Compared to the near-IR night sky, the optical night sky is relatively free of emission lines towards bluer wavelengths with the strong neutral oxygen lines (see Figure below) being the exception. Moon light does affect the sky background (dependent on phase) by reflecting sunlight onto molecules in the atmosphere thereby increasing the sky background. Even in the absence of the Moon, Zodiacal light which is sunlight reflected off dust grains located in the inner solar system can also increase the brightness off the sky background. Zodiacal light predominantly affects observations during dark time with objects located towards the ecliptic and is most dominant in the V-band. The zodiacal light is one of the major contributors to the total HST sky background, together with Earthshine light and geocoronal line emission (originating from photochemical reactions involving hydrogen and oxygen atoms in the Earth’s exosphere) (Giavalisco et al. 2002). The geocoronal line emission, also known as airglow, changes on timescales of years to minutes getting systematically fainter as the night progresses (Krisciunas 1997).
