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.
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