Dusk

A hard science exists to explain the terms dusk and twilight.  The terms reflect the geometric dance of the planets.  “Civil twilight” describes both light and the space of time between when the Sun first touches the horizon at sunset and the precise moment when the geometric center of the disk reaches six degrees below the horizon.

That exact place, at six degrees below the horizon, is known as “civil dusk.”  Civil dusk, is a seemingly perfect time when shades of pink and yellow and orange and sometimes purple are sent glancing off cloud bottoms and painted across the expanse.  This is the time of a photographer’s dream.  And there is still enough light that we might run at full speed and not stumble on something unseen.

The next six degrees of light and time are known as nautical twilight.  This is when shadows around us connect together to form a whole.  The sky itself darkens and turns into a cobalt blue.  Upon reaching the next marker, at twelve degrees below the horizon, we have reached "nautical dusk."

Those of us still standing in place now enter “astronomical twilight,” which represents the final six-degree plunge into full darkness.  The plunge ends at "astronomical dusk."  More commonly known as night.

In the mountains, given the constant changes in elevation at all sides, the horizon is a tricky item to either see or define.  We must abandon science, and stand on the nearest rock for a sensory appraisal.  We may require something more romantic and less precise.  The famed Montana-born writer Norman Mclean wrote in A River Runs Through It about a certain light at the end of summer day in Montana:

 “Like many fly fishermen in western Montana where the summer days are almost Arctic in length, I often do not start fishing until the cool of the evening. Then in the Arctic half-light of the canyon, all existence fades to a being with my soul and memories and the sounds of the Big Blackfoot River and a four-count rhythm and the hope that a fish will rise.”

—Mitchell Hegman