Geodetic Greenwich Meridian
On October 21st, 1884, at "The International Meridian Conference", in Washington DC, it was established that the zero-degree meridian would be that which passed right through the Greenwich observatory. A century later, it was moved 102 meters east to match the "geodetic" meridian, which passes through the Earth's center of mass.
Source: Tyson, N. d., & Lang, A. (2018). Accessory to war: The unspoken alliance between astrophysics and the military. W.W. Norton & Company.
Image by Charlie Sanabria under CC BY 4.0
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!