Determination of one's longitude at sea has perplexed sailors for many centuries. The significant uptake of world trade in the 17th and 18th Centuries rendered the increasingly urgent need to solve the 'longitude problem' an issue of strategic national importance. Historical accounts of these efforts often focus almost exclusively on John Harrison's role in 18th Century Britain. This book starts instead from Galileo Galilei's late-16th Century development of an accurate pendulum clock, which was first achieved in practice in the mid-17th Century by Christiaan Huygens in the Dutch Republic. The open, tolerant and transparent conditions in the 17th Century Dutch Republic allowed the nation to play a pivotal role in the international network of humanists and scholars before and during the 'scientific revolution.' The primary means of communication among the educated intellectuals consisted of a prolific exchange of letters. This book is therefore primarily based on collections of letters that have not been combined into a single volume before, but which represent a well-defined body of work on one of the key scientific and practical issues of the time in the 17th and 18th Centuries. Extensive introductory chapters on the history of mapmaking, the establishment of the world's reference meridian at Greenwich Observatory, and the rise of the scientific enterprise provide the appropriate context for non-expert readers to fully engage with the book's main subject matter.