<P> See diffuse sky radiation for more details . </P> <P> The Sun's electromagnetic radiation which is received at the Earth's surface is predominantly light that falls within the range of wavelengths to which the visual systems of the animals that inhabit Earth's surface are sensitive . The Sun may therefore be said to illuminate, which is a measure of the light within a specific sensitivity range . Many animals (including humans) have a sensitivity range of approximately 400--700 nm, and given optimal conditions the absorption and scattering by Earth's atmosphere produces illumination that approximates an equal - energy illuminant for most of this range . The useful range for color vision in humans, for example, is approximately 450--650 nm . Aside from effects that arise at sunset and sunrise, the spectral composition changes primarily in respect to how directly sunlight is able to illuminate . When illumination is indirect, Rayleigh scattering in the upper atmosphere will lead blue wavelengths to dominate . Water vapour in the lower atmosphere produces further scattering and ozone, dust and water particles will also absorb selective wavelengths . </P> <P> On Earth, the solar radiation varies with the angle of the sun above the horizon, with longer sunlight duration at high latitudes during summer, varying to no sunlight at all in winter near the pertinent pole . When the direct radiation is not blocked by clouds, it is experienced as sunshine . The warming of the ground (and other objects) depends on the absorption of the electromagnetic radiation in the form of heat . </P> <P> The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet . Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). As the orbital eccentricity changes, the average distance from the sun (the semimajor axis does not significantly vary, and so the total insolation over a year remains almost constant due to Kepler's second law, </P>

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