daylight saving time (DST), time observed when clocks and other timepieces are set ahead so that the sun will rise and set later in the day as measured by civil time. The amount of daylight on a given day of the year at a given latitude is fixed, but over the year the hours of sunrise and sunset vary from day to day. During the summer months, the sun rises earlier and sets later and there are more hours of daylight. If clocks and other timepieces are set ahead in the spring by some amount (usually one hour), the sun will rise and set later in the day as measured by those clocks. This provides more usable hours of daylight for activities that occur in the afternoon and evening, such as outdoor recreation. Daylight saving time can also be a means of conserving electrical and other forms of energy. In the fall, as the period of daylight grows shorter, clocks are set back to correspond to standard time.

Benjamin Franklin, when serving as U.S. minister to France, wrote an article recommending earlier opening and closing of shops to save the cost of lighting. In England, William Willett in 1907 began to urge the adoption of daylight saving time. During World War I the plan was adopted in England, Germany, France, and many other countries. In the United States, Robert Garland of Pittsburgh was a leading influence in securing the introduction and passage of a law (signed by President Wilson on Mar. 31, 1918) establishing daylight saving time in the United States. After World War I the law was repealed (1919). In World War II, however, national daylight saving time was reestablished by law on a year-round basis. National year-round daylight saving time was adopted as a fuel-saving measure during the energy crisis of the winter of 1973-74. In late 1974, standard time was reinstituted for the winter period. In 1987 federal legislation fixed the period of daylight saving time in the United States as the first Sunday (previously the last Sunday) in April to the last Sunday in October; it was expanded in 2005 (effective 2007) to extend from the second Sunday in March to the first Sunday in November. Arizona and Hawaii do not use daylight saving time.

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.

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

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 ):

Planet	Distance from Sun (AU)	Relative solar intensity
Mercury	0.387	6.68
Venus	0.723	1.913
Earth and Moon	1	1
Mars	1.524	0.431
Ceres	2.765	0.131
Jupiter	5.203	0.0369
Saturn	9.539	0.0110
Uranus	19.189	0.00272
Neptune	30.060	0.00111
Pluto	39.439	0.00064

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 more than 60% of the solar light it receives. The actual illumination of the surface is about 5000-10000 lux, comparable to that of Earth during a dark, very cloudy day.

Daylight on Mars would be more or less like daylight on Earth wearing sunglasses, and as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus it would give perceptions and "feel" very much like Earth daylight.

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
• 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