The Types of Twilight

    On a recent backpacking trip along the Oregon Coast, I learned that there are multiple phases of the everyday lessening of daylight as the Sun sets. We were looking out over the Pacific Ocean, admiring the view from atop a cliff. A lighthouse stood on a rock a little off the coast. The light from the Sun continued long after it had disappeared. This first, brightest time after sunset is called Civil twilight, and occurs when the Sun has set, but its center is at most six degrees below the horizon. On average, it lasts about 30 minutes after sunset. (Side note: it is legal to leave your car's headlights off for the first 30 minutes after sunset as well) We could still hike around and see the trail with some clarity. We found a concrete bunker that was one of the many installations the US army had built during World War II as protection from possible mainland invasion. 

    On our way back to our campsite the trail was a lot darker and a little more difficult to discern tripping hazards. I noticed a decrease in both color and visual definition. We were entering Nautical twilight. The first stars began to appear when the Sun was between six and 12 degrees below the horizon, which was still visible. The temperature began to drop noticeably. This time of year, nautical twilight lasts about 40 minutes. 

    We headed back to camp to start a fire. By the time it was joyfully crackling away, most of the daylight was gone. This was Astronomical twilight, which means the Sun was between 12 and 18 degrees below the horizon. It usually lasts around 30 minutes as well depending on the time of year. If we had a telescope along (unlikely on a backpacking trip, but who knows?) we could begin to discern galaxies and nebulae. Beyond this point it was difficult to discern when astronomical twilight ended and true night began. We were mostly staring into the fire.  

The Pale Blue Dot

In 1990, at the suggestion of astronomer Carl Sagan, NASA used the Voyager 1 space probe to take a picture of Earth as the probe was leaving the Solar System. From 3.7 billion miles away, the Earth appeared as a pale blue dot against the vastness of space, among bands of light reflected by the camera. During a lecture at Cornell University in 1994, Sagan shared his thoughts about the photograph and its larger implications:

"We succeeded in taking that picture [from deep space], and, if you look at it, you see a dot. That's here. That's home. That's us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives. The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every inventor and explorer, every teacher of morals, every corrupt politician, every superstar, every supreme leader, every saint and sinner in the history of our species, lived there on a mote of dust, suspended in a sunbeam. 

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that in glory and in triumph they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of the dot on scarcely distinguishable inhabitants of some other corner of the dot. How frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds. Our posturings, our imagined self-importance, the delusion that we have some privileged position in the universe, are challenged by this point of pale light. 

Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity – in all this vastness – there is no hint that help will come from elsewhere to save us from ourselves. It is up to us. It's been said that astronomy is a humbling, and I might add, a character-building experience. To my mind, there is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly and compassionately with one another and to preserve and cherish that pale blue dot, the only home we've ever known."

T Rex, you gots to chill!

Photo by Zach Reynolds/Royal Gorge Dinosaur Experience