Atop a steep hill in western London, overlooking the River Thames, is the U.K.’s Royal Observatory Greenwich. The modest complex of brick buildings, tucked away in Greenwich Park, doesn’t seem like much of an observatory to anyone up-to-date with modern astronomical technology. It doesn’t have a massive reflector telescope, or an array of radio telescopes beside it. Indeed, with all of the light pollution around London, it wouldn’t be very useful as an observatory anyway. Nowadays, it isn’t a functional observatory at all.
Instead, it houses a small museum dedicated to its rather storied past. One of its buildings housed, until 1955, the Astronomer Royal, a position created in 1675 alongside the construction of the core part of the Royal Observatory. The position, and the observatory, were necessitated by the increasing ability and desire of Europeans to explore the world.
The goal of the Observatory and its staff was to utilize astronomy to allow for much more accurate navigation by, as King Charles II said, “rectifying the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so-much desired longitude of places” The longitude of a location is defined by the line which goes through the location, starts at the north pole and ends at the south pole. The Prime Meridian, which cuts directly through the Royal Observatory, is the “zero-th” longitudinal line, and all other longitudes are defined by their distance to the Prime Meridian.
Since the actual distance between two longitudinal lines changes as they go north or south, the difference between them is measured in hours and minutes. We know that it takes the Earth a full 24 hours to rotate, and so we can draw 24 longitude lines which each exist at a different hour in the day. Using some fancy math, sailors at sea could calculate their longitude by observing the stars and the sun, but this method required complex and accurate star-charts.
Thus, the first mission of the Royal Observatory was fairly simple: create the most accurate and up-to-date star charts possible. A much simpler way was known as well, which was to keep track of what time it is on the ship, what time it is back home, and subtract. However, there were two problems with this method. The first, and easiest to overcome was the requirement for a “fixed” time, against which the local time could be compared. Thus, the invention of Greenwich Mean Time. The time observed at the Royal Observatory was declared the “zero-th” hour, and all times were (and are) measured relative to it. The second, more serious challenge was keeping accurate time aboard a ship.
Although sailors could determine their own local time by the sun, knowing the time at Greenwich was nearly impossible. Watches weren’t accurate enough, and conventional mechanical clocks are completely useless on ships since the swaying messes with the pendulum. Ultimately, the Royal Observatory helped to develop a sequence of increasingly effective ways of keeping track of GMT anywhere in the world. Throughout the years, the Royal Observatory worked to keep star-charts updated and pioneer new ways of tracking time. Though it is now a museum, the Royal Observatory once served as the lynch-pin of global commerce and trade, broadcasting the current GMT time across the world and enabling the rapid expansion of international trade which came with the industrial revolution.