Why is the Average Day Longer Than 12 Hours?

At a latitude of 51 degrees north (e.g. southern England), the length of the average day is 12 hours and about 16 minutes. Why is it not exactly 12 hours? We are defining a day here as the time between sunrise and sunset. By the 'average day' we mean the average over a whole year.

The explanation lies in the combination of three effects: the non-zero diameter of the sun, the refraction of light by the atmosphere, and the elliptic orbit of the earth which leads to the planet having more long summer days than short winter days (in the northern hemisphere). The fact that these effects account for the ~16 minutes excess is demonstrated in the pdf below.

Read the detailed derivation of the Length of the Average Day

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Red Dye Crowned in Milk: This crown is formed by the splash and droplets of a 2mm drop of red dye impacting on a thin layer of milk. The coronet initially forms without the droplets. The rim of the coronet then breaks up in a number of satellite droplets determined by the most unstable wavelength of the Rayleigh-Plateau instability.[Credit: Wim van Hoeve, Tim Segers, Hans Kroes, Detlef Lohse, Michel Versluis of the Physics of Fluids Group, University of Twente, The Netherlands, 2008]