To know your exact position on the globe, it is necessary to be able to measure latitude and longitude. Today, it is possible to pinpoint location within one centimetre using modern technology, but until the eighteenth century, calculating exact whereabouts was largely guesswork. Since 600 BCE, people understood how to determine latitude: on land, a traveller needed only to measure the height of the sun above the horizon, while sailors used special instruments to measure the inclination of the stars to perform the same calculation while at sea. Longitude, however, was far more difficult to establish because, unlike latitude, its lines are not parallel. It was not until the late seventeenth century that the Italian astronomer Cassini showed how the timing of astrological events such as eclipses or stars rising in the sky could be used to determine longitude on land. Unfortunately, sailors could not time such celestial events with sufficient precision while at sea, due to the ship's constant motion. As global sea trade was becoming vital to the economies of the European empires, prizes were offered to anyone who could solve the problem of calculating longitude on board a ship. John Harrison, an amateur clockmaker, realized that an accurate timepiece could solve the conundrum. It took him decades, but eventually, in 1762, he engineered a marine chronometer that could reliably keep the time set at the start of a voyage. With Harrison's device, sailors could finally calculate the degrees of longitude they had traversed since their departure point and thereby ascertain their exact position on the planet.