On a blustery afternoon, a multinational clot of tourists waited for a turn to straddle some of the most ephemeral notions - time and space.
They had come to the Royal Observatory, perched on a steep grassy knoll in Greenwich on the outskirts of London, to stand by a brass line that runs through a courtyard. A preteen played hopscotch along the 4-inch-wide line, her braids bouncing; a family posed solemnly for a snapshot.
The line in the paving stones at the historic British observatory, now a museum, symbolizes the Prime Meridian of the world, or Longitude 0.
The Prime Meridian is the north-south line from which every place on Earth is measured in terms of its distance west or east. And, thanks to an international agreement, the Prime Meridian is the world's timing lodestone, the source of Greenwich Mean Time (GMT), from which all time zones are calculated.
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In a world now so precisely timed, it's hard to remember that even in the mid-1800s there was no internationally standardized measurement of time. Cities and regions kept their own time.
But as the world globalized - with railroads, telegraphs, telephones and so on - a unified time system was vital.
An international conference in 1884 chose Greenwich as the place from which world time would be standardized.
It was another feather in the cap of the Royal Observatory, which was founded in 1675 when Britain, a seafaring superpower, wanted to improve navigation by finding a way to more exactly measure longitude at sea - which would let mariners figure out where they were from east to west.
The world's lines of longitude - meridians - run from pole to pole and divide up the world. To calculate longitude (which, when combined with the more easily measured latitude, pinpoints exactly where you are in the world), accurate timekeeping was essential.
Through calculating the difference in time at a fixed point, such as Greenwich, and the local time, the longitude can be determined. (Originally, mariners tried to calculate longitude using astronomical calculations - hence the Royal Observatory was created to make charts of the heavens.)
To spur the invention of accurate marine clocks for measuring longitude - timekeepers that wouldn't lose time over months-long sea voyages and could withstand a ship's movement and humidity - the British government created the Board of Longitude. It offered a reward, worth millions of dollars today, for the best marine timekeeper.
The prize was won by English clockmaker John Harrison, in the late 1700s, after decades of toil.
Harrison's intricate marine chronometers are enshrined at the Royal Observatory. (The working observatory moved elsewhere decades ago.)
Climb up the steep hill to the museum, tucked amid the broad lawns and groves of Greenwich Park, and get lost in time. Stroll past historic telescopes and through small galleries packed with centuries of clocks, from old-fashioned pocket watches to massive gleaming brass clocks with whirling parts and pendulums.
Harrison's intricate clocks - including his fourth timekeeper that won the longitude prize (called H4 and built in the mid-1700s) are on display.
In Flamsteed House, the observatory's original 17th-century building, cozy rooms with period furnishings show where John Flamsteed, the first astronomer royal, and subsequent astronomers and their families lived.
The building was designed by Sir Christopher Wren, the architect of London's landmark St. Paul's Cathedral and an ardent scientist.
Meander into the Royal Observatory's planetarium for a state-of-the art look at the heavens and interactive displays on space missions, gravity and more.
Finish in the courtyard, straddling the brass line representing the Prime Meridian that defines time and divides the world into Western and Eastern hemispheres. You can have a foot in both worlds.