UV index and UV dose: varying Sun-Earth distance
data version 2.0

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UV radiation

UV index
UV dose

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Varying Sun-Earth distance

Because the Earth follows an elliptic orbit around the Sun, the Sun-Earth distance and this has to be accounted for in the UV index and UV dose calculation. With D_0 the average distance, the correction factor is:
         f(D) = ( D / D_0 ) ^2
This factor is computed with Kepler's first law from the date of the measurement and from the perihelion date and the eccentricity of the Earths orbit for the measurement year. The latter two are determined from the table for years 1901-2100 found in this external file (the eccentricity is 1 minus the distance at perihelion). The value of f(D) varies between about 1.034 on 3 Jan. and about 0.967 on 3 July.

We assume the correction factor f(D) to have no error, i.e.~that the elleptiticy of the Earths orbit is very well.


last modified: 19 February 2019
data product contact: Jos van Geffen & Michiel van Weele & Ronald van der A
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