What is astronomical twilight? Noctilucent Clouds?
Astronomical twilight is a period of time in which the sun is below the horizon but some of its light is still being scattered between air molecules and bounced back to an observer on the surface.
Astronomical twilight generally happens twice each day: It begins in the morning before dawn or ends in the evening after sunset when the geometric center of the sun is 18 degrees below the horizon, according to the National Weather Service (NWS). When the sun is at or above 12 degrees below the horizon, it's no longer astronomical twilight.
During astronomical twilight in an area without light pollution, the horizon disappears from view and even the dimmer planets and some less visible stars can be spotted with the naked eye, according to the NWS. For casual observers and those in urban or suburban areas, the sky may appear fully dark, and viewers can get a good look at objects like planets and stars that appear as a bright point of light. In higher latitudes, rare formations called noctilucent clouds are sometimes visible against a dark twilight sky. However, astronomers wait for truly dark skies to observe galaxies, nebulas and anything else that creates diffuse and tricky-to-spot light sources.
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All images are taken from Google Images for educational purposes only.