Daylight Saving Time

System for uniformly advancing clocks, especially in summer, so as to extend daylight hours during conventional waking time. In the Northern Hemisphere, clocks are usually set ahead one hour in late March or in April and are set back one hour in late September or in October. In the U.S., Daylight Saving Time begins on the second Sunday in March and ends on the first Sunday in November. In most of the countries of western Europe, it starts on the last Sunday in March and ends on the last Sunday in October.

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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.

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 , or volcanic ash .

Daylight intensity in different conditions

Illuminance	Example
120000 lux	Brightest sunlight
110000 lux	Bright sunlight
20000 lux	Shade illuminated by entire clear blue sky, midday
10000 - 25000 lux	Typical overcast day, midday
<200 lux	Extreme of darkest storm clouds, midday
400 lux	Sunrise or sunset on a clear day (ambient illumination).
40 lux	Fully overcast, sunset/sunrise
<1 lux	Extreme of darkest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

Illuminance	Example
<1 lux	Moonlight
0.25 lux	Full Moon on a clear night
0.01 lux	Quarter Moon
0.001 lux	Moonless clear night sky
0.0001 lux	Moonless overcast night sky
0.00005 lux	Starlight

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

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
• Daylighting

Notes

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
• Daylight Phonebook of the World Map