Extreme-ultraviolet observation of the full Sun showing bright active regions, displayed in yellow.

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Preparing the 3D view

Preparing lighting and the first complete frame…

Model scale

Parallel sunlight at 1 AU. 1361 W/m² normal to the beam. local noon meridian.

0-90°

217-60°

375-30°

4330°

37530°

21760°

090°

Selected latitude: local noon 847 W/m² · daily mean 270 W/m². Incidence angle 51.5° from the surface normal. Declination 0.00°. These are horizontal top-of-atmosphere values at fixed 1 AU.

Sun and Earth

Explain how latitude and the Sun’s declination affect the solar energy received at noon and across a whole day.

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Why it happens

The same sunlight spreads across a larger area when it strikes a surface at a shallow angle. Latitude, solar declination and day length therefore affect the energy received by a horizontal surface.

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SDO's Atmospheric Imaging Assembly, 18 December 2015. This false-colour ultraviolet observation reveals the hot solar atmosphere, not the Sun's visible colour.

Try a comparison

Change declination to each solstice limit and compare noon with daily mean.

Reading the result

The calculation describes sunlight above the atmosphere; clouds and atmospheric absorption would change the energy reaching the ground.

A common misunderstanding

Sunlight above the atmosphere supplies the same energy to every square metre of ground.

The angle of the rays changes how incoming energy is spread over a horizontal surface. At a lower Sun angle the same beam covers more ground, and day length also changes daily energy receipt. Clouds, reflection and atmospheric absorption further modify the energy reaching the surface.

Cases

Exam language

Solar constant

Total solar irradiance near Earth's mean orbital distance, about 1,361 W m⁻² on a surface perpendicular to the rays above the atmosphere. It varies slightly over time.

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NASA SDO observes the solar photosphere and sunspots. The Sun supplies incoming solar radiation (insolation). The solar constant is irradiance near 1 AU on a plane perpendicular to the rays, about 1,361 W m⁻²; it is not measured by this visible-light photograph.

Insolation

Incoming solar radiation received at a surface. Its amount depends on the angle of the Sun, daylight duration and atmospheric conditions. Insolation is an energy input; air temperature also depends on energy losses, storage and transport.

Incidence

The angle between incoming radiation and the surface normal. A larger incidence angle spreads the same beam over more ground; solar altitude is measured above the horizon instead.

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Sunlight passing through Earth's atmospheric limb in an ISS photograph. At low solar angles, light crosses a longer atmospheric path and spreads across a larger surface area.

Sunspot

A relatively cool, dark region of the solar photosphere associated with strong magnetic fields.

Top of atmosphere

A reference level above most atmospheric absorption and scattering, used when describing incoming and outgoing radiation. Solar irradiance also needs a specified distance and surface orientation.

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NASA CERES observations, 2000–2015 mean. Colours show sunlight reflected by the Earth system in W m⁻². This is outgoing shortwave flux, not incoming solar irradiance or albedo itself.

Latitude

The angular position of a place north or south of the equator, expressed in degrees. Latitude helps determine solar altitude and seasonal day length. Lines of equal latitude are parallels, unlike meridians, which measure longitude.

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NASA Blue Marble monthly composite in an equirectangular latitude–longitude grid. Latitude measures angular distance north or south of the equator; parallels run east–west. The geographic reference lines are mathematical, not marks on the land.

Photosphere

The visible layer of the Sun from which most of its observed light escapes. Sunspots are relatively cooler, magnetically active regions within it.

Adjust parameters

Latitude51.5°Solar declination0.00°