There comes a period of time during sunrise or sunset when the sun is just 6 degrees below the horizon. At this time, the outline of the horizon is distinctly visible assuming atmospheric conditions are ideal, without intrusion of moonlight or other external light sources. One could comfortably engage in typical outdoor activities under this type of faint natural lighting.
Moreover, the range between 6 and 12 degrees below the horizon presents a distinctly different phase of sunrise or sunset. During this stretch, the horizon remains noticeably noticeable, and it may be possible to distinguish the silhouettes of objects without the need for artificial lighting. However, usual outdoor activities may become challenging without additional illumination.
Moving forward, as the sun descends to an angular distance of 12 to 18 degrees below the horizon, its contribution to the brightness of the sky dwindles. Its impact on sky illumination is no longer notable before this time in the morning, or after this equivalent period in the evening.
The onset of what’s known in astronomy as ‘twilight’ occurs in the wee morning hours. Conversely, a similar twilight phase also concludes the lit portion of the day in the evening. The level of illumination in the sky during these times is incredibly low, and might escape the detection altogether.
A meaningful comparison can be made between ‘Civil Sunset’ and ‘Civil Sunrise’. Specifically, the times of Civil Sunset and Civil Sunrise help bracket the time frame that is generally viewed as acceptable for conducting outdoor activities without the aid of artificial lights.
Going further, it’s worth looking at two additional comparisons: the moment of ‘Actual Sunset’ and the moment of ‘Actual Sunrise’. These times indicate the precise moments when the sun dips below the horizon in the evening and emerges above the horizon in the morning, respectively.
In addition to understanding these stages of sunrise and sunset, there’s also value in observing the daily shift in daylight duration. The difference in daylight hours between an individual day and the next is also typically noted, providing a broad sense of how much usable daylight we can expect on any given day.