Everything about Daylight totally explained
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 isn't considered daylight as it occurs outside of the hours that one would consider "daytime".
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 isn't 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 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.
| Illuminance |
xample |
| 0.00005 lux |
Starlight |
| 0.0001 lux |
Moonless overcast night sky |
| 0.001 lux |
Moonless clear night sky |
| 0.01 lux |
Quarter Moon |
| 0.25 lux |
Full Moon on a clear night |
| <1 lux |
Moonlight |
| 400 lux |
Sunrise or sunset on a clear day. |
| 32000 lux |
Sunlight on an average day (min.) |
| 100000 lux |
Sunlight on an average day (max.) |
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
(External Link
)):
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's 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 s 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.
Further Information
Get more info on 'Daylight'.
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