UV index and UV dose: varying Sun-Earth distance
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Varying Sun-Earth distanceBecause 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_0the average distance, the correction factor is:f(D) = ( D / D_0 ) ^2This 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.
19 February 2019
data product contact: Jos van Geffen & Michiel van Weele & Ronald van der A
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