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The old Tartu Observatory is the first point on the Struve Geodetic Arc, a chain of survey triangulations undertaken by Friedrich Georg Wilhelm von Struve in the 19th century to determine the size and shape of the Earth. Ossuusso sent me a card of the Observatory, which is one of 34 sites that make up the Struve Geodetic Arc UNESCO listing.

The card comes with an Estonian stamp that commemorates the Arc and shows the Observatory:

Although only 34 points are on the UNESCO list, the Arc consists of over 200 points spread through 10 countries, from Norway to Ukraine. I have no idea how the math works that they could accurately measure the size of the Earth from these observations, but it’s pretty amazing that 100 years before we made it into space, we could figure out how large the Earth is.