Casting Light on Twilight Exploring the Diverse Shades of Dusk

By Carol Wolf

Sunrise and sunset hold a magical allure, marking the moments when the sun’s upper limb graces a level horizon. These instances represent the first appearance or final disappearance of the sun for observers at sea level and over a sea horizon. However, it is crucial to acknowledge that factors such as trees, hills, buildings, and observer elevation introduce slight variations in timing for other locations.

The twilight period, which precedes sunrise and follows sunset, is divided into three distinct stages: Civil Twilight, Nautical Twilight, and Astronomical Twilight. While the following descriptions pertain to evening twilight, they also apply in reverse to morning twilight.

CIVIL TWILIGHT:

Civil Twilight concludes when the sun rests 6° below the horizon. Beyond this point, ordinary outdoor activities become impractical without artificial light. Traditionally, streetlights were switched on at the end of Civil Twilight, a practice still observed in some rural areas. However, many regions now employ light meters for control, typically activating streetlights approximately 10 minutes before Civil Twilight concludes.

NAUTICAL TWILIGHT:

Nautical Twilight ends when the sun descends 12° below the horizon. At this stage, the horizon is no longer visible, except in the direction of sunset, where a faint twilight may persist. Nautical Twilight marks the onset of ideal conditions for astrophotography, except in western sky areas. For most practical purposes, it would be deemed “dark.”

ASTRONOMICAL TWILIGHT:

Astronomical Twilight concludes when the sun dips 18° below the horizon. By this point, residual sunlight falls below the threshold to illuminate the night sky.

In Western Australia, the duration of each twilight stage exhibits minimal yearly fluctuations, differing only by a few minutes. Roughly, Civil Twilight begins or ends approximately 25 minutes before sunrise or after sunset, with the other two stages preceding or following at intervals of approximately 30 minutes.

These descriptions apply to clear, normal conditions. However, heavily overcast skies can significantly diminish illumination. Thus, meteorological conditions must be taken into account. In the absence of meteorological variations, an assessment of illumination at a specific date, time, and location can be obtained by visiting the site on another day at the equivalent time before sunrise or after sunset.