There is a unique cluster of four medieval round churches, linked by a simple geometry, on Bornholm Island in the Baltic Sea. Why so many and why so close together? Immediate simple answers are "Just by chance" and "For no reason." Why are the churches round? "Defense." This essay proposes another hypothesis for this unique situation: the churches are astronomical observatories, meant to solve a scientific problem (Is the Earth really spherical?) and a practical problem (How far is it to sail west to the Orient?). The capacity and desire to find answers, together with other practical needs related to astronomy, can better explain these round churches' special architecture. The geometry that connects them fits the ideal pattern with an angular accuracy of 1 minute of a degree. The round churches may be the earliest astronomical observatories in Christian Europe; other hypotheses have been shown to be untenable. Their location provides for a good method to estimate the Earth's extent in the east-west direction, seemingly the earliest such measurements.