« 2018-06-25
Back to Home Page
2018-06-20 »

It's longer than you think it is
(permalink)

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


The web edition of KGB Report is published Monday-Thursday, except on holidays. Follow KGB Report and my personal account on Facebook for frequent daily updates.


Feedburner RSS Subscribe  Home   Commentwear   E-Mail KGB


Donate via PayPal


Older entries, Archives and Categories       Top of page


Like KGB Report on Facebook and follow us on Twitter

« 2018-06-25
Home Page
2018-06-20 »