Daylight

  Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime (and perhaps twilight). This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected from the Earth and terrestrial objects. Daytime is the period of time each day when daylight occurs. While perceived moonlight is in fact light from the sun reflected towards the Earth, and is hence to be considered "indirect sunlight", it is not considered daylight as it occurs outside of the hours that one would consider "daytime".

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Definition

Daylight is present, to some degree, whenever that 50% of the earth is in view of the sun, but the outdoor illuminance can vary from 100,000 lux for direct sunlight at noon, which may cause eye discomfort, to less than 5 lux for the thickest storm clouds with the sun at the horizon, which may make shadows from distant street lights visible. It may be darker under unusual circumstances such as a solar eclipse or very high levels of atmospheric smoke.

Daylight intensity in different conditions

Sky light intensity at night in various conditions is given for comparison.

Daylight intensity in the Solar System

Different bodies of the Solar System receive light proportionally to the square of their distance from Sun. A rough table comparing the amount of light received by each planet on the Solar System (and the dwarf planets Ceres and Pluto) follows (from data in [1]):

The actual brightness of daylight that would be observed at the surface depends also on the presence and composition of an atmosphere. For example Venus' thick atmosphere reflects up to 60% of the solar light it receives, so the actual illumination of the surface is comparable to that of Earth.

For comparison purposes, daylight on Saturn is somewhat slightly brighter than Earth daylight on the average sunset or sunrise. Even on Pluto the Sun would be still bright enough to almost match the average living room. To see the Sun shine as dim as the full Moon on the Earth, a distance of about 500 AU is needed: there is only a handful of objects in the solar system known to orbit farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.

Effects

Daylight is widely accepted to have a positive psychological effect on the human being, and consequently more cases of mental health problems are registered during the winter months than during the summer months due to the shortened periods of daylight. Cases of depression specifically linked to limited daylight are referred to as seasonal affective disorder.

Daylighting is lighting an indoor space with openings such as windows and skylights that allow daylight into the building. This type of lighting is chosen to save energy, to avoid hypothesized adverse health effects of over-illumination by artificial light, and also for aesthetics.

In recent years, work has taken place to recreate the effects of daylight artificially. This is however expensive in terms of both equipment and energy consumption and is applied almost exclusively in specialist areas such as filmmaking, where light of such intensity is required anyway.

See also

• Rights of Light
• Twilight
• Moonlight
• Daylight saving time

Notes

1. ^ Petzl reference system for lighting performance (html). Retrieved on 2007-04-24.
2. ^ Bunning, Erwin; and Moser, Ilse (Apr. 1969). "Interference of moonlight with the photoperiodic measurement of time by plants, and their adaptive reaction". Proceedings of the National Academy of Sciences of the United States of America 62 (4): 1018–1022. doi:10.1073/pnas.62.4.1018. Retrieved on 2006-11-10.