The light of twilight

The light of twilight
twlight--wikimedia

Around the summer solstice,  the tilted orbit of our earth around the sun brings early morning light  and lingering evenings. These are the longest days and shortest nights in the Northern Hemisphere. This is also the time of twilight, the light of  dusk and dawn.

Did you know there are three designations of twilight?  NOAA defines twilight this way--

Civil Twilight
Begins in the morning, or ends in the evening, when the geometric center of the sun is 6 degrees below the horizon. Therefore morning civil twilight begins when the geometric center of the sun is 6 degrees below the horizon, and ends at sunrise. Evening civil twilight begins at sunset, and ends when the geometric center of the sun is 6 degrees below the horizon. Under these conditions absent fog or other restrictions, the brightest stars and planets can be seen, the horizon and terrestrial objects can be discerned, and in many cases, artificial lighting is not needed.

Nautical Twilight
Begins in the morning, or ends in the evening, when the geometric center of the sun is 12 degrees below the horizon. In general, the term nautical twilight refers to sailors being able to take reliable readings via well known stars because the horizon is still visible, even under moonless conditions. Absent fog or other restrictions, outlines of terrestrial objects may still be discernible, but detailed outdoor activities are likely curtailed without artificial illumination.

Astronomical Twilight
Begins in the morning, or ends in the evening, when the geometric center of the sun is 18 degrees below the horizon. In astronomical twilight, sky illumination is so faint that most casual observers would regard the sky as fully dark, especially under urban or suburban light pollution. Under astronomical twilight, the horizon is not discernible and moderately faint stars or planets can be observed with the naked eye under a non light polluted sky. But to test the limits of naked eye observations, the sun needs to be more than 18 degrees below the horizon. Point light sources such as stars and planets can be readily studied by astronomers under astronomical twilight. But diffuse light sources such as galaxies, nebula, and globular clusters need to be observed under a totally dark sky, again when the sun is more than 18 degrees below the horizon.

Twilight is also called the blue hour, a time favored by photographers because of the quality of the light and absence of shadows.

It is the most active time of dusk and dawn creatures such as birds, cats and fireflies.  And, alas, mosquitos!

Because of our tilted planet, the sun at the summer solstice is at its most northerly point in our sky, along the line of the Tropic of Cancer.  Above the Arctic Circle, the sun never sets below the horizon.  Further south, there are different degrees of twilight lighting the summer nights.  Here is an explanation from EarthSky.

 

Filed under: seasons, weather

Tags: solstice, summer, twilight

Comments

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  • This is delightful! I never knew that there are three kinds of twilight, and this is a great time to enjoy them all... at least if I can get close enough to the lake.

  • In reply to Margaret H. Laing:

    Thanks for reading! Love and light to you

  • the 4th kind of Twilight is the one with sparkly vampires that no one really likes

  • In reply to Michael Messinger:

    Thanks for reading! Yes, there's that Twilight, and don't forget the Twilight Zone...

  • I had no idea, I loved those descriptions. As for the mosquitoes at twilight, it ruins such a great time of day!

  • In reply to Kathy Mathews:

    Thanks for reading, Kathy! Aren't those descriptions from NOAA great? So glad you enjoyed them, too.

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