2021, May 21: Twilight Lengthens

May 21, 2021:  At the weather warms, daylight and twilight lengthen to diminish nighttime hours.  As the summer solstice approaches far northern latitudes do not have periods of darkness.  From the most northern latitudes, the sun does not set – the Land of the Midnight Sun.

Twilight is the period of time before or after sunset when the upper atmosphere is illuminated by the sun that is below the horizon.

by Jeffrey L. Hunt

At this time of the year, children object about preparing to sleep for the night when “there’s light outside!”

At the mid-northern latitudes, daylight lengthens as the weather warms.  From Chicago at the beginning of May, daylight was a few minutes longer than 14 hours.  Complete natural darkness was about 6 hours, 20 minutes long.  The remainder of the 24-hour period, over 3 hours, 30 minutes, was twilight, split between the period before sunrise and after sunset.

Nighttime is defined to be when the sun is not in the sky – twilight and darkness together.

Twilight is from sunlight illuminating the upper atmosphere after sundown.

By May’s end, daylight lasts, 15 hours; twilight, 4 hours, 12 minutes; and darkness, 4 hours, 48 minutes.

By mid-June, daylight lengthens another 13 minutes and twilight adds another 11 minutes.  So that’s 19 hours, 36 minutes where the sky has some natural illumination.  The sum of the two periods is longer for latitudes farther north.

During June, from north of latitude 48.6°, twilight does not end after sundown.  There’s only daylight and twilight. There is no darkness.  From north of latitude 65.7° only daylight occurs – the Land of the midnight sun, where the sun does not set.

On the summer solstice, June 20th this year in Chicago, morning twilight begins at 3:04 a.m. CDT.  Sunrise occurs at 5:16 a.m. CDT, followed by sundown at 8:29 p.m. CDT.  Evening twilight ends at 10:41 p.m.  For locations farther west in their time zones, the times could shift by and hour or more.

For Sawyer, Michigan, about 50 miles across Lake Michigan and a time zone away, morning twilight begins at 3:59 a.m. EDT, while evening twilight ends at 11:37 p.m. EDT.

The result of longer daylight during late spring and early summer also adds longer periods of twilight and shorter darkness periods, the opposite of what the northern hemisphere experiences near the winter solstice.

“Bedtime is bedtime,” is the reply to those youngsters wanting to stay awake until the sky is dark.


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Categories: Astronomy, Sky Watching

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