In 1623, a German astronomer invented the earliest known mechanical calculating machine revealed centuries later in two unknown letters written to Johannes Kepler: How Wilhelm Schickard developed 'calculating clock'?

In 1623, a German astronomer invented the earliest known mechanical calculating machine, known as the calculating clock. Wilhelm Schickard designed it for addition, subtraction, multiplication and division. Details of the machine were found in letters he wrote to Johannes Kepler. The discovery changed the history of mechanical calculation, although later research raised questions about its operation, completion and influence on later calculating machines.

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