2019, September 23: Equinox: Equal Night or Equal Day and Night



(Image Credit: NASA)

The sun reaches the Autumnal Equinox (as considered from the Northern Hemisphere) on September 23, 2019 at 2:50 a.m.  CDT. 

Since our planet is tilted about 23.5 degrees, the sun appears higher in the sky during the summer season and lower during the winter.  The sun’s rising and setting positions change throughout the seasons as well.   During late March and late September, the sun rises at the east cardinal point and sets at the west cardinal point.  Its arc across the sky is halfway between the extremes of Summer and Winter.

The sun’s light shines most directly at the equator.  Those folks living there see the sun pass overhead.  Noon is nearly shadowless. because of the sun’s path as seen from the equatorial region.

The day is called “Equinox,” sometimes translated as “equal night.”  The sun is in the sky for approximately 12 hours from the equator to higher latitudes.  Sometimes we tell school children that we have “12 hours of daylight and 12 hours of night.”

Consider this: During a 24-hour period, there are three cycles of light.  Daytime, the period when the sun is above the horizon and brightly illuminates the ground.  Dark, the period when the sky is completely without sunlight and twilight and it is as dark as it gets naturally.  Twilight, the period when the sun is below the horizon, but it illuminates the sky.  As the sun sets, the sky is bright and it’s easy to see terrestrial features.  This is known as Civil Twilight.  During this twilight phase, streetlights turn on.  If you know where to look, Venus and Jupiter can be seen

The sky continues to darken.  From a location with a good horizon, Nautical twilight occurs when the horizon is barely visible.  This is easier to at sea and so the name.  Brighter planets and stars become visible.  This occurs about 60 minutes after sunset.  Then the sky continues to darken until the last light shines from the western horizon.  This is Astronomical Twilight.  This takes about 90 minutes.  Until the process reverses in the morning, the sky is dark, as dark as it gets naturally.

So when I look at this for my latitude, on September 23, daylight is 12 hours, 8 minutes long; twilight, 3 hours, 7 minutes; and darkness, 8 hours, 45 minutes.

At my latitude “Equal Darkness,” when the daylight hours are equal to the dark hours, is October 30. Daylight and darkness are equal at 10 hours, 26 minutes. Twilight takes up the balance that is divided between the pre-sunrise hours and post-sunset hours.


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