The time slot when the sun falls a mere 6 degrees or less below the horizon, at either dawn or dusk, defines a particular atmospheric condition. This is a period where the horizon line appears sharply drawn and the most luminous of the stars become perceptible, granted the sky exhibits favorable atmospheric characteristics, such as the absence of moonlight or other external light sources. Interestingly, it’s during this interval that one could typically continue with standard outside tasks.
The term also applies to the timeframe when the sun moves between 6 and 12 degrees beneath the horizon, again observable at either daybreak or sunset. A well-consolidated horizon can be noted and the silhouettes of items are potentially discernible without needing synthetic light sources. Everyday outdoor pursuits might become a challenge at this point, necessitating supplemental illumination.
Extending this further, consider the phase during which the sun is under the horizon by an angle ranging from 12 to 18 degrees, applicable both at sunrise and sunset. The contribution of the sun, during these hours, towards the lighting up of the sky, is practically non-existent, whether it’s early in the morning or later in the evening.
At the inception of morning astronomical twilight, as well as its conclusion in the evening, the light from the sky is incredibly faint and may be hard to discern. Life outside of the closed interiors becomes an interaction with the subtly-lit expanses of nature, a period that sky-watchers and astronomers relish and mark as a distinct, atmospheric phase.