Daylight

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For other uses, see Daylight (disambiguation).

World map showing 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.

1 Definition
2 Daylight intensity in different conditions
3 Daylight intensity in the Solar System
4 Effects
5 See also
6 Notes
7 External links

[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 explantion 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.

[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

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^[1]
0.25 lux	Full Moon on a clear night^[2]
0.01 lux	Quarter Moon
0.001 lux	Moonless clear night sky
0.0001 lux	Moonless overcast night sky
0.00005 lux	Starlight

[edit] Daylight intensity in the Solar System

It has been suggested that this article or section be merged with sunlight. (Discuss)

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]):

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 up to 60% of the solar light it receives, so the actual illumination of the surface is comparable to that of Earth.

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.

[edit] Effects

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, 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.