King Charles II ordered the construction of the Royal Observatory in 1675. It was located in the royal park at Greenwich, just downstream from London beside the River Thames. The observatory was intended to advance the science of astronomy as a tool for navigators, for what the king described as the “exact care and diligence to Rectifying the Tables of the Motions of the Heavens and the Places of the Fixed Stars, in order to find out the so much desired Longitude at Sea, for perfecting the Art of Navigation.”
Astronomy and navigation were inseparable – sailors knew how to use the Earth’s movement in relation to the Sun, planets, and constellations to find out where they were and the course they needed to steer. As the English expanded their trade and the reaches of the Royal Navy, and as explorers sought to plot points on their charts, they looked to the relationship between astronomy and navigation. This would help them in their quest for longitude, to measure an east-west position on the globe. This was of great importance to vessels sailing across the Atlantic and Pacific Oceans, for without an accurate measure of longitude, they faced the constant danger of losing ships and crew by running into the coast sooner than the navigators had expected. The Spanish and the Dutch launched similar efforts towards determining position when out of sight of land.
John Flamsteed was appointed the first Astronomer Royal in the 1670s. On the hill at Greenwich, away from the smoke of the city, he oversaw the construction of a small observatory building where he set up instruments purchased with his own money. These instruments included an enormous sextant with a radius of 2.1 metres that required a stepladder to operate, as well as telescopes and clocks. These were similar in purpose to the navigational tools carried aboard an ocean-going vessel. Flamsteed devoted himself to creating accurate catalogues and tables of the movement of celestial bodies by carefully recording his observations made at night. His work showed that it was possible to calculate longitude by observing the relative positions of the moons of Jupiter. He corresponded with Sir Isaac Newton of the Royal Society, who encouraged him to publish his catalogues.
Nevil Maskelyne, a successor to Flamsteed as Astronomer Royal, focused his work on navigation and published the British Mariner’s Guide in 1763. His tables proved important to Captain Cook, who took them on his Pacific voyages. Astronomy was practiced by many expeditions that set up portable observatories on shore to observe planets, comets, and other celestial features.
