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World map showing the areas of the Earth receiving daylight around 13:00 UTC, April 2nd.

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. Sunlight scattered or reflected from objects in outer space (that is, beyond the Earth's atmosphere) is generally not considered daylight. Thus, moonlight is never considered daylight, despite being "indirect sunlight". Daytime is the period of time each day when daylight occurs.

[edit] Definition

Daylight is present at a particular location, to some degree, whenever the sun is above the horizon at that location. (This is true for slightly more than 50% of the Earth at any given time. For an explanation of why it is not exactly half, see the section labeled "introduction" on the "day" article). However, the outdoor illuminance can vary from 120,000 lux for direct sunlight at noon, which may cause eye pain, to less than 5 lux for thick storm clouds with the sun at the horizon (even <1 lux for the most extreme case), 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 (See New England's Dark Day), dust,^[1] or volcanic ash.^[2]

[edit] Daylight intensity in different conditions

Artificial image showing a nightfall over Europe and Africa. The solar terminator is shown for UTC July 5, 2005 18.45.00

For comparison, nighttime illuminance levels are:

For a table of approximate daylight intensity in the Solar System, see sunlight.

[edit] Effects

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Daylight in January. Some people choose not to live in polar regions because of the extreme differences in the amount of daylight in summer and winter.

Daylight is widely accepted to have a positive psychological effect on the human being^{[citation needed]}, 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^{[citation needed]}. 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. Artificial lighting energy use can be reduced by simply installing fewer electric lights because daylight is present, or by dimming/switching electric lights automatically in response to the presence of daylight, a process known as daylight harvesting.

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.

[edit] See also

• Twilight
• Moonlight
• Daylight saving time
• Daylighting
• Right to light

[edit] Notes

[edit] External links

• Royal Institution of Chartered Surveyors - Rights to Light Determination Homepage
• Daylight Chart shows sunrise and sunset times in a chart, for any location in the world.
• Daylight Google Map and Satellite View