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It's longer than you think it is
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Published Thursday, June 21, 2018 @ 11:39 AM EDT
Jun 21 2018

Here in Pittsburgh on the summer solstice, the elapsed time from sunrise to sunset today is 15 hours, 3 minutes and 49 seconds. Sunrise was at 5:49 am; sunset will be at 8:53 pm. Solar noon, when the sun reaches its highest point in the sky (73.0°) will be at 1:21 pm. While it's the longest day of the year, the latest sunset will not occur until next Thursday, June 28 at 8:54:28 pm.

Sunrise and sunset times are misleading. It's not like flicking a light switch. The sky gets lighter before rising above the horizon at sunrise, and stays light later than when the sun dips below the horizon at sunset... this period is called twilight, and includes dawn and dusk.

Twilight is the time between day and night when the Sun is below the horizon but its rays still light up the sky due to sunlight scattering in the upper atmosphere.

There are three phases of twilight: civil, nautical, and astronomical.

Morning civil twilight begins when the geometric center of the sun is 6° below the horizon and ends at sunrise. Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6° below the horizon. Under clear weather conditions, civil twilight approximates the limit at which solar illumination suffices for the human eye to clearly distinguish terrestrial objects. Enough illumination renders artificial sources unnecessary for most outdoor activities. Civil twilight in Pittsburgh began today at 5:17 am, 32 minutes before sunrise; it ends at 9:27 pm, 34 minutes after sunset.

So we actually have an hour more daylight for outdoor activities than the sunrise and sunset times would suggest.

Nautical dawn is the moment when the geometric center of the Sun is 12 degrees below the horizon in the morning. It is preceded by morning astronomical twilight and followed by morning nautical twilight. Nautical dusk is the moment when the geometric center of the Sun is 12 degrees below the horizon in the evening. It marks the beginning of evening astronomical twilight and the end of evening nautical twilight. Sailors can take reliable star sightings of well-known stars, during the stage of nautical twilight when they can distinguish a visible horizon for reference. Under good atmospheric conditions with the absence of other illumination, during nautical twilight, the human eye may distinguish general outlines of ground objects but cannot participate in detailed outdoor operations.

Morning astronomical twilight (astronomical dawn) begins when the geometric center of the sun is 18° below the horizon in the morning and ends when the geometric center of the sun is 12° below the horizon in the morning. Evening astronomical twilight begins when the geometric center of the sun is 12° below the horizon in the evening and ends (astronomical dusk) when the geometric center of the sun is 18° below the horizon in the evening. In some places- away from urban light pollution, moonlight, auroras, and other sources of light- where the sky is dark enough for nearly all astronomical observations, astronomers can easily make observations of point sources such as stars both during and after astronomical twilight in the evening and both before and during astronomical twilight in the morning. However, some critical observations, such as of faint diffuse items such as nebulae and galaxies, may require observation beyond the limit of astronomical twilight. Theoretically, the faintest stars detectable by the naked eye (those of approximately the sixth magnitude) will become visible in the evening at astronomical dusk, and become invisible at astronomical dawn. However, in other places, especially those with skyglow, astronomical twilight may be almost indistinguishable from night. In the evening, even when astronomical twilight has yet to end and in the morning when astronomical twilight has already begun, most casual observers would consider the entire sky fully dark. Because of light pollution, observers in some localities, generally in large cities, may never have the opportunity to view even fourth-magnitude stars, irrespective of the presence of any twilight at all, and to experience truly dark skies.

(Sources: Wikipedia, timeanddate.com)


Categories: Astronomy, Solstice, Summer, The Daily KGB Report, Twilight


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Quotes of the day: Summer
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Published Sunday, May 24, 2015 @ 10:13 PM EDT
May 24 2015

A man says a lot of things in summer he doesn't mean in winter.
-Patricia Briggs

Ah, summer, what power you have to make us suffer and like it.
-Russell Baker

Every man makes his own summer. The season has no character of its own, unless one is a farmer with a professional concern for the weather.
-Robertson Davies

How ungenerously in later life we disclaim the virtuous moods of our youth, living in retrospect long, summer days of unreflecting dissipation.
-Evelyn Waugh

I almost wish we were butterflies and liv'd but three summer days- three such days with you I could fill with more delight than fifty common years could ever contain.
-John Keats

I know I am but summer to your heart, and not the full four seasons of the year.
-Edna St. Vincent Millay

I was thinking that we all learn by experience, but some of us have to go to summer school.
-Peter De Vries

In the depth of winter I finally realized that there was in me an invincible summer.
-Albert Camus

Many a person has been saved from summer alcoholism, not to mention hypertoxicity, by Dostoyevsky.
-Roy Blount, Jr.

Memory can glean, but never renew. It brings us joys faint as is the perfume of flowers, faded and dried, of the summer that is gone.
-Henry Ward Beecher

One swallow does not make a summer, nor does one day; and so too one day, or a short time, does not make a man blessed and happy.
-Aristotle

People wait all week for Friday, all year for summer, all life for happiness.
-Unattributed

Rest is not idleness, and to lie sometimes on the grass under the trees on a summer's day, listening to the murmur of water, or watching the clouds float across the sky, is by no means a waste of time.
-John Lubbock

Sound loves to revel in a summer night.
-Edgar Allan Poe

Summer will end soon enough, and childhood as well.
-George R.R. Martin

The climate is just like it always was. Hot in the summer, cold in the winter and brainless in Washington.
-Steve Goddard

The quarrels of lovers are like summer showers that leave the country more verdant and beautiful.
-Susanne Necker

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(May 25 is also the birthday of Bennett Cerf and Ralph Waldo Emerson.)


Categories: Quotes of the day, Quotes on a topic, Summer


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