Sky brightness

During daytime, when the Sun is above the horizon, the direct scattering of sunlight is the overwhelmingly dominant source of light.

During twilight (the duration after sunset or before sunrise until or since, respectively, the full darkness of night), the situation is more complicated, and a further differentiation is required.

Airglow is the collective name of the various processes in the upper atmosphere that result in the emission of photons, with the driving force being primarily UV radiation from the Sun.

How sodium gets to mesospheric heights is not yet well understood, but it is believed to be a combination of upward transport of sea salt and meteoritic dust.

The effect is however not noticeable to the human eye, since the glare of directly scattered sunlight outshines and obscures it.

Zodiacal light varies quite a lot in intensity depending on the position of the Earth, location of the observer, time of year, and composition and distribution of the reflecting dust.

After the Sun has also set for these altitudes at the end of nautical twilight, the intensity of light emanating from earlier mentioned lines decreases, until the oxygen-green remains as the dominant source.

When astronomical darkness has set in, the green 557.7 nm oxygen line is dominant, and atmospheric scattering of starlight occurs.

(The S10 unit is defined as the surface brightness of a star whose V-magnitude is 10 and whose light is smeared over one square degree, or 27.78 mag arcsec−2.)

The intensity of zodiacal light depends on the ecliptic latitude and longitude of the point in the sky being observed relative to that of the Sun.