2024, September 18:  Autumn’s Diminishing Daylight

September 18, 2024:  As Autumn nears, daylight noticeably decreases from week to week.  At the same time darkness, part of nighttime increases.

Autumn Leaves
Autumn Leaves

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by Jeffrey L. Hunt

Autumn Begins Soon

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Photo by Pixabay on Pexels.com

With the beginning of astronomical autumn imminent, study the effects of the changing of the seasons.  Autumn begins in four days at 7:44 a.m. Central Time.  The sun appears over Earth’s equator, heading southward.

In the simplest interpretation, daylight and nighttime are equal at 12 hours.  With the definition of sunrise and refraction of light, this occurs on the 25th.

The Chart

Autumn’s Diminishing Daylight
Image Caption – The lengths of daylight, darkness, and twilight are graphed from September 1, 2024, through April 1, 2025, for Chicago, Illinois (Data from the US Naval Observatory)

After the equinox, daylight decreases rapidly as nighttime increases.  A closer look at a table like one supplied in US Naval Observatory’s data, nighttime can be further divided into periods of twilight and darkness.

Data points, beginning on September 1, 2024, and ending April 1, 2025, are graphed on the accompanying chart showing the daily lengths of daylight, twilight and darkness.  Equinoxes, mid-season points, and solstice points are noted along with other seasonal features.

Twilight occurs after sunset or before sunrise, when the sun is within 18° of the horizon.  We see the sky near sunset or sunrise display colorful hues. For this study, twilight’s length is the sum of the length of predawn twilight and post sunset twilight.

When the sun is lower than 18° below the horizon, the sky is at its natural darkness.

The three values, daylight, twilight, and darkness, add to 24 hours.

For Other Cities

At Chicago’s latitude, daylight and darkness are equal on October 30, 2024, near autumn’s midpoint (November 6th).  This date of equal daylight and darkness varies by latitude. The difference is at its maximum at the winter solstice (December 21st).  By February 10th, values are equal again.

The interval when darkness is longer than daylight is 102 days long.  This period varies by latitude. At Cocoa, Florida’s latitude the period lasts 59 days; Tucson, Arizona, 80 days; Fayette, Missouri, 98 days; Seattle, Washington, 112 days; and Juneau, Alaska, 120 days.

It should be noted that the beginning and ending dates when darkness is longer than dates are different for the cities listed above.  The durations of the intervals are centered on the winter solstice.

Daylight Changing with the Season

During the cold season at more northerly latitudes, there is simply no daylight to save, especially if you have a traditional eight-hour job and a modest commute.  The sky is either dark in the morning or during the evening.  One end of the human activity period is without the sun.

More southerly latitudes do not have a daylight problem.  Cocoa, Florida has over 10 hours of daylight at the winter solstice, while Chicago has nine hours and eight minutes.

Watch the seasons change with the less daylight.  The sunlight does not seem as bright and certainly less intense, that is reflected in the temperature.  The hummingbirds that battled over the feeder have left.  Certainly, the first frost is on its way for suburban and rural areas. 

Have a wonderful fall while watching Venus appear higher in the evening western sky and eventually Jupiter and Mars are visible after sundown as well.

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