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MSG/SEVIRI -- Reflectance

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#3 : d(t), Sun-Earth distance in Astronomical Unit (AU).

If t is the day of the year, then

M = 0.9856*float(t-4)* pi / 180.
d(t) = 1.-0.01673*cos(M)

AU, which stands for "astronomical unit", is a unit for measuring distance. One AU is the average distance from the Sun's center to the Earth's center. It is equal to 149,597,871 km (92,955,807 miles).