When the sun is just below the horizon, we often find ourselves immersed in a state of illuminated darkness. It's a part of our day where we can still see our way outside. Many people drive to work in the early morning or come home in the nighttime and children playing outside are called in to dinner or woken up for school the next morning. It’s a part of the day we all find as a comforting end or beginning. A passage from one day to a new day. During the twilight hours, we all go about our business without a single thought of how and why this occurs. Unaware, that if we didn’t have an atmosphere, twilight wouldn’t exist.
A beam of light passing through a substance is called transmitted light. When that light is transmitted through varying densities, it either speeds up or slows down. When light hits a dense substance at an angle, it bends. The bending of light is called refraction. Because our atmosphere is so thick with varying densities, the light that is transmitted by our sun bends, slows down and speeds up as the sun carries out its rotation across the sky. When the sun is at different angles in the sky, it can change in light intensity or directness. At twilight, the sun sinks below the horizon. At this particular angle, the light is no longer being directly transmitted because it is blocked by the horizon. So, how is it we can still see light at this time of day?
Due to refraction - the bending of light through our dense atmosphere - the light transmitted from the sun can still reach that part of the world. The more atmosphere the light has to penetrate through, the more it gets refracted. When the sun is below the horizon, light has to penetrate through the atmosphere at it thickest. Therefore by the time the light reaches your area before or after sunset, it has been bent around the circumference of the sun and angled toward you. This is how it still seems like it's partially light out after the sun dips below the horizon.
When light travels through space, it is travelling through a vacuum. Space is a vacuum because it is lacking in matter, density, and pressure. When light travels through a vacuum it has nothing to slow it down or bend it. If our atmosphere didn’t exist, it would just be replaced with outer space. There would be no bending of light through different densities. Therefore, once the sun sets behind the horizon, it becomes completely dark. Twilight would not exist.
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.