Many publications note that at the time of the June solstice, for Northern Hemisphere localities, the days are the longest and the nights are the shortest. But in the strictest sense, this statement is only half true — it gets more complicated when you factor twilight into the equation.
Summer in the Northern Hemisphere began with the solstice on Monday (June 20) at 6:34 p.m. EDT (2234 GMT), which was the earliest arrival of the summer solstice in 120 years — a celestial occurrence that likely got a good deal less attention than the full moon commanded later that very same evening.
After all, you can see the moon. [Delicious 'Strawberry Moon' Photos: Rare Solstice Lunar Show Wows Stargazers]
The solstice, on the other hand, is a calculation — that moment when the sun shines directly down over the Tropic of Cancer (23.5 degrees north latitude).
With the naked eye, skywatchers can't even note the change in the daylight span on the solstice, which is virtually the same on Sunday and Tuesday (the days before and after the solstice), and the length of the day diminishes by just half an hour by the end of the next month.
The days are said to be the longest and the nights the shortest in the Northern Hemisphere at the time of the summer solstice, but discerning the time when the sky is actually dark is more complex. As an example, from New York City, at 41 degrees north latitude, the sun on June 20 was above the horizon for 15 hours and 6 minutes. But can we really refer to the remaining 8 hours and 54 minutes as "night"?
Not if we consider twilight.
Actually . . . three twilight zones
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 the sun emerges from the eastern horizon in 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.
Over 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. Astronomers categorize twilight into three specific stages:
Civil twilight is defined as when the center of the sun is geometrically between the horizon and 6 degrees below the horizon — the sun reaches 6 degrees about 30 to 40 minutes before sunrise or after sunset. (Your clenched fist held at arm's length measures roughly 10 degrees in width.) Civil twilight is the limit at which illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished.
Under a clear sky, at the beginning of morning civil twilight or the 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.
On some newspapers' weather page, you'll find a notation about when to turn on your car's headlights. That time is usually the end of civil twilight. Before lights became ubiquitous at most major-league baseball parks, an extra inning afternoon game that ran past sunset would be cut short on account of darkness around the time of the end of civil twilight. Of course, the precise moment was dependent on sky conditions; under a heavily overcast sky, it would get dark much sooner.
Nautical twilight is defined as when the center of the sun is geometrically between 6 degrees and12 degrees below the horizon — the sun reaches 12 degrees 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. [Amazing Night Sky Photos by Stargazers]
Astronomical twilight is defined as when the center of the sun is geometrically between 12 degrees and 18 degrees below the horizon — the sun reaches 18 degrees 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 — but 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 New York City, at the June solstice, the entire span of twilight lasted 2 hours and 6 minutes before sunrise and for a similar interval after sunset. So, of the nearly 9 hours that the sun was below the horizon, it was totally dark for just 4 hours and 43 minutes.
Big differences in the North and South
For every degree you go north of 40 degrees north latitude, the length of twilight around the June solstice increases by an average of almost 12 minutes. At 48.7 degrees north latitude, twilight lasts 3 hours and 44 minutes, and north of there, twilight lingers through the entire night. From London, (51.5 degrees north latitude), twilight persists through the entire night from May 23 to July 18. From Edmonton, Alberta (53.6 degrees north latitude), it runs from May 13 through July 28.
In Fairbanks, Alaska (64.8 degrees north latitude), perpetual twilight runs from April 8 through Sept. 2. 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 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.
Yet, in stark contrast to these far-northern locations, if you were to travel south to the tropics for a summer vacation in the coming weeks, you likely be struck by how quickly it gets dark after sundown. Let's say, for example, you plan to be in Honolulu on Independence Day. Sunset that evening is at 7:17 p.m. Hawaii time, and astronomical twilight will end at 8:41 p.m. Yet only about an hour after sunset, a good number of stars will already be visible in the darkening sky.
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.