Are you scared of the dark? “Of course not!” you might say. But let’s be realistic. Even if you don’t worry about ghosts in the closet or monsters under the bed, you probably still get a little nervous when the sun sets and darkness creeps over the horizon.
After all, it’s perfectly natural. All of us from time to time feel a cold chill when light gives way to night. Whether it’s the fact that our senses are hindered by the lack of light, the spooky sounds that often accompany the darkness, or the mysteriousness that we’ve learned to associate with twilight, there’s a reason that scary scenes in movies take place at night — because 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 as we WONDER about twilight, dusk, and the science of these times of day. Although some people use sunset, twilight, and dusk interchangeably, scientists know that each of these terms has a special meaning.
Twilight is the time of day between daylight and darkness. Twilight occurs twice each day, between darkness and sunrise and again between sunset and darkness. Exactly what time of day these occur varies by the time of year and your geographical location. You can consult a weather website to determine when sunrise and sunset will occur locally in your area today.
Scientists, such as astronomers, who want to observe the stars need full darkness. So they came up with more strict definitions of the stages of twilight. These definitions depend upon a measurement (in degrees) of how far below the horizon the sun has traveled.
The first stage of twilight is called civil twilight. This stage starts just after the sun passes the horizon and lasts until the sun is 6 degrees below the horizon. During civil twilight, there’s still enough light to see, but everyone knows darkness will soon arrive. At civil twilight, drivers turn on their headlights and you may notice streetlights and other lights operated by light sensors turn on.
When the sun is between 6 and 12 degrees below the horizon, this period of time is known as nautical twilight. During this time, it gets fairly dark outside. You can no longer read outside without artificial light. Nautical twilight 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 astronomical twilight and also dusk — is when the sun is between 12 and 18 degrees below the horizon. 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 vestiges of light eventually fade 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.