2021, October 29:  Equal Daylight, Equal Darkness

October 29, 2021:  Today is the date for equal daylight and equal darkness for about 42° north latitude. This is not to be confused with the autumnal equinox.

Astronomy
Sunrise from the International Space Station. (NASA Photo)

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by Jeffrey L. Hunt

Today at 42° north, daylight and darkness are equal.  This is not to be confused with the equal daylight and equal nighttime that occurs near the equinox.

Daily notes for October 2021

This concept is considered from the three phases of light that occur on the planet during a twenty-four-hour cycle.  When the sun is in the sky, we call this daytime.  The solar system’s central star brightly illuminates the ground, even on cloudy days.

Nighttime, the time when the sun is not in the sky can be broken into two parts, twilight that is divided into that which occurs before sunrise and for a period after sunset.

Darkness is the final part.  This occurs between the end of evening twilight and the beginning of morning twilight.  The sky is as dark as it gets and there is no further progression.

Metaphors, such as “Black as midnight” or “Darkest before the dawn” are inaccurate in the reality of nature’s light.

From Chicago’s latitude, daylight and darkness last for 10 hours, 26 minutes.  After this date and until the equal daylight/darkness date of February 10, darkness is longer than daylight.  This occurs for 104 days.

From November 26 through January 15, daylight is less than 9 hours, 30 minutes, for 59 days.

In the US as Daylight Saving Time is coming to an end on November 7, the clocks are returned to standard time.

During standard time, clocks more closely match the sun’s place in the sky.  The sun is nearly south when the clock reads noon.  During daylight time, the clock is an hour ahead of the sun’s place in the sky. During these days and months, extra daylight is not created.  It is merely shifted to the evening hours, when families are reunited after a day at school or work.  Outdoor activities can occur without artificial illumination.

Likely the pro year-round-daylight-time crowd will begin its crowing about changing clocks soon.  A year ago, an article appeared here about why clocks should not bounce back and forth between standard time and daylight time.  Standard Time should govern the year, and localities can make their own decisions about how to handle their clocks. 

National legislation regulating whether clocks should be changed has little or no value.

Let’s look at some daylight and darkness summaries from information pulled from the U.S. Naval Observatory’s Multi-Year Interactive Computer Almanac (MICA) program.  The software provides data, such as sunrise/sunset times, moon rise/set times, moon phases, planet conjunctions, and much more.  Several locations were selected, as well as daily sunrise and sunset data, and the times of the beginning of morning twilight and the end of evening twilight.  The length of daylight was calculated as well as the length of darkness – after the end of evening twilight and before the beginning of morning twilight.

In the table, the date of the equal daylight/darkness is noted, along with its length.  Further the shortest length of daylight, at the winter solstice is listed.  Finally, the number of days darkness exceeds daylight is in the final column.

Some examples:

CityLatitudeEqual Daylight/Darkness Date (2021)Daylight (hr:min)Shortest Daylight (hr:min)Days
Cocoa, FL28.3°November 2110:3810:2059
Tucson, AZ32.2°November 1110:3610:0280
Fayette, MO39.1°November 210:299:2498
Chicago, IL41.8°October 2910:269:08104
Juneau, AK58.3°October 229:486:23120

It is clear that more southerly latitudes, have shorter durations of lengthy darkness.  The daylight is longer and the loss of daylight during the duration is shorter.  Cocoa, FL only loses 18 minutes of daylight during their short duration of more darkness.  It starts the “dark season” with more daylight than locations farther north and it loses less daylight.  Perhaps, after all, the “Sunshine State” moniker is more than an advertising campaign.

On the other extreme, Juneau, AK starts with less daylight, when the length of daylight equaled that of darkness on October 22.  The latitude loses 3 hours, 25 minutes of daylight until the solstice. The location takes longer to recover to the equal daylight/darkness date.

On the other hand, beginning April 11 (without DST), the sun rises in Juneau before 5 a.m.  By the summer solstice it rises nine minutes before 3 a.m.

Additionally, beginning April 27 and lasting until August 15, there is no darkness in Juneau.  Twilight occurs after sunset, until sunup the next morning. At the solstice the sun is in the sky for 18 hours, 17 minutes.  The remainder of the “night” is bright twilight.  Clearly there’s no sense for daylight time in Alaska.  The state has an overabundance of sunlight during spring and summer and none to save during the remainder of the year.

Florida is leading the way in a national directive for year-round Daylight Saving Time. The table indicates Florida’s short duration of shorter daylight and minimal loss of daylight.  Year-round daylight time makes no sense there. There is ample daylight for some outdoor activities after sunset during the “dark” season.

North of America’s population center (the Fayette, MO data), the length of daylight falls off dramatically and the duration of short daylight lasts longer.  Clearly there is no daylight to save.  As one of my friends said regarding this issue, “The blanket for the bed is clearly too short on one end.”

Daylight time makes little sense for the northern latitudes of North America.  Farther south, there is ample daylight for outdoor activities across the entire year.

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