Today’s Wonder of the Day was inspired by Norah. Norah Wonders, “Why are people scared of the dark?” Thanks for WONDERing with us, Norah!
Are you scared of the dark? “Of course not!” you might say. Let’s be realistic, though. Maybe you don’t worry about ghosts in the closet or monsters under the bed. But you probably still get a little nervous when the Sun sets. As darkness creeps over the horizon, anyone can feel a bit spooked!
After all, it’s perfectly natural. It could be the fact that the lack of light hinders our senses. Perhaps it’s the spooky sounds that often come with the darkness. Or maybe it’s the mystery that we’ve learned to associate with twilight. There’s a reason scary movie scenes take place at night—that’s where danger lurks and the unknown can happen!
But today we’re going to let our curiosity take us fully into the dark. We’ll WONDER about twilight, dusk, and the science of these times of day. Some people use sunset, twilight, and dusk interchangeably. However, each of these terms has its own meaning.
Twilight is the time of day between daylight and darkness. It occurs twice each day—once between darkness and sunrise and again between sunset and darkness. Exactly what time each occurs depends on the time of year and your location. A weather website can tell you when sunrise and sunset will occur in your area each day.
Scientists who want to observe the stars need full darkness. So they came up with more strict definitions of the stages of twilight. These are based on a measurement (in degrees) of how far below the horizon the Sun has traveled.
The first stage of twilight is called civil twilight. This starts just after the Sun passes the horizon. It lasts until the Sun is six degrees below the horizon. At this time of day, there’s still enough light to see, but everyone knows darkness will soon arrive. At civil twilight, drivers turn on their headlights. You may also notice streetlights turn on.
When the Sun is between 6 and 12 degrees below the horizon, it’s nautical twilight. During this time, it gets fairly dark outside. It becomes difficult to read outside without artificial light. This period ends when the distant line of the sea horizon can’t be distinguished from the background of the sky.
The darkest part of twilight—called dusk—is when the Sun is between 12 and 18 degrees below the horizon. Dusk is also known as astronomical twilight. Once the Sun drops farther than 18 degrees below the horizon, it’s officially fully dark. As this final stage of twilight passes, all the remaining light fades from the sky until it’s completely dark.
Once it’s fully dark, astronomers can begin their nightly observations. Of course, the position of the Sun is just one factor that affects their ability to see the stars. If it’s cloudy or if there is ambient light from artificial lights, such as streetlights in large cities, it may be impossible to see much in the night sky.
Do you ever get spooked at dusk? Or are you more excited by the mysterious mood in the air? Whatever your feelings as the Sun goes down, we’re sure you’re thankful each time it rises again!
Standards: CCRA.L.3, CCRA.L.6, CCRA.R.1, CCRA.R.2, CCRA.R.4, CCRA.R.10, CCRA.SL.1