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Astronomical twilight: a weather geek's delightful discovery!
5:08 AM EDT 9:54 PM EDT for Orlando, FL at: http://www.wunderground.com/q/zmw:32801.1.99999
I always thought of the words "early dawn" and "almost dark" when I thought of this phenomenon.
https://en.wikipedia.org/wiki/Twilight#Astronomical_twilight
Morning astronomical twilight is defined to begin when the center of the sun is geometrically 18 degrees below the horizon and to end at sunrise. Evening astronomical twilight is defined to begin at sunset and end when the center of the sun is 18 degrees below the horizon.[1] From the end of astronomical twilight in the evening to the beginning of astronomical twilight in the morning, the sky (away from urban light pollution, moonlight, auroras, and other sources of light in the sky) is dark enough for nearly all astronomical observations. In some places, especially those with sky glow, astronomical twilight may be almost indistinguishable from night.
Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars. However, some critical observations, such as of faint diffuse items such as nebulae and galaxies, may require that the sun be more than 18° below the horizon, beyond the limit of astronomical twilight.
Theoretically, the dimmest stars ever visible to the naked eye (those of the sixth magnitude) will become visible in the evening once the sun falls more than 18° below the horizon (i.e., at astronomical dusk), and become invisible when the sun moves to within 18° of the horizon in the morning (at astronomical dawn). However, because of light pollution, some localities, generally those in large cities, may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all, and to experience truly dark skies.
Most casual observers would consider the entire sky fully dark even when astronomical twilight is just beginning in the evening or just ending in the morning, and astronomers can easily make observations of point sources such as stars. However, some critical observations, such as of faint diffuse items such as nebulae and galaxies, may require that the sun be more than 18° below the horizon, beyond the limit of astronomical twilight.
Theoretically, the dimmest stars ever visible to the naked eye (those of the sixth magnitude) will become visible in the evening once the sun falls more than 18° below the horizon (i.e., at astronomical dusk), and become invisible when the sun moves to within 18° of the horizon in the morning (at astronomical dawn). However, because of light pollution, some localities, generally those in large cities, may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all, and to experience truly dark skies.
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