Geometry
Civil, nautical and astronomical twilight use the Sun’s center at roughly −6°, −12° and −18° altitude. These geometric conventions make planning repeatable.
What does the Sun’s altitude mean for the sky you photograph?
Move the Sun below the horizon and connect the −6°, −12° and −18° twilight boundaries to practical changes in sky brightness. These are geometric definitions, not fixed exposure recipes.
Solar altitude—
Twilight stage—
Why
Civil twilight ends at −6°, nautical twilight at −12°, and astronomical twilight at −18° solar elevation. The boundaries describe solar geometry; actual sky brightness also depends on atmosphere and local conditions.
In the field
Watch the histogram and foreground, not just the clock. As twilight fades, shutter time usually lengthens and the balance between sky, artificial light and foreground changes quickly.
Starting point
Use solar altitude to choose the kind of light you want, then meter the real scene. For faint Milky Way work, −18° is a useful darkness boundary, but Moon and light pollution can still dominate.
Try it
Move from −4° to −10° to −19°. Notice how the planning question changes from balancing twilight to protecting faint night-sky contrast.
Source & limitation
Twilight thresholds follow NOAA definitions. Atmospheric refraction, aerosols, clouds, Moon, airglow and light pollution are not modeled in this teaching view.
Why twilight is defined by solar altitude
Atmosphere
Real sky brightness does not jump at those angles. Aerosols, humidity, clouds, elevation and light pollution change how quickly the sky darkens and how color develops.
Photography meaning
Higher twilight can retain landscape detail and color; deeper twilight increases contrast for faint stars. The useful point depends on whether your subject is foreground, Moon, bright stars or faint Milky Way structure.
Field workflow
Use solar altitude to predict the phase, then judge the real sky. Check histogram, foreground readability, Moon altitude and local light dome before deciding exposure strategy.