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Daylight is the combination of all direct and indirect
Length of Day and Twilight – Deriving the formulas to calculate the length of day
{{Authority control Atmospheric optical phenomena Light Parts of a day Visibility
Daylight is the combination of all direct and indirect sunlight
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
during the daytime
Daytime or day as observed on Earth is the period of the day during which a given location experiences Daylight, natural illumination from direct sunlight. Daytime occurs when the Sun appears above the local horizon, that is, anywhere on the ...
. This includes direct sunlight
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
, diffuse sky radiation
Diffuse sky radiation is solar radiation reaching the Earth's surface after having been scattering, scattered from the direct solar beam by molecules or particulates in the Earth's atmosphere, atmosphere. It is also called sky radiation, the ...
, and (often) both of these reflected by Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
and terrestrial objects, like landform
A landform is a land feature on the solid surface of the Earth or other planetary body. They may be natural or may be anthropogenic (caused or influenced by human activity). Landforms together make up a given terrain, and their arrangement ...
s and buildings. Sunlight scattered or reflected by astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...
s is generally not considered daylight. Therefore, daylight excludes moonlight
Moonlight consists of mostly sunlight (with little earthlight) reflected from the parts of the Moon's surface where the Sun's light strikes.
History
The ancient Greek philosopher Anaxagoras was aware that "''the sun provides the moon with its ...
, despite it being reflected indirect sunlight.
Definition
Daylight is present at a particular location, to some degree, whenever theSun
The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
is above the local horizon
The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...
. This is true for slightly more than 50% of the Earth
Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
at any given time, since the Earth's atmosphere refracts some sunlight even when the Sun is below the horizon.
Outdoor illuminance
In photometry (optics), photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate ...
varies from 120,000 lux
The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the irradiance, as perceived by ...
for direct sunlight
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
at noon
Noon (also known as noontime or midday) is 12 o'clock in the daytime. It is written as 12 noon, 12:00 m. (for '' meridiem'', literally 12:00 midday), 12 p.m. (for ''post meridiem'', literally "after midday"), 12 pm, or 12:00 (using a 24-hour cl ...
, which may cause eye pain
Pain is a distressing feeling often caused by intense or damaging Stimulus (physiology), stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sense, sensory and emotional experience associated with, or res ...
, to less than 5 lux
The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the irradiance, as perceived by ...
for thick storm cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant street light
A street light, light pole, lamp pole, lamppost, streetlamp, light standard, or lamp standard is a raised source of light on the edge of a road or path. Similar lights may be found on a railway platform. When urban electric power distribution b ...
s visible. It may be darker under unusual circumstances like a solar eclipse
A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season i ...
or very high levels of atmospheric particulates
Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspension (chemistry), suspended in the atmosphere of Earth, air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate ...
, which include smoke (see New England's Dark Day
New England's Dark Day occurred on May 19, 1780, when an unusual darkening of the daytime sky was observed over the New England states and parts of eastern Canada. The primary cause of the event is believed to have been a combination of smoke f ...
), dust, and volcanic ash.
Intensity in different conditions
For comparison, nighttime illuminance levels are: For a table of approximate daylight intensity in theSolar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, see sunlight
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
.
See also
References
External links
Length of Day and Twilight – Deriving the formulas to calculate the length of day
{{Authority control Atmospheric optical phenomena Light Parts of a day Visibility