City Creek Nature Notes – Salt Lake City

September 21, 2016

September 20th

Filed under: Astronomy, Colors, Light, Seasons, Twilight — canopus56 @ 3:10 am

Twilight

7:45 p.m. It is dusk. Twilight is becoming shorter as compared to its longest duration in the summer. In our scientific age, these things have precise astronomical definitions. Civil twilight is the time between when the upper limb of the Sun first disappears below the horizon and when the Sun is 6 degrees below the horizon. Astronomical twilight is when the Sun’s disk falls below 12 degrees of the horizon. But such things also have older commonplace and more human definitions. Civil twilight is the time after sunset that one can comfortably read a book. Astronomical twilight is when the Sun’s rays reflecting off the upper atmosphere no longer obscure stars. After that time, the maximum number of stars are visible to the naked eye until the next morning. In Salt Lake City at the vernal equinox in March, the duration of civil twilight is about 28 minutes. At the summer solstice it increases to about 34 minutes. At the autumnal equinox which occurs on September 22nd, twilight again shortens to 27 minutes and then increases to 33 minutes at the winter solstice. But our perception of the change in the duration of twilight is overwhelmed by the more rapid and larger change in the length of days as the seasons change. Between the summer and winter solstices, the length of the days varies by 8 hours, and changes by 3 or 4 minutes per day.

Our lighted cities make us insensitive to the length of civil twilight because we no longer sit reading at dusk, and we do not go through a ritual that governed the lives of people for the previous 5,000 years: lighting a candle as it becomes too dark to read. In the canyon, one can reconnect with these changes in light. Tonight in the canyon, the twilight sky turns from blue to subtle shades of purple, blue gray and slate gray. Between civil and astronomical twilight the canyon grows quiet.

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