Twilight is defined by a period of time after sunset or before sunrise that the sky remains illuminated enough to still be able to carry on outdoor activities without the help of artificial light. It is caused by the refraction of light through the densities of our atmosphere from transmission of sunlight. Dawn is the term for twilight that occurs before the sun rises in the morning and dusk is the term for after sunset. Depending on the season or latitude, twilight can last anywhere from 70 to 100 minutes. In the mid latitudes, twilight tends to be longer during the summer and shorter in the winter months.
It isn’t common knowledge, but there are three different stages of twilight. The National Weather Service (NWS) defines these stages as civil, nautical and astronomical twilight. Each stage is determined by the sun’s solar angle when it’s below the horizon. Civil twilight occurs from the time the sun sets or rises and when the sun’s geometric center is at a six-degree angle below the horizon. During this time, twilight is characterized by the appearance of larger bright planets and stars in the sky. Some left over pinks and oranges displayed from the sunset can still paint the sky. Artificial lighting isn’t needed to see your way around outside. The horizon is still able to be seen.
Nautical twilight is when the sun’s angle is between six and twelve degrees below the horizon. During this time, you will be able to see more light from stars and planets through the naked eye. Most, if not all the displays of color from the sunset have disappeared. The horizon is still visible but artificial lighting may be needed to continue activities. The ability to start seeing stars during this stage of twilight is helpful to sailors who start setting course by the position of the stars. This is how nautical twilight gets its name.
Astronomical twilight is the last stage of twilight before it gets completely dark. Here, the sun's angle is between twelve and eighteen degrees below the horizon. Most celestial stars and objects can be seen in the sky at this point. During this final stage, it is hard to distinguish a horizon and artificial light pollution in populated areas tends to block on the remaining light of the day. This stage gets its name from astronomers observing objects in the sky. Brighter objects, such as stars and planets, can begin to be viewed by astronomers at this time. Other objects, such as nebula and galaxies, are easier to see during complete nighttime when the sun goes further below 18 degrees.
There is so much to know about twilight and why it exists. It’s interesting to know there are three stages of twilight that are defined by different sun angles that demonstrate different characteristics before it becomes full night. Next time you find yourself driving to work in the morning or calling the kids inside for the night, reflect on the stage of twilight you are experiencing. It’s good to take a breath and reflect on our surroundings every once in a while.
To continue reading about twilight in part two and for more weather education, click here.
© 2020 Meteorologist Alexandria Maynard
Ahrens, C. Donald. Workbook/Study Guide to Accompany Meteorology Today: an Introduction to Weather, Climate, and the Environment. Brooks/Cole, CengageLearning, 2009.
US Department of Commerce, and Noaa. “Definitions of Twilight.” National Weather Service, NOAA's National Weather Service, 16 Mar. 2015, www.weather.gov/fsd/twilight.
“What Is Astronomical Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/astronomical-twilight.html.
“What Is Civil Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/civil-twilight.html.
“What Is Nautical Twilight?” Timeanddate.com, www.timeanddate.com/astronomy/nautical-twilight.html.
“What Is Refraction of Light?” Timeanddate.com, www.timeanddate.com/astronomy/refraction.html.