 80 minutes before sunrise or after sunset. 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 most outdoor activities are not possible, and the horizon is indistinct.

Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the sun is geometrically 18 degrees below the horizon, about 90 to 120 minutes before sunrise or after sunset. 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. Admittedly, for a considerable interval after the beginning of morning twilight and before the end of evening twilight, any sky illumination is quite faint and somewhat difficult to perceive. Still, not until after astronomical twilight ends in the evening, or just before it begins in the morning can we say that the sky is indeed totally dark.

From Philadelphia, at the June Solstice, the entire span of twilight lasts 2 hours and 2 minutes before sunrise and for a similar interval after sunset. So of the nine hours that the sun is below the horizon, it is totally dark for just 4 hours and 56 minutes.

And for every degree that one goes north of latitude 40, the length of twilight increases by an average of almost 12 minutes. At latitude 48.7 degrees north, twilight lasts 3 hours and 44 minutes, and north of there, twilight lingers through the entire night. From London, England (latitude +51.5 degrees), twilight persists through the entire night from May 23 to July 18. From Edmonton, Alberta, Canada (latitude +53.6 degrees) it runs from May 13 through July 28.

And at Reykjavik, Iceland (latitude +64.2 degrees), perpetual twilight runs from April 9 through September 1. In fact, from May 20 through July 23, the sky remains so bright that few, if any bright stars or planets can be seen. Just a little farther to the north, lies the Arctic Circle, the southern extremity of the "midnight sun." North of the Arctic Circle, the sun remains above the horizon for 24 continuous hours at least once per year.

Finally, in stark contrast to these far-northern locations stand the tropics, and those of you who might be heading south to those climes for a vacation during the next few weeks will likely be struck as to how quickly darkness falls after sundown. Indeed, from places like Honolulu or San Juan, twilight lasts about 90 minutes, and about an hour after sunset a good number of stars will already be visible in the darkening sky.

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